Flexible and rigid-flexible circuits are difficult to work with since they are not as easy to understand as traditional printed circuit boards are.
There are not nearly as many variations, properties, or design methods to traditional circuit boards as flexible circuit boards or rigid-flexible circuits.
However, you will likely be working with flexible and rigid-flexible circuits more and more in the future.
It's best to familiarize yourself with how you can arrive at a clear decision about which you should use.
This guide will cover rigid-flexible circuits first, then flexible courses.
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I - Rigid Flexible Circuits
This is a rigid-flexible circuit board.
1 What is a Rigid-Flex PCB?
As the name suggests, a rigid-flexible circuit combines some of a traditional circuit board's qualities and a flexible circuit board.
To be more specific, a rigid-flexible circuit contains multiple layers of rigid and flexible technologies.
It allows the electronics that utilize it to function at an optimal level without the risk of breaking.
Something you should understand about rigid-flexible circuits is that they do not stay in a fragile state. Instead, they always resemble a curve.
One of the reasons why rigid-flexible circuits are worth learning is that many current electronic devices operate better on rigid-flexible channels than traditional PCBs.
It is why you need to know how to design, assemble, and learn about the rigid-flex circuit's production processes.
Just like traditional PCBs were the industry standard for circuits and electronics decades ago, rigid-flex courses are slowly becoming the industry standard for the current day.
It is also true of flexible circuits, which will be covered later.
For now, let's explore the different kinds of designs that rigid-flexible circuits follow and how they are related to their assembly.
2 - Rigid-Flex PCB Design
Assembling rigid-flexible PCBs takes a lot of creativity and effort, mainly due to the various layers of a rigid-flexible circuit.
Designing a rigid-flexible circuit requires intense attention to detail, and this is what can turn some off from them.
The first thing that you should understand when designing such a PCB is that while each layer does have its design, the layers also need to be assembled. They again cannot intersect.
The most disturbing aspect of the design and assembly process of rigid-flex PCB is this aspect.
Combine this with the fact that technology changes and evolves continuously, and you'll discover very quickly that there is no one absolute way to make a rigid-flex PCB.
Another thing that you need to understand is that the purpose of this rigid-flex PCB can also affect how you should approach its design.
It is the first thing you need to determine before ordering the materials needed to assemble a rigid-flex circuit.
Ask yourself just what kind of equipment to use flexible PCB. It will guide you in the assembly and design process.
2.1 - Materials
Two common materials need to make rigid-flex PCB.
The first of these is copper. As a universal conductor in all circuits, you'll never be able to get away with not using copper in your rigid-flexible courses.
Polyimide adhesive is the second universal material. It is necessary to join the rigid and flexible layers together.
Working with polyimide adhesive can take some practice, but you will need to get used to it if you ever hope to construct an excellent rigid-flex PCB.
2.2 - Rigid-flex PCB assembly
While there are only two universal materials necessary to create a rigid-flex PCB, the additional elements you have to choose from are endless.
It is what makes creating a rigid-flex PCB so complicated.
There are so many variables to consider here that can lead to a lot of mistakes. These mistakes can destroy the circuit and even the application.
However, some design "templates" do exist that you can follow if you are new to crafting rigid-flex PCBs.
Flip chip, wire harness, ultrasonic welding, and in-circuit testing are a few proven methods that can effectively guide you to assembling your PCB.
These methods are used in conjunction with other ways to create a circuit board that is both durable and flexible, which is the entire point of a rigid-flexible PCB.
2.3 - Design Considerations For Rigid-Flex PCBs
Electromechanical design is the first consideration.
Electromechanical factors are made up of the bend to thickness ratio, the bend line, and the rigid-flex PCB curve.
The more thick the wire of your board is, the larger the curve needs to be. While this gives you a little more room to work, it also increases the risk of failure.
Your rigid flex PCB will have a bend line. It affects their routine, and you should do all you can to place any components on or around it.
Curve traces are responsible for routing the power within your rigid-flex PCB.
Do all you can to route your traces in the form of curves instead of straight lines. Doing this will prevent the board from bending too much.
These are all the things you should consider when designing rigid-flexible PCBs in terms of methods and materials.
You need to familiarize yourself with things when it comes to the production process of the rigid circuit board.
3 - The Rigid Flexible PCB Technology and Production Process
When it comes to technologies surrounding rigid-flex PCBs, one of them is "stack up."
3.1 - Flex Rigid PCB technology (the "stack up")
Although some rigid-flexible PCBs can contain one layer, may include at least two layers.
The more layers the board has, the more uses it will have in its applications.
These layers can think of as ingredients for a recipe. If they combined in the right way, they would create a beautiful dish.
If only one thing is wrong with one of the ingredients or if one of the parts is missing, it will negatively affect the dish's quality.
If you know which material can place in any position in the stack, it will contribute significantly to the rigidity - the overall production of flexible circuit boards.
The stack-up creates a "base" of sorts for your rigid-flexible PCB, but there is more to it than that.
3.2 - Embedded flex manufacturing technology
You can also use embedded technology to manufacture flex-rigid PCBs.
As the term suggests, doing this entails embedding flexible circuits onto rigid units to continue with the layering process.
Doing this means you will use buried and blind vias for connecting the layers. You will not be able to use interconnections for rigid and flexible circuits in the same layer here.
One of the best benefits of this approach is that you will use fewer materials since you won't waste too much substrate material.
You can also reduce the overall size of the board while keeping a high level of performance.
Both of these allow you to save money and resources.
It does not mean that embedded technology is optimal. There are some circumstances where you will need to utilize stack up technology.
In any event, check if your application can handle embedded technology before attempting it.
3.3 - Flex-rigid PCB types of methods
There also exist a few methods about how the manufacturing of these PCBs can finalize. Consider these shortcuts of sorts for either stack up or embedded technologies.
3.3.1 - The Window Opening Method
In this method, lamination by way of mold punching is done in an attempt to remove the rigid core while at the same time retaining the properties of a rigid-flexible circuit.
3.3.2 - Copper Foil Etching Method
This method utilizes copper foil structures for making the flexible portion of the rigid-flexible PCB.
3.3.3 - Positive And Negative Depth Control Method
A blind groove is created on the rigid board right next to the flexible board in this method.
This groove is then laminated and combined with the rigid plate when molding.
The rigid plate is then removed in the window position, leaving the flexible portion exposed so that you can make whatever modifications you need to make.
3.4 - Flex-rigid PCB processes
Rigid-flex PCBs solve contact and muscular heat-related issues that are caused by the wiring harness and connectors.
It improves the reliability of the devices they use.
Rigid-flex PCBs are at a premium because contact and sturdy heat management itself is at a premium.
The technology that surrounds rigid-flexible circuits is a little different than the technology that other circuit boards follow.
The following are a few technologies that have proven themselves useful.
3.4.1 - Drilling Technology
Obtaining a perfect drill wall in your rigid-flexible PCB is a little more complicated than it is with other circuit boards.
To create a perfect drill wall, check the drill bit's quality and the other parameters first.
You need to do this because rigid-flexible PCBs require both a hole cover and a base plate.
There are two specific things about the drill bits that you need to pay attention to the sharpness and the rotation and feed speed.
Drill bits that are the least bit dull will cause the nail head to appear on the flexible substrate.
It is why you should probably use a different drill bit for every hole that you create on the board.
The feed and rotation speeds of the drill also need to be just right. You'll produce a lot of heat and waste if they are too slow.
Go too fast, and you might create a nailhead on the board.
3.4.2 - Desmear
You need to be especially aware of rigid-flexible PCBs that it is twice as likely to produce contamination on the hole wall because it uses rigid and flexible materials.
Understand that the more materials you need, the possible contamination that might occur.
It is the reason why the completion of each drill holes in the rigid-flex PCB requires decontamination.
3.4.3 - Electroless Plating
Electroplating of most conventional circuit boards usually results in electric shock.
However, because flexible layers are used in rigid-flexible circuits, the plating needs to be done using oxidation.
It thickens the metal layer of a hole in the board, allowing copper ions to become oxidized.
It reduces shock and will prevent the rigid-flex PCB's already sensitive flexible materials from getting too hot.
3.5 - Benefits of rigid-flex PCBs
It is perhaps an essential part of this guide thus far.
Why do people use rigid-flex PCBs?
An easy answer to this question is because they are versatile.
As accurate as a statement as that is, we need to explore rigidity - flexible PCB's versatility.
Rigid-flex PCBs are far more customizable than their generic counterparts. Unlike generic PCBs, designers can customize the shape and size of a rigid-flex PCB to fit the device they are working with correctly.
If you happen to make a design that you like but have trouble making a PCB for it, you can craft a rigid-flex PCB to fit the model better.
Rigid-flex PCBs also have a compact and flexible design. It means the size of various accessories can reduce.
More components in devices are also allowed in a rigid-flex PCB, which significantly improves the overall product.
One of the best examples of this is in smartphones. Rigid-flex PCBs are practically made for smartphones, as they give the designers the liberty to make a PCB that caters precisely to the designs and specifications they had in mind.
Nothing beats rigid-flexible PCBs when it comes to heat absorption. The majority of their substrates are made of polyimide, which is extremely heat resistant and UV rays and radiation.
After being processed and made, rigid-flex PCBs wind up being much thinner than their counterparts.
3.6 - Rigid-flex PCB applications
Rigid-flexible circuits have various applications in different types of accessories and equipment that we use daily.
The most prominent example of this is in smartphones. Today's smartphones need to be very thin as well as functional. It can only achieve using rigid-flex technology.
Various computer electronics have been using rigid-flexible circuits for years, if not decades.
For example, hard drives over the past few years can be smaller and smaller in size yet can keep up with demanding write speeds.
Hard drives used to resemble a brick. It is no longer necessary thanks to rigid-flexible technology.
The medical field has also taken advantage of rigid-flexible circuits. The most prominent example of this is an invention called the pill cam, allowing a medical professional to see inside a patient's stomach.
These are just a few examples of rigid-flexible PCB applications.
3.7 - Rigid-flex PCB cost and pricing
The overall cost of a rigid-flex PCB will vary depending on the type you get and where you get it.
Keep in mind that rigid-flex PCB can possess 1 to 4 layers, with each layer consists of different materials. It is what makes the overall cost vary so much.
Another major factor in the cost and pricing of rigid-flex PCBs is the manufacturer. This factor is a little more relevant than the type of rigid-flex PCB you want since it is the manufacturer's job to turn your design into a reality.
3.8 - Tips For Choosing The Right Rigid-flex PCB Manufacturer
More and more circuit board manufacturers realize that rigid-flexible PCBs are the way of the future.
Because of this, there are more and more to choose from than ever before.
Understand that rigid-flexible PCBs are a little more complicated than traditional circuit boards and these complications go right down to how they manufactured.
There are nine questions that you need to ask before choosing a rigid-flex PCB manufacturer.
All of these are important and will affect the quality of not only the rigid-flex circuit board itself but also their related applications.
3.8.1 - Is the manufacturer following the proper standards, and are they certified?
It is the very first thing you should consider, and it is probably the most important.
No matter how much or how little a manufacturer costs, they need to be certified.
For a manufacturer to be certified, they must follow specific rules and regulations.
The following four are hard-and-fast regulations that act as minimum requirements for any given manufacturer:
-ISO 9001:2008 standards
-UL Standards
-IPC-A-600G Class 1 and 2 standards
-ISO 9001:2015 standards
Choose a different manufacturer if the manufacturer you are interested in does not follow all four of these regulations.
Make sure the manufacturer has a lot of certifications. The more credentials it has, the better. It shows that the manufacturer experienced it.
3.8.2 - How experienced is the manufacturer?
Speaking of experience, this is another essential thing to consider.
It is true that newer manufacturers might be cheaper or might appear more impressive. But do not be fooled. "New" does not mean "good" in this field.
When dealing with rigid-flex circuit boards, you're going to want the most experienced manufacturers out there.
Experienced manufacturers can adapt to industry changes, can produce several varieties of rigid-flexible PCBs, and they possess other advantages that newer manufacturers just do not have.
3.8.3 - Does the manufacturer have a proven track record in quality?
Checking for quality matters a lot in any given production, but it matters even more, when it comes to rigid-flexible PCBs.
We all know what happens if failures are found in your boards after installation: Significant amounts of time and money get wasted.
Ensure that all components in the rigid-flexible circuit boards are of the best quality.
3.8.4 - Does the manufacturer provide excellent customer support?
Any positive or negative thing that said about any given organization revolves around customer support.
Rigid-flex PCB manufacturers are no different.
At the base of customer support lies three things: Finding a solution, knowledge about the product, and availability.
You will want your manufacturers to provide reliable customer support if you are new to rigid-flexible PCBs.
3.8.5 - Does the manufacturer use quality machinery?
Rigid-flex PCBs use tiny parts, parts that are smaller than you might think.
Make sure that your manufacturer uses machinery that assembles these small parts correctly.
If possible, take a tour of their plant, either in person or virtually.
Ask the manufacturer what kinds of machinery they use and then compare it to other manufacturers' mechanism.
It is something that absolutely cannot ignore.
3.8.6 - Does the manufacturer Possess a diverse product line? Can they produce in bulk?
Large volume scales must consider for any given circuit board manufacturer, including those who work with rigid-flex PCBs.
Choose another manufacturer if the one you are interested in cannot produce several requests at one time.
Multitasking is at a premium when it comes to fast production. It is why the ideal manufacturer should have a diverse product line.
3.8.7 - Does the manufacturer store their completed circuit boards properly?
Three things can utterly ruin any circuit board, especially rigid-flex ones: Humidity, hot temperatures, and open air.
Make sure your potential manufacturer knows better than to store circuit boards under these conditions.
While hot temperatures typically cannot be helped, humidity and open-air can micromanage.
It does by way of vacuum-sealed packages that consist of silica gel.
Make sure your manufacturer knows about this and adheres to it.
3.8.8 - Is the personnel of the manufacturer qualified?
It seems like it should be common sense, but it overlooked all the same.
The best manufacturers operate as a unit all the time. They are also adequately trained and have excellent communication skills.
If you have qualified personnel, you can easily handle production details.
3.8.9 - Does the manufacturer conduct regular inspections?
Quality control is at a premium when it comes to the manufacturing of rigid-flexible circuit boards.
The only way quality control can manifest by conducting inspections, to make sure that you want to work with the manufacturers.
Make sure the manufacturers conduct their inspections in-house, as well. Outsourcing inspections are not reliable when it comes to rigid-flex PCBs.
Now you know what a rigid-flexible printed circuit board is, the technology behind them, how they made, and what you need to do when sourcing them.
It is now time to explore flexible circuitry and how they differ from the rigid-flexible chip.
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II - Flexible Printed Circuit Boards
1 - Flexible Circuitry – What Is It？
Although rigid circuit boards still have a place in the electronics field, they are gradually replaced by another more optimized circuit board: flexible printed circuit boards.
The general definition of a flexible circuit board is a circuit board that can bend. However, it is not that simple. You won't use the same layout for flexible PCBs as you can with their rigid counterparts.
A dielectric layer connects with a metallic coating of traces in flexible circuitry—this dielectric layer is made of either polyester or polyimide.
Flexible PCBs use copper to conduct just like rigid circuit boards do. However, different from wooden circuit boards, copper can be oxidized easily in a flexible circuit board.
Because of this, solder or gold is used to protect it.
It covers the general design and the foundation of the flexible PCB.
But why would you ever need to use them if rigid circuit boards can accomplish similar things?
Why is Flexible Circuitry Necessary?
Have you ever heard the phrase "they don't make them like they used to?"
It refers to the observation that new technology tends to fail faster than old technology.
Why do you think that is?
It's because current-day technology is still trying to use methods that are decades old in that designers still think that using rigid PCBs that haven't changed since the 1970s can work properly in the latest electronics.
The following devices can all be optimized using flexible circuits: antennas, laptops, mobile phones, LCD TVs, calculators, cameras, hearing aids, satellites, and printers.
That is why flexible circuitry is necessary. It is a lot more durable and reliable than any given rigid circuit board, especially in the above applications.
1.1 Fundamental Types of Flexible Circuitry
Size, configuration, and functionality is one way to classify flexible circuits. These can summarize as different "templates" of flexible PCBs
Single-sided
Single-sided flexible circuit boards contain a single layer of metal trace arranged on only one side of the dielectric layer.
It is made possible by bonding a flexible polyimide film to a metallic copper sheet.
After that, a copper layer chemically etched to develop your desired circuit pattern.
After some time, the polyimide dielectric layer bonds, providing environmental protection and insulation of the circuit.
Single-sided with dual access.
It is essentially the same as a single-sided flexible circuit, and only copper is accessible from both sides of the board instead of just one.
To accomplish this, laser techniques skive open the dielectric layer to get dual access to the single conductive layer.
Double-sided
Double-sided flexible circuits have a single dielectric layer with metal layers on both sides.
It is made possible by affixing conductive copper layers to substrate film by way of a through-hole.
To create proper trace patterns here, simply follow whatever requirements are necessary for both sides of the polyimide film.
After doing this, it is best to connect it to two copper layers using plated through-holes.
Multi-layered
Multi-layered flexible circuits go where other flexible courses don't.
They combine many complex single and double-sided circuits in a multi-layer fashion. Some particular ones also have laminations throughout its structure.
Multi-layered circuits are your solution if you are struggling with impedance requirements, unavoidable crossovers, eliminating crosstalk, high component density, and additional shielding in your PCB design.
Rigid-flexible circuitry
These were already explored in great detail earlier, but the one thing you must understand about rigid-flexible circuits is that they all use multiple layers.
They can also be incorporated internally, externally, or both.
Now that you know about the types of flexible circuitry, it is time to explore the benefits of these circuits.
1.2 - Benefits of flexible circuitry
As mentioned previously, flexible circuits can be used in almost any commonly used device today.
Here are some specific benefits that they have over traditional rigid and cabling circuit boards.
1.2.1 Decreased assembly costs and time
Flexible circuitry reduces wrapping, routing, and soldering wires because many of them are completed in pre-production.
Manual labor is also minimized when it comes to flexible circuitry.
1.2.2 Error reduction during assembly
Wire harnesses in rigid circuit boards often need to be built by hand. It can cause a lot of human-made errors.
These errors are reduced and, in some cases, eliminated since the production of most flexible circuit boards is automatic.
1.2.3 More design options
Flexible circuits are multi-dimensional, giving you a lot more design options than their rigid counterparts do.
1.2.4 Flexibility in installation
Flexible circuits are also multi-dimensional in terms of their installation, connecting with at least two different planes.
They can also deploy several times when being executed without any electronic faults.
1.2.5 Enhanced airflow
This benefit is massive; it is one of the biggest reasons why flexible circuits are the way of the future.
Flexible circuits follow a modern design, allowing air to flow more freely through their electronic components.
1.2.6 High density uses
Electronics of today operate on a high density. Flexible circuitry makes creating these high-density electronics much more comfortable.
1.2.7 Improved dissipation of heat
The surface-to-volume ratio is something that affects the thermal path of any given circuit board.
The compact design of flexible circuitry yields a higher surface-to-volume ratio than rigid circuits do.
Flexible circuits that have a slim design also permit heat to dissipate from both sides of the course.
1.2.8 Durability and reliability
Most electronics today contain a lot of moving parts. It is not an issue with flexible circuits, making them extremely reliable.
Flexible circuits can stand up to high temperatures as well, thanks to their polyimide base.
1.2.9 Repeatable routings
Hand wiring and solder that are used in rigid circuit boards cannot repeat.
Flexible circuits effectively replace this, making routings repeatable.
1.2.10 Reduced weight and size of packages
Traditional circuit boards contain many systems that add weight and take up more space.
It is not an issue when using flexible circuits. The polyimide substrates they use are extremely thin, much thinner than any given substrate in any given rigid circuit board.
As fantastic as these benefits are, working with flexible circuits is different from working with rigid circuits.
1.3 - What is the best way to use flexible PCBs?
It would be best to use flexible PCBs whenever you need a high adaptability level without compromising performance.
It is entirely possible to swap out a rigid circuit board for a flexible one, and there are four examples of this.
The first example is in computer electronics such as printers, gaming systems and consoles, microwaves, and flat-screen televisions. All of these use flexible PCBs, or at least they should.
Numerous modern vehicles also use flexible PCBs. Airbag controllers, braking, and dash systems in cars made over the last decade or so all utilize flexible PCBs.
The third and most significant example of this is in smartphones. They make smartphones light and portable.
The last example of this is in the medical industry. There have been a lot of developments in this industry that utilize flexible PCBs.
Educating yourself about the assembly technology surrounding the flexible PCB is the most important thing you can do before using them in your applications.
2 - How to Choose The Right Assembly Technology of Flexible PCBs
Choosing the right assembly technology of flexible PCBs requires you to do two things.
First, it requires you to get educated about the general structure of the board. Flexible PCBs follow a different format than rigid PCBs do.
Second, you need to choose the right materials. Flexible PCBs use different materials than rigid PCBs do.
2.1 - The structure of flexible PCBs
As mentioned earlier, flexible circuits use more than just one layer.
You have received training on the kinds of flexible circuit boards that exist, but now you need to be informed of these types' structure details.
Single-sided boards can fabricate quickly. The structure is not very complicated, either.
Double-sided boards have a more complex structure. You'll need a decent amount of attention to detail when assembling and designing these.
Multi-layered boards require an amount of attention to detail that will often require you to seek out the views of other designers to see if everything about your design is perfect.
2.2 - Choosing the right material for a flexible PCB
Flexible PCBs work from a draw of four significant materials. All four of them used, no matter what the structure of the PCB is.
Any given flexible PCB is made up of a substrate, a conductor, an adhesive, and a cover layer.
Substrates themselves use three different materials: Polyimide, polyester, or FR4.
While FR4 is only used in rigid circuit boards, you might be able to get away with using its slight variations, especially if you are attempting to design a rigid-flexible board.
It leaves polyimide and polyester. Both of these are acceptable for flexible PCBs. However, both are different in terms of solderability temperature, tensile strength, and cost.
Polyester is very cheap, and you will save money when using it. But it cannot withstand high temperatures at all.
Polyester can only handle temperatures up to 110 degrees celsius. It is half of what polyimide can withstand, which is 220 degrees.
Polyimide is also much more durable than polyester when it comes to tensile strength. Soldering, it is much easier compared to polyester.
Use polyimide for your flexible PCBs if you have the money to spend.
Polyester is an option if you want to save money, but you need to be very good at soldering and control temperatures to have an easy time with it.
2.2.1 - Which materials should you use as flexible PCB substrates?
Three popular material types usually chose for the applications of a flexible PCB: Polyimide, polytetrafluoroethylene, and polyether ether ketone.
Polyimide, or PI, is by far the most popular of these. It is the cheapest of the three and has adequate reliability and performance.
The flexible circuit boards of almost any given smartphone, tablet, calculator, or even camera probably contain a PI substrate because it is accessible and easy to use.
Polytetrafluoroethylene, or PTFE, offers an acceptable temperature range, a high level of stability, as well as low dissipation.
Applications for electronics in the aerospace and automotive industry, such as guidance control, driving assistance control, and antennas, can support flexible circuit boards with a PTFE substrate.
Then there is polyether ether ketone or PEEK. PEEK substrates offer extreme chemical resistance, handle radiation very well, and have an impressive heat range.
PEEK-based flexible circuit boards are almost exclusive to applications in the medical industry.
There is a good chance that the X-Ray machines in your local medical facility operate on a flexible PCB with this kind of substrate.
Cover layers are another of the four foundational materials in any given flexible PCB. These cover layers use something called cladding components.
2.2.2 - Cladding components
The purpose of a cladding component is to keep the entire board protected by utilizing sufficient insulation.
It best does by way of something that engineers refer to as a copper-clad laminate or CCL.
For any given CCL, the best resin you can find is polyimide. It reduces the risk of softening as well as maintains its flexibility and elasticity.
Even though it is more expensive, choose a higher quality film for this kind of resin. It will decrease the absorption of humidity and reduce tear strength.
The following foundational material necessary for a flexible circuit board is adhesive for the CCL.
However, this used to be a foundational material, before experts discovered ways around it.
2.2.2.1 - Why should you go adhesive-free in CCLs?
Something discovered about using adhesives in CCLs was that they caused limitations in electrical performance and dimensional stability.
Thus, removing adhesives was at a premium. CCLs could contain glue by constructing two layers of CCL instead of just one, but this changed the assembly process and made things more expensive.
Despite this, there are options you have when it comes to going adhesive-free in CCLs. They are not expensive, either.
Lamination is the right choice if your board has two sides.
Film coating works well if you need to mass-produce CCLs.
Electroplating is the most affordable option as it is as close to being adhesive without actually being adhesive.
2.2.3 - What role do halogen-free substrate and liquid crystal polymer play?
As proven as it is for both a substrate and a CCL resin, polyimide is not perfect.
It begs to be improved, and it can grow by creating something called a liquid crystal polymer, or an LCP.
And LCP is made by placing copper foil on top of a liquid crystal polymer film. It is then hot pressed to create a resin for a CCL.
The result is something with a humidity absorption rate of 0.04%.
The CCL you get from this can be used in high-frequency circuits with digital characteristics without worry.
One of the best things about LCP is that it can be recycled, making it eco-friendly.
Halogen use is something else that should not only be improved but must be.
The European Union began creating laws and regulations in 2003 that restricted hazardous materials in electronic devices.
Halogen is one of these hazardous materials.
It changed the circuit board industry in Europe, as those who wanted to manufacture and sell circuit boards in Europe needed to stop using halogen.
Therefore, halogen-free substrates started emerging, making polyimide almost obsolete.
2.2.4 - Other flexible PCB materials you should also know.
Any given flexible circuit board needs a conductor.
Copper and copper foil has been the industry standard for conductive materials for decades.
However, depending on the application you need flexible PCB for, you may need to explore other options.
These options are aluminum, nickel, gold, silver, or any other conductive alloy.
As far as copper foil, there are two types that you can use.
Rolled and annealed, or RA has a column array shape. It implies that the structure is flat and even.
RA copper foil has excellent flexibility and is a very popular flexible PCB conductor.
Electro-Deposit or ED copper foil has a fish scale structure. It can secure any required smoothness or toughness that is necessary.
Keep in mind that you can't roughen ED copper foil or put it through any kind of etching process.
However, ED copper foil is the right choice if you need to make a high-density flexible circuit board.
You might also see a silver paste alongside conductive properties used as a printed layer in flexible boards.
These can apply to a project to improve reliability and flexibility.
Another way you can improve these is to use photosensitive polyimide as an overlay. It also enhances density and stability.
2.2.5 - Can you make a rigid-flex circuit board with the same materials as a flexible circuit board?
As you already know, rigid-flexible circuit boards combine rigid circuit boards and flexible ones.
Because of this, you can undoubtedly use polyimide as a material for the flexible half of these boards, but you'll need stuff for the rigid half.
RO3000 and RO4000 are two popular choices for this.
RO3000 is a PTFE variation that is infused with ceramics. It secures electronic and mechanical stability.
Multi-layer units used in radars and satellites can take advantage of what RO3000 materials have to offer.
RO4000 is an affordable option if you need reliable performance in high frequencies. It is a hydrocarbon ceramic laminate, which contributes to its dimensional stability.
2.2.6 - What factors should you consider when choosing the right materials for your flexible PCB?
Designing an optimal structure for a flexible circuit board all falls on choosing the right materials.
Knowing about all the materials and how they come together are a few pieces of this puzzle, but the crucial final part is choosing a good company.
The manufacturer you select needs to have the right equipment and experts to ensure that every detail of the process is handled adequately.
Look for an organization that can meet or exceed your expectations while also saving you money.
New organizations or organizations that have an appealing "culture" to them are just that; attractive.
But can they exceed your expectations or save you money?
Probably not.
OurPCB doesn't try to be flashy. They succeed in providing information, containing engineers with years of experience in the industry.
You cannot go wrong with material selection for your application when contacting OurPCB.
OurPCB also has the staff and equipment that makes it possible to assemble even the most advanced flexible boards quickly.
Another thing OurPCB offers is free information about what goes into the manufacturing process of flexible circuit boards.
3 - The manufacturing process of flexible PCBs: Blanking and drilling
Blanking is the very first thing done in the assembly process.
It entails identifying all the materials that you need and has at your disposal.
It also entails including a separation film and pastes used for lamination or anything else you might need from auxiliary materials.
Drilling is the second thing that takes place. It is difficult because you will be dealing with soft and thin materials.
Experts recommend stacking the boards first. It makes drilling a little easier.
You can do to achieve this to take the cladding layer and stack up all of its pieces, making the materials resemble a book.
4 - Desmear and pitting
Just like with rigid-flex PCBs, you will have some residue left on the material after drilling.
Do all you can to remove this residue before moving on. Be extremely careful when doing this since you are dealing with small and thin components.
This process is called "decontamination." It needs to complete before the "pitting" process, which entails pattern plating and plated-through hole, which circuit engineers refer to as PTH.
There are three pitting options that you have at your disposal.
The first of these is electroless plating. It requires you to have a pre-treatment solution.
The best substance to use here is acidic colloid palladium. It performs better than alkaline ionic palladium since it has much better speed and reaction time.
It will prevent materials from swelling or making unnecessary holes.
Electrical plating is the second option you have.
Copper panels in flexible PCBs tend to be susceptible to damage from thermal shock. It is because they have reduced elasticity and other mechanical properties.
Electrical plating solves this by thickening the panel. It secures the hole wall coating integrity for later in the assembly process.
The final option is imaging. Imaging in flexible circuit boards is a little similar to how it is in rigid circuit boards in that the technology is identical.
The only difference here is how the board surface should clean. Use electrolytic and chemical cleaning solutions to prevent the cover from warping.
After this, apply some dry film and wait for it to develop. This method of polymerization will make the surface brittle.
You might also notice a decrease in the copper foil's binding force, but that's alright.
5 - Etching and lamination
The following two processes that represent the penultimate step in assembling a flexible PCB.
5.1 - Lamination
There are three general tips for lamination.
First, check the heating rate. You'll want the temperature to reach 173c in 10 to 20 minutes.
When it comes to pressure, use 150n-300n/cm2. It will let you acquire full force within 5 to 8 seconds.
After doing these two things, the overall timing of the lamination should be 1 hour, assuming full pressure.
It even works if you want to design a flexible PCB at home. As this video shows, you can convert any household laminator into one for PCBs.
You'll need professional lamination equipment for advanced uses or applications, however.
Choose the right gasket material for all of this. What you are looking for is a gasket with low fluidity and excellent formability.
Go with materials that won't be shrunk by the cooling process, as well. Polyethene or PVC film is the most popular, but any other content that has thermoplastic characteristics will do.
5.2 - More general tips
The etching is the next technology used in the manufacturing process of flexible PCBs.
Something that you'll notice about substrates is that they have numerous parallel wires at bending locations.
You can use this observation to maximize etching results. Focus on maintaining the etching solution direction during this process. You can use the wires you see as a guide.
The goal in etching is to attach the rigid substrate that you are using to the flexible one. Make sure to do this in front of the flexible substrate. It will pull it forward.
Treat the surface to boost the binding potential once this is all finished.
After doing this, take the cladding layer and position it. It will get you ready for the final step.
The final step in etching is to optimize humidity absorption. To do this, bake the flexible board and the cladding layer.
Put both in an oven for 34 days, but ensure that the stack-up height is shorter than 25 millimeters.
6 - Finalizing the process of flexible PCB assembly
The final steps of flexible PCB assembly entail baking, hot melting, and sharp processing.
These three things do get rid of excessive moisture, dry the board, and keep the board from getting wet in the future.
Packaging must consider as well. Using vacuum packaging technology is practically the only safe way to pack multiple boards.
The best shippers know this and do everything in their power to package PCBs safely and use a reliable service to ship them.
7 - What makes OurPCB the right choice for your flexible PCB?
OurPCB does a lot more than offering free information about the manufacturing, production, and assembling of flexible PCBs.
We also meet all of the expectations you should have from your flexible circuit board manufacturer.
Here are the things you should expect from any given PCB manufacturer.
It can read your order carefully.
Manufacturers might even offer advice if there are any better alternatives, which there usually are.
The best organizations will also help you choose the best materials and other details of your board.
Fast quotes
You also want to receive quotes quickly so that the organization can begin working on your PCBs as soon as possible with competitive rates.
Reliable components
Another thing the best organizations do is acquire components only from reputable distributors.
The manufacturer should already have the necessary components to meet your order specifics. You should not need to perform any guesswork.
Quick turnaround
Assembly processes should be finished in the shortest possible time frame, no matter how many flexible boards you need. The best organizations are aware of and adhere to this.
Fast delivery time
It sounds obvious that fast delivery time is also the goal that the best organization promises and achieves.
OurPCB does all of these currently, did them in the past, and will continue to do them.
You are now entirely educated on all the basics surrounding flexible PCBs.
However, some other things are worth knowing about flexible PCBs, such as the opportunities and challenges surrounding them.
3 - How to Deal With Flexible PCB Opportunities And Challenges
Because flexible PCBs have taken over the industry by storm, they present many opportunities and challenges.
Knowing about these opportunities and challenges will educate you about flexible circuit boards more than most experts.
3.1 - Why are flexible PCBs so popular?
Flexible circuits go back as far as the 1950s.
Since then, they have been produced more and more and will eventually eclipse their rigid counterparts.
Flexible PCBs can bend, which is the most prominent and essential benefit.
They also have improved reliability, containing fewer connectors, solder joints, and other interface connections.
Being a lot thinner and lighter than rigid PCBs, flexible PCBs can also fit into restricted spaces.
Another thing about flexible PCBs is that they have improved thermal management that resists acids, oils, and UV exposure.
Flexible PCBs also cost a lot less than rigid PCBs, even if they have a higher density. It is because they are much smaller.
And finally, flexible PCBs are proven to be very durable. It is why some of them are used in medical and military applications.
As fantastic as flexible circuit boards are, they do face some challenges. One of the most common of these challenges is in wearable products.
3.2 - The challenges of designing flexible PCBs for wearable products
Wearable technology has caused a massive demand for flexible PCBs. It is because the two practically made for each other.
The challenge here has to deal with how compact the flexible circuit board is. Do you want the circuit board of your smartwatch to be physically visible? Of course not.
It is where the issue of being compact comes in. This issue demands one to take on different design strategies that might not have considered before.
Whether the board's design needs to be round or elliptical, they are certainly not "common." It is one challenge.
The other challenge has to do with routing and placement. Even if space is limited, everything needs to be placed correctly.
3.3 - Materials and technologies for flexible PCBs
The performance of any given printed circuit board has a lot to do with the materials used in its design.
It is why identifying new and existing materials is one way to enhance a flexible PCB.
Small innovations can go a long way because of this. Even something like enhancements in metal foils, adhesives, or cover layers will enhance a flexible PCB.
Something you need to understand is that thermal management and the ability to bend the material is at a premium when designing a flexible PCB.
These are non-issues in rigid PCBs, and it can be a challenge to go from one mindset to another.
Coming up with new technologies for the design and assembly process is also essential. One of the latest innovations in this realm is microtia technology.
Using the micro via approach can be a challenge. However, it is slowly becoming common knowledge as many manufacturers have perfected it.
There is an excellent example of this video that shows how to convert it into public knowledge.
3.4 - Common issues in flexible PCB design
Designing flexible circuit boards will be utterly alien to you if you have spent years developing rigid ones.
Here are some common issues that occur during a flexible PCB design that you can refer to if you are new.
The first is not opening the solder mask or overlay openings enough.
Understand that different steps in this process might change how a flexible circuit board looks.
Etching and plating can and usually will adjust the board's shape, which is why you should make sure your overlay openings are beautiful and full.
Another common issue has to do with the proximity of the solder joints and the bending point.
To fix this, follow this simple rule: Ensure the solder joint is at the required distance of the bending location.
If these are too close, the solder pod may break, or delamination could occur. Err on the side of being too far and work from there.
The distance between solder pads and traces is another issue. Make sure there is enough distance between these. You'll experience lamination loss if you do not.
3.5 - Current challenges in flex-rigid PCB development
As mentioned at the beginning of this guide, flex-rigid PCBs combine the best of both worlds when it comes to rigid and flexible PCBs.
You need to choose materials even more carefully for these circuit boards since you will be connecting rigid and flexible layers.
It is especially true when selecting a substrate. This substrate needs to match both portions.
You also absolutely need to ask your manufacturer if they possess the technology to assemble a flex-rigid PCB at all.
The issue here is that designers are keen on using flexible CCLs without adhesion as long as flex-rigid boards have eight layers.
It means designers may not work with you if the flex-rigid PCB you conceived has less than eight layers.
Embedding the resistor and capacitor in the board's rigid section has proven to leave the flexible portion unaffected, which is fantastic.
However, a lot more research needs to be done on how to do this correctly.
The above is just one of many pressing issues that flex-rigid PCBs face.
3.6 - Future flexible PCB trends
Here are some predictions that you might be privy to over the next few years.
Circuit boards are going to be smaller, thinner, and flexible PCBs will be the industry standard.
PCBs will be capable of improving data-processing speed since more and more devices rely on data processing.
PCBs of any type will be able to achieve even higher densities.
The ability to use circuit boards in even higher heat will be possible.
Flexible circuits are going to be the industry standard. You need to get ready for this.
How do you get ready?
Choosing a reliable, flexible PCB manufacturer specializing in flexible PCBs is something you need to do sooner, not later.
3.7 - Why do you need a reliable, flexible PCB manufacturer?
The short answer to this question is easy: Because flexible PCBs are the way of the future, and you'll be working with flexible PCB manufacturers whether you like it or not.
A more detailed answer is because you'll want to ensure that the assembly process goes smoothly for your flexible circuit boards.
Check to see if the company has the proper equipment to assemble the board. Then see if the staff experienced enough to handle the challenge of manufacturing them.
Your manufacturer should familiarize yourself with every issue facing flexible PCBs, as previously described in this very guide.
Contact OurPCB using the chatbox located in the lower right if you need a flexible PCB.
If we are knowledgeable enough to provide all of this information for free, we can certainly help you find a flexible PCB for any application you need.
Now that the problems and challenges related to flexible PCB have been solved, it is time to introduce its design method.
4 - Nine Factors You Need to Consider When Designing a Flexible PCB
Designing a flexible PCB is an entirely different animal from their rigid counterparts.
However, you'll have a much easier time if you understand these nine factors that go into designing a flexible PCB.
The first thing you should know about flexible PCBs is that they all provide electronic interconnection.
They are made of polymer films made up of circuit patterns that are either etched or affixed.
It is the "base" of any given flexible circuit.
Consider this a fundamental guide to a flexible circuit design that revolves around these two above facts.
4.1 - The flexible circuit conductors must arrange
Your conductors' arrangement is a crucial distinguishing factor that represents the first significant difference between rigid and flexible circuit boards.
Don't forget that flexible circuit boards can twist and bend. The results can be hazardous if too much stress is put on the conductors.
Make sure the conductors pass through curved areas to minimize this stress.
4.2 - Correctly set the bending area at the initial stage
Be mindful of the bending capabilities of your revolution.
Above all else, be well aware of the number of times the board will be turning and the council's bendability.
The bending area needs to be set right as soon as possible in the design phase. It will give you useful information about what materials you'll need for the rest of the board.
4.3 - Notches and slots in curved areas
The most crucial design elements of the PCB are gaps and curved areas.
Because of this, we are creating the right bridging sections to place the slots in is necessary.
It can do by familiarizing yourself with drawing tools related to gaps and curved areas, such as the arc tool.
Making small holes around the narrowest region will make extra room for the slots as well.
A good board outline also anticipates and prepares for a more flexible arrangement.
Using bridge sections can help with this. They can help avoid bottlenecks and help you achieve better edges as well.
You can also use a circular board on your outline layer. You can make half-circles to determine the size required by the individual notches.
4.4 - Flexible routing
Routing in a flexible PCB is tricky, especially when constructing the wire routing and the path.
You should remember that routing prepares for the next step, which is placing active elements on the board.
When routing, avoid beans, and make sure your copper circuits don't get too thin. Ignoring this will lead to discontinuities.
4.5 - Risks of flexible through holes
The through-hole method of flexible circuit design involves mounting the board with holes drilled onto the board.
The pads on both sides then use these holes, and the automatic machine completes them.
It is an excellent mounting method, but it has its risks.
Make sure the holes are within a safe distance from areas that you know will bend or flex.
4.6 - Flexible layer placement for multi-layer boards
Layer placement is at a premium when using multi-layer flexible boards—these types of circuit boards for applications requiring high circuit density.
One mistake here can put the full functionality of the board in jeopardy.
Making accurate placements and avoiding placing the circuit parts on the board's solder side will prevent this.
4.7 - Do not specify the thickness of adhesives on the drawing
Unlike all the above factors, it is best to save adhesive width later in the design process.
It is because before determining it, you'll need to select the overall width of the board as well as the dielectric components first.
Adhesive width is not as critical as you think. Overspecifying it is usually a waste of time, energy, and even money.
4.8 - Do not stack conductors on multi-layer boards
You need to avoid designing flexible circuit boards but will also be tempted to create an "I beam" effect.
It happens when line pairs are staggered to minimize impact from an electromagnetic reduction.
It is caused by stacking on multi-layer boards, which is a way to make life a little more comfortable in design.
Resist the temptation to do this. It will lead to a thicker board, which is not optimal.
4.9 - Selection of cover material
The cover material will determine the overall performance and life of the entire board.
When choosing cover material, look for expected performance, specifications of the application, and conditions related to the plating.
4.9.1 - Circuit traces
Keep a distance of at least four mils for the solder mask when placing circuit traces.
4.9.2 - Overlay confusion
Overlay confusion is another common mistake. Take care not to confuse overly with solder masking.
The former is plastic made of one substance, and the latter is a substance made of plastic. Learn to distinguish from the state of nature.
4.9.3 - Hole sizes
Also, know your DA sizes. The correct DA size for an overlay is ten mils. It is four mils for a solder mask.
When determining hole sizes, err on creating holes that are too small then work from there. Spots that are too big will cause problems in mounting and will lead to pointless soldering.
4.9.4 - Solder resist pads
Make sure you have the exact amount of solder resist pads that you need. Too little or too few can cause complications in manufacturing.
4.9.5 - Incorrect landing patterns
Incorrect landing patterns are the result of drawing them manually.
It is a very common shortcut, and it never works out well. Resist the temptation to do it.
Now that you are educated on the design of flexible circuit boards, it is time to familiarize yourself with flexible circuit prototypes and their suppliers.
5 - Knowing Your Flex PCB Prototype Supplier From 8 Aspects
If you are a buyer or a distributor of flex PCBs, you'll want to know about what your suppliers are capable of. This way, you can save on time, resources, money, and energy.
These eight observations will allow you to make a sound decision on a supplier.
Think of this as a checklist of what to look for in a supplier.
Also, do not forget that your prototype flex PCB should be as close to ready for production as it can before selecting a supplier.
5.1 - Understand the qualifications of the flex PCB supplier
Because there exist different kinds of flexible circuit boards, this means that other suppliers have different skillsets.
Qualifications of flex PCB suppliers vary. Because of this, you'll need to get as much information about your prototype flexible PCB as you can.
Asking questions like "how many times will need the board flex" or "how thick or thin will the overall board be" is useful for seeing what suppliers have to offer.
Remember that prototyping is often a game of trial and error. The clearer you are on your prototype, the brighter the qualifications of the supplier appear.
5.2 - Production equipment that the flex PCB supplier can provide
To produce a quality flexible PCB prototype, your supplier will have the necessary equipment to make the boards.
These include drills, exposure machines, gold plating machines, and bare board testers.
If the supplier is missing just one of these, move onto the next one.
5.3 - The flex PCB supplier can usually provide materials
Never trust a supplier who claims that they cannot provide at least some materials.
Portable electronics have been in high demand for about a decade. Because of this, PCB suppliers need to make provisions. It is one more way that they can turn a profit.
Suppliers who provide materials will limit technical issues with your prototype.
5.4 - The supplier must understand the flexible PCB manufacturing process
Move right onto the next supplier if the one you are interested in does not understand that different flex PCBs have other manufacturing processes.
They need to know that the manufacturing of a double-sided flexible board is a whole different animal than a single-sided one.
5.5 - Send the prototype to the supplier and get their analysis
One of the best ways to "feel out" a supplier is to send them a file of your prototype.
Let the supplier tell you if they can make this prototype and how they intend to make it, and you'll often get a lot of information that would typically take hours to uncover.
Choose a supplier who has a good reputation and a lot of experience when doing this.
5.6 - Make sure the supplier has a variety of flexible circuits in a standard supply
There is a lot more variety in flexible circuit boards than in rigid ones when it comes to materials.
Ensure your suppliers know this and make sure they have a standard supply for each type of flexible circuit board.
Whether your prototype is single-sided, double-sided, or multi-sided, make sure your supplier has what you need or at least can acquire it quickly.
5.6.1 - Single-side flexible circuit
This particular PCB contains a single conductive layer, a conductive metal-filled polymer that gets mounted on a flexible dielectric film.
Assembly for these takes place on only one side of the board.
5.6.2 - Double-sided flexible circuit
This flex PCB contains two conductor layers.
You can fabricate this with or without plated through holes, but it is common to drill plated holes in the boards.
5.6.3 - Multi-layer flexible circuit
This flex PCB contains at least two conductor layers that can be interconnected by plated through holes.
Make sure your manufacturer knows about all of these types. This way, you can get materials that support your prototype at the lowest cost possible.
5.7 - The supplier must understand inspection standards
There exist all kinds of inspection standards when it comes to flex PCB products.
General standards, industrial standards, technical requirements, military rules, and other inspection standards are something your supplier should know without needing to reach for a manual.
The automatic/automated optical inspection (AOI) and x-ray inspection method of assessing the quality and reliability of flex PCBs is also something your supplier should know.
If they do not know these, they are not the ones.
5.8 - The supplier needs to have a capable, flexible PCB production team
The success of your flex PCB design will depend on the production team that works on the boards.
Some suppliers do not even have a production team, which will be a problem if you select such a group as a supplier.
If you end up with a prototype design that you cannot even manufacture, then you have just wasted money and resources.
Avoid this by choosing suppliers who also produces and has a record of production as good as to supply.
Now that you know what to look for in a supplier, it is now time to explore the soldering and welding methods that flex PCBs follow.
6 - Flex PCB Soldering
The technology behind soldering and welding has advanced a lot over the last decade. However, there is still room for improvement.
Soldering engineers still need to settle for outdated processes and techniques, and the research behind new and innovative technologies is still incomplete.
It affects the world of flexible circuits more than you know.
You see, flex PCB soldering yields decent results if it is conducted adequately.
However, there is not enough information about flexible welding to yield said decent results consistently.
It does not mean that there isn't any information on how to do this properly, however.
You can still arm yourself with some knowledge to take on flexible soldering and welding as best you can.
6.1 - Five standard flex PCB soldering technologies
Different from rigid PCBs, the soldering techniques for a flexible board utilizes adjustable welding.
This results in fewer interconnects, contact crimps, solder joints, and connectors.
However, you will need to educate yourself if you are used to rigid welding, which is not adjustable.
The following processes will make things easier for you.
6.1.1 - Low-temperature solder paste in flex PCB soldering
Solder paste.
Solder paste required during the manufacturing of any given printed circuit board.
Solder paste is also known as solder cream. It is tantamount that you know about this substance because you will be using it a lot.
Whether the board is rigid or flexible, it will interact with solder paste at one point.
The solder paste's purpose is to attach the surface mount component to the pads located on the board.
It happens because as the solder paste melts, it becomes more adhesive.
Flexible PCBs use solder paste at low temperatures.
It is necessary because the components of the flexible board aren't able to tolerate high temperatures.
Doing this is a simple adjustment in most cases: Just make sure your solder paste's temperature is not as high as you usually have it when soldering rigid circuit boards.
The exception to this is if you are using a tin-based soldering alloy. In this case, add some indium or bismuth to the solder paste to lower the temperature.
6.1.2 - Flash technology in PCB soldering
Flash technology is not yet accessible in the world of welding, but it will be soon. It uses a high-powered homogeneous light flash to solder the components to the copper tracks.
Use a lead-free solder paste when attempting this. You can also use foil substrates with polyethylene terephthalate.
The temperature increase in flash technology is enough to create a soldering effect. You would think that this would risk the functionality of the board, but it does not.
You will hear a lot about flash technology in the future as it might even replace physical welding.
6.1.3 - Reflow soldering technology
Reflow soldering is a lot more simple than flash technology.
Reflow uses solder paste for temporary attachments. You can then place tiny electrical components and their contact pads within contact of the solder paste.
The assembly then becomes permanent under the effect of controlled heat. The solder paste then "reflows" in the form of molten liquid, creating durable joints in the process.
It makes it easy to solder different types of surface mount components.
If you have chosen reflow as your technique, remember that you need to choose a solder alloy. Make sure this alloy has a comfortable eutectic temperature.
There are three different reflow methods you can use: Infrared, convention, and hot air.
Infrared reflow makes soldering iron components a breeze since it provides extra fixturing and support.
The only thing you need to take into account with this method is material limitations and thermal mass.
Conventional reflow is also known as semi-automatic welding. While you need to conduct some manual welding here, you can also use equipment to control the conditions.
Using hot air as a reflow is entirely automated. As long as you know, your machines are working as they should, all you need to do is sit back and wait for everything to complete.
6.1.4 - Flux selection in flex PCB soldering
Flux is necessary for soldering. It will guarantee good results.
You can take a chance on not using flux, but the options are good that if you don't, your work will be futile.
Spreading the flux on the soldered metal will keep the surface tidy until the molten solder alloy makes its way to the soldered metal.
While you can use flux in generous amounts on a rigid board, you won't be able to use it in generous amounts on a flexible board. Have a flux pen handy this way; you do not overdo it.
You should also consider solder flux. Solder flux will eliminate oxides and different kinds of impurities from the board.
It can also accelerate wetting during reflow soldering, which is very useful.
Although there are many kinds of flux available on the market, the best ones are low in acid content.
The most common kinds of flux are rosin, inorganic acid, and organic acid.
Organic acid flux will remove oxides very fast. Consider inorganic acid flux when working with durable metals such as copper or brass.
6.1.5 - Hot-pressure welding technology
Hot-pressure welding allows you to form bonds between the components and the board.
This method only uses heat and pressure. It does not involve any metals at all.
To do this, apply heat to both surfaces and then attach them by using pressure.
The primary heat source in hot-pressure welding comes from oxyfuel torches.
It allows for preciseness during application. Just don't forget to remove the heat source once the two surfaces reach the right temperature.
As soon as the obstacles are clear, you can bring the bodies closer and attach them under pressure. It is known as the open joint process.
In some instances, the surfaces can already touch under pressure when the heat is applied. It is known as the closed joint process.
Choose the right material for this when using this method for a flex circuit board. The content needs to have forge ability properties to accommodate the new fixture.
6.2 Flex PCB Soldering - 5 Things You Need to Pay Attention to When Flex PCB Soldering SMD
Surface mount devices (SMD) attachments on flexible circuit boards are handy, and they can also affix by way of soldering.
The basic principles surrounding soldering SMD components are similar for rigid and flexible boards.
However, some considerations need to be made in the case of flexible circuit boards.
These considerations revolve around material handling and flexibility.
The following are five strategies for soldering SMD components onto a flexible circuit board that revolve around the above two considerations.
6.2.1 - Substrate support
Remember that substrates are what hold entire circuit boards together.
Choosing the right substrate that holds the components during the soldering process is the first step towards creating a high-quality circuit board.
There are three accessible substrate materials to choose from when soldering SMD attachments. The first is fiberglass. It is effortless to solder SMD components onto this substrate.
Those who are making flexible circuit boards will want to go for RO3000 and RO4000. These substrates have a low dielectric loss and are easy to solder SMD components.
6.2.2 - Overheating the flex material
Most PCB materials can withstand some amounts of heat or temperatures.
But what happens when the temperatures get too high? We all know the answer to that question: Reduced performance.
Soldering SMD components are not exempt from this.
Circuit materials will expand when SMD components are soldered. It changes the form of transmission lines and can alter the functionality of the board.
Using thermal interface materials that offer excellent thermal conductivity can micromanage this.
Another good thing about thermal interface materials is that most flexible printed board designers prefer them as well.
6.2.3 - Proper component layout and orientation
Flexible PCB design is 10% routing and 90% layout and orientation.
It means that before you can even think of routing, you need to make sure that the board's components follow a comfortable layout.
Putting layout and orientation first also makes things much easier for designers to sell SMD components.
Something that you can do to make this much easier is to orient the same parts in the same direction.
Also, make sure to use the correct orientation for excellent placement and soldering.
Ensure you avoid following designs surrounding the solder side of SMD components, where the PHT leads are.
Instead, place these SMD components and parts on the same side of the board.
Any components that need to be glued should be at least 3 millimeters apart for soldering. There needs to be sufficient space between the elements to ensure that the heat sink does not touch other parts.
6.2.4 - Solder printing
Solder printing is one of the most vital processes for getting high-quality products when it comes to surface mount assembly.
This process uses a stencil to apply solder paste onto the printed circuit board.
One of the main reasons for doing this is to ensure that the electrical and mechanical components continue to work as desired after soldering.
The biggest reason this doesn't get done as often as it should is that it takes time to do it. Solder printing just cannot be done if a project is time-sensitive.
There are a few principles you need to follow if you have decided to solder print.
First, you need to come up with a perfect stencil and program the screen printer as desired.
Then you need to make sure you have applied the correct amount of solder paste ready.
After that, make sure to use the right processor to achieve a good print.
Solder printing requires a lot of attention to minimizing defects that may occur, but it is worth it. At least 60% of errors can eliminate in soldering by solder printing.
6.2.5 - The planarity problem
Planarity happens when you have a flat surface or level that continues or goes in all directions.
It can cause deformations, affecting the overall flatness of the circuit board.
Planarity is also known as "bow and twist" characteristics that also affect the board's functionality.
There is a good chance you'll encounter planarity when soldering SMD components.
To avoid planarity or bow and twist characteristics, use materials that are RoHS compliant. You can also add pre-preg material to your solder paste before soldering.
Avoiding heavy thermal shocks is also useful, as they can also affect the flatness of the board.
Now you know practically everything there is to know about flexible printed circuit boards.
The last thing to explore is how flexible PCBs measure up against the even newer rigid-flexible PCBs.
III - Flexible vs. Flex-Rigid PCBs
Both flexible and flex-rigid PCBs have their place in a multitude of applications.
After exploring the differences between the two and their respective advantages, you will better understand which one you need.
1 - Differences between flexible and rigid-flex PCBs
As you have learned throughout most of this guide, flexible PCBs get their name because they are a printed circuit board that is flexible.
The flexibility of this PCB allows it to adapt to the design of almost all applications.
As its name implies, flex-rigid circuit boards combine both the classic rigid PCB design and a flexible one.
They use both types of circuitry and usually have multiple layers that use flexible and rigid insulation.
The choice between these two depends on your preferences as well as your product's needs.
Both boards perform very well. They both have their specific draw of benefits, as well.
2 - Benefits of using flexible and flex-rigid PCBs
Both of these circuit boards have their own set of benefits. Take note of these if you are wondering whether you need a flexible or a flex-right PCB.
Flexible PCBs are a godsend if you have limited space. It is because they bend, making them usable in desired angles.
You can order flex-rigid PCBs in custom shapes as long as you have unambiguous specifications.
You'll be able to get the best of both worlds if you know your dimensions well.
Go with flexible PCBs all day if you are working on a product or device that needs to be light and of compact size. Portable projects and flexible PCBs are practically made for each other.
Flex-rigid PCBs win the day when it comes to smartphones and tablets. Rigid circuit boards are just not a reliable solution for these.
The one thing that truly determines if you should use a flexible PCB is that if the device warps in any way.
A device that warps demands adaptability and this is where flexible PCBs come into play.
Three standard devices use flexible circuit boards.
The first of these is in modern computer electronics.
Any given printer, gaming console, microwave, or television made in the last 15 years or use flexible PCBs.
Hard drives benefit from this the most. Thanks to flexible PCBs, hard drives can remain light and small while withstanding the high temperatures involved in their usage.
Modern vehicles also use flexible PCBs. Airbag controllers, breaking, and dash systems all use flexible PCBs.
Today's smartphones practically require flexible PCBs.
The smartphone that you use every day contains several flexible and flex-rigid PCBs.
There is no way that you can fit a rigid board in such a restricted space, but you can provide a wooden board into such a space.
Some smartphones or smartwatches contain flexible screens now, and this is because of how advanced flexible circuit boards have become.
And finally, flexible PCBs have their place in the mighty medical industry. The PillCam is a very recent invention that uses a flexible circuit board.
The PillCam is a miniature camera that a patient can swallow, allowing the healthcare worker to see what is taking place inside the patient's body.
Brilliant advances in medical technology, all thanks to the flexible PCB.
The PillCam one of the medical industry's latest inventions.
3 - When should flex-rigid PCBs use?
As fantastic as flexible PCBs are, flex-rigid PCBs also have their place.
Flex-rigid PCBs are juggernauts in terms of durability. They may lose a bit of adaptability, but this doesn't mean that flex-rigid circuit boards don't have their place.
You should use rigid-flexible circuits when your application is faced with generating a lot of heat in a compact area.
It is perhaps the most common use of rigid-flexible circuits.
4 - Here are some industries in which flex-rigid boards are popular
You'll be surprised to see what industries utilize flex-rigid PCBs.
Three primary industries make the most of rigid-flexible circuits.
Health gadgets: Smartwatches, fitness trackers, and heart rate monitors all use flex-rigid PCBs.
Industrial applications: Increased durability while still being somewhat flexible is a godsend in industrial machinery that encounters extreme temperatures.
Commerce: Online and offline retail stores have always used devices to track inventory. In recent days, these devices were as big as a computer.
It is not the case any more thanks to flex-rigid PCBs. Nearly every retail store's inventory trackers resemble a PDA, and these devices use flex-rigid PCBs.
Conclusion
You will not come across a guide as comprehensive as this one regarding flexible and flex-rigid printed circuit boards.
You now have all the tools you need to make a well-educated decision about which board you need for your application.
When you are ready to have your flexible or rigid-flex PCB assembled, contact us. You can use the chatbox located in the lower left of the page or reach us by telephone or email.
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