PCB Soldering-The Ultimate Guide to PCB Soldering

Soldering is an essential process when fastening components onto a PCB. It has a significant impact on the overall quality and performance of a PCB. There exist different soldering processes, solders, and techniques. Hence, it is not uncommon for the process to the overwhelming to beginners and even experts.

This article presents the ultimate guide to PCB soldering. You will learn how to choose the best solder for circuit boards, common PCB problems, and how to avoid them. You will also learn about solder balls and how to make them.

PCB soldering

9 Ways to Choose the Best Solder for Circuit Boards

1. Lead-Based or Lead-Free Solders?

Lead-based solders are alloys comprising of Tin and Lead. Nowadays, lead-based welds are no longer conventional because of the health risks associated with their usage due to inhalation or ingestion of lead.

As a result, most standardization bodies suggest the usage of Lead-free solders. Nevertheless, lead-free solders have high melting temperatures, so they are quite challenging to work with, but usually not an issue.

2. Based on the Chemical Contents of the Solder

Lead-based solders are alloys comprising about 60 % tin and 40 % lead. Lead-based welds have a melting temperature between 180 ºC – 190 ºC. Because PCBs are sensitive to high temperatures, tin's primary function is to lower the alloy's melting temperature. Lead-based solder is most suitable for use in aerospace or medical electronics.

Lead-free solder is an alloy of tin and copper. It has a higher melting temperature than lead-based welds. Although it results in the Formation of whiskers, it is still ideal for many electronics due to its low health risk.

Solder wires have a hollow core running through the center. The hollow core houses the flux chemical. The flux is the chemical that prevents oxidation when oxygen in the air reacts with the hot metal. Flux helps to improve the electrical contact and mechanical support in the soldered joints.

3. The cost of solder

Lead-free solders are generally costlier than lead-based welds because of their desirable properties, such as strength. Nevertheless, you could still achieve these beneficial properties with lead-based solders by coating the solder with silver and other compounds. Silver coating provides more strength than tin.

4. What Should Be the Correct Dimension?

Solder wires come in different diameters, which is determined by the gauge number. For example, gauge numbers 18, 20, and 21 correspond to diameters of 1.22 mm, 0.914 mm, and 0.813 mm.

It is essential to know the suitable solder wire width needed before handling the work. Thick solder wires help weld larger joints quickly but could be difficult when used for smaller joints. Nevertheless, solder wires with a diameter of 0.711 mm are regarded as the best option for beginners.

Also, a solder wire with a diameter of 1.22 mm (gauge 18) is suitable for Plastic Dual In-line Package (PDIP) with a pin-spacing of 0.1 inches. You should use smaller wire diameters for smaller PCBs.

5. Solder Based on Your Projects

Different projects require different kinds of solder. While some solder types pose no health risks in specific environments, other solder types could be dangerous for use in that same environment. Here are some of the solders you should use in different environments

When to choose Lead-based solders

Although there has been debate over the health risks associated with lead-based solders, there is not enough evidence to back up the claims. Hence, lead-based welds are the most preferred choice in aerospace and medical electronics.

Lead-free solders

Lead-free solders have been gaining ground over the conventional lead-based solders. They are used in almost all kinds of electronics.

Amateur application

Because of the high cost of lead-based solders, beginners and amateurs can opt for the standard Sn60Pb40 welding.

Lead-based solder used in PCB

6. How to Choose a Solder based on the Electronic product?

It is crucial to also pay close attention to your electronic product's nature before choosing a solder. Selecting the wrong welds can pose serious challenges, such as corrosion and failure due to fatigue. For instance, if you plan on soldering gold-plated components, you should choose 70Pb30. It provides high resistance to fatigue during thermal cycling.

7. What is the Perfect Solder Diameter for You?

Before choosing the diameter of solder wires to be used in a project, you must be well-versed in gauge numbers. The gauge number is the industry term used to describe the thickness of solder wires.

The big gauge numbers for solder wires are 16, 18, 20, 21, and 22. They correspond to 1.63 mm, 1.22 mm, 0.914 mm, 0.813 mm, and 0.711 mm. As you can see, the larger the gauge number, the lesser the wire diameter.

Gauge 22 solder wire is suitable for smaller PCBs because the pins are much closer. Whereas for older circuit boards having larger pin spacing, you should use gauge 16 solder wire.

8. Solder in Circuit Board Repair

When repairing PCBs, you must take note of factors like pin spacing and thermal resistance. As a rule of thumb, you can use lead-free solders with small diameters for almost any circuit board repair activity. Thin wires allow you to maintain control, allowing you to produce excellent work.

Circuit board repair Source:

9. Focus on the Shelf Life of the Solder

Alloys used in a solder have a crucial role it performs. It reduces the corrosive effect and enhances electrical characteristics. Also, it determines the shelf life of solder wires (flux-cored). Alloys containing over 70 % of lead remain usable for about two years after manufacture, while other alloys have a shelf life of about three years.

How to Solder a Circuit Board

What Is the Best Temperature for Soldering Circuit Boards?

Solder contains different metals that melt at different temperatures. Leaded solder has a relatively lower melting point than the lead-free type, so if you’re using the former, set the soldering iron’s temperature to about 316 to 343°C (600-650°F).

A technician repairing a drone’s circuit using a soldering iron

But if you’re dealing with lead-free solder, heat the soldering iron to between 343 and 371°C (650-700°F). Check the specification of the solder you are using to get the exact melting temperature point on the packaging.

How to Solder Components on PCBs

The first step is to prepare the surface by cleaning it using 3M Scotch Brite pads, acetone, and compressed air. This step ensures the solder joint created later is strong and has little electrical resistance between the component and the pad.

Next, place the component on the board then heat the soldering iron to slightly above the melting point of the solder you are using.

After that, apply a tiny amount of solder to the iron’s tip and hold this tinned tip against the PCB pad and component lead for a second or two.

The idea is to elevate the temperature of these two to ensure the solder wets and sticks to them. Otherwise, it will pool and refuse to stick due to the extreme temperature difference.

Component soldering on a PCB

After that, apply solder to the joint by touching the wire’s tip to the hot solder pad and component lead, not to the iron. The solder should melt, flow freely, and release smoke. Touch some more wire until the pad gets completely covered, and the solder forms a mound around the lead with concave sides. Keep the joint steady until the solder cools and solidifies.

Inspect the solder joint, especially if it is on a through-hole component. The solder should fill the hole to the other side and form a pyramid-like shape. Trim the component lead that extends to the other side if it is too long, but don’t trim the solder joint.

Lastly, clean the soldering area using flux remover if you used a rosin-activated, rosin-core, or aqueous flux.

How to Solder Wires to a Circuit Board

The first step is to peel the insulation from the wire’s end and then insert the strands into the through-hole. If soldering to a surface pad, leave the peeled wire first and tin the soldering iron.

The tanning process involves applying a tiny amount of solder to the hot iron tip, similar to when soldering components. In this case, the wire is the component, so secure it on the PCB pad or through a hole, then touch the hot tinned tip to these two for one or two seconds to elevate their temperatures.

Solder iron tinning

Next, feed the solder wire to the hot joint until it melts, covers the pad, and protrudes to the other side (through-hole). Leave the joint to cool, then inspect it, trim the excess wire, and clean the surface.

How to Solder a Circuit Board Without a Soldering Iron

The purpose of the soldering iron is to apply heat to the joint, which then melts the solder wire as you feed it. If you don’t have this tool, you can use a long pointy piece of metal, such as one arm of a pair of scissors, screwdriver, nail, knife, or even scrap metal.

Preheat this metal in a heat source such as an alcohol burner, open campfire, candle, or oil lamp for 10-20 seconds. But we recommend butane lighters or torches because the blue flame is super hot and soot-free.

Heat the metal, then follow the procedures above for soldering components and wires, beginning with tinning.

How to Desolder a Circuit Board

Desoldering is essential if you want to remove a component or wire from a PCB either for upgrades, replacements or if the soldering was done incorrectly. Here are four ways to do it.

Desoldering With a Soldering Iron

Using a soldering iron is the easiest of the four methods, and you’ll only need tweezers and a pair of pliers. Begin by heating the joint using the iron until the metal melts, then try to nudge the component leads to move away from the joint using the iron.

Once they are free, remove the component from the board using a pair of pliers by pulling the pins, not the component body. You can damage the component if you pull the body. If you want to remove a through-hole component, try sticking a safety pin inside to release the solder and free the pin.

Desoldering With a Soldering Wick/Desoldering Braid & Iron

Desoldering braids are thin copper wire tools that have the strands braided and covered in flux. They come in rolls of different lengths and are also known as soldering wicks when cleaning the excess solder from the pads or desoldering wicks when removing solder from through-hole component pins (not effective for SMDs).

Soldering wick or desoldering braid

Some desoldering braids don’t have flux, so you have to dip them in this substance before applying it to the pad. Ensure this wick is the same size as the contact area, handle it using a pair of pliers to position and hold it, and then place the soldering iron over the wick.

As the solder melts, it will be attracted to the wick. Remove that filled part or trim it and apply fresh braid then repeat the process until all the solder is out.

Desoldering With a Soldapullt Pump and Iron

A Soldapullt pump is a vacuum desoldering pump that acts as a suction cup to pull out molten solder from the through holes. You have to preheat the joint using the iron first to melt the solder, then squeeze the pump’s handle and put its tip on the solder. Release the handle and let the pump suck the solder from the joint.

PCB component desoldering using a soldering iron and a vacuum plunger

Desoldering With a Power Desoldering Station

A power desoldering station is an automated, temperature-controlled desoldering device that is ideal for removing hundreds of components from one of more boards. The main power desoldering station types include thru-hole stations, hot-tweezer stations (for surface-mount components), and hot air stations, also for SMT parts. This last type blows hot air (427°C or 800°F to 538°C or 1000°F) to melt the solder from the joint.

15 Common PCB Soldering Problems to Avoid

People make many mistakes during PCB soldering; here is a list of the most common ones. If you want desirable and excellent results, you must avoid them.

1. Solder Bridging Bad solder joints.

Solder bridging is mostly associated with smaller and compact components during PCB soldering. This problem occurs when unwanted connections between two or more joints are made. It leads to short circuits, which eventually damage the components.

This problem is particularly challenging since the bridge could be too small and unnoticeable. However, if you can find the bridge, you can quickly fix the issue. All you need to do is melt the solder in the middle using a soldering iron and remove the excess solder using a solder sucker.

Solder Bridging

2. Excessive Solder

Beginners are quick to apply as much solder to a pin as they can. This act is a common mistake that leads to excessive buildup on the nail and causes solder bridging.

Another side effect of excessive solder is that it deters proper wetting of both the pin and the pad. The best way to avoid this issue is to apply enough solder to wet the pad and the pin during soldering.

3. Solder Balling

Solder balling is a common problem that occurs during the soldering of circuit boards. As the name implies, solder balls are spherical soldiers that adhere to the circuit board.

Solder typically occurs during the reflow process when you choose the wrong reflow temperature. It also occurs when there is moisture in some of the components. You should adopt the correct soldering procedures to avoid this common mistake.

4. Cold Joints

A cold joint simply means a poor connection between the components and the PCB board. When the soldering temperature is too low, this type of common form occurred when you did not allow the soldering iron to heat up adequately. If unattended to, it leads to crack and, eventually, failure of the entire element.

5. Overheated Joints

In contrast, overheated joints occur when the PCB soldering temperatures are too high or when the solder does not flow. It also causes failure of the entire element; hence it must be avoided.

6. Tombstoning

Tombstoning is a common problem in PCB soldering. It occurs when a passive component such as a resistor partially lifts from a pad at one end. And as the name suggests, it resembles the slabs you will often find in the cemetery.

This issue arises when the solder on one pad fails to complete the wetting process. To avoid this issue, you should ensure to check the pad dimensions and utilize better PCB finishing.

One way you can avoid this is by checking the pad dimension. When one pad dimension is larger than the other, it will complete the wetting process faster because the extra copper will act as a heat sink.

7. Insufficient wetting

Wetting is an ideal situation where the solder applied to the board has reached a fluid state, making it attach properly to a pad or component.  When this process is not sufficient, the solder will not bond properly with the element or pad, leading to weak joints.

Engineers cause this when they fail to apply enough heat to the pen and pad, or when they fail to give the solder enough time to flow. Cleaning of the PCB and heating both the pad and the pin would help prevent this issue.

8. Solder skips

Solder skips are unsoldered surface mount joints. It occurs when the solder skips over a surface mount pad, which causes the area or pad to be unconnected. As a rule of thumb, you should avoid placing uneven pad sizes for an SMD component.

9. Lifted Pads

As the name suggests, lifted pads occur when the pad of a component lifts from the circuit board. It is typically caused when you try to remove a part that was mistakenly soldered. High soldering temperature or excessive forces on one of the joints also leads to lifted pads.

These problems make the pad challenging to work with since the pad becomes fragile.  Some specific boards are prone to this problem, especially ones designed with a thin copper layer.

10. Solder Starved Joints

A solder starved joint is a joint that does not have enough solder, which causes the joint to lack reliable electrical contact. It is caused when insufficient heat is applied to the lead.

Although this joint might still perform its function, it comes with the drawback of a weaker joint. Stress cracks will develop over time, causing the joint to fail. You will have to reheat the joint to solve this problem.

11. Solder Splashes

Application of excessive flux, or inadequate preheat, causes solder splashes. Solder splashes may lead to solder bits sticking onto solder masks in splatters. As a rule of thumb, you should ensure that the PCB surface is clean before soldering. This act will help you to prevent solder splashes.

12. Pin Holes and Blow Holes

These problems usually arise during wave soldering, and they are easy to detect because they appear as holes in solder joints. These holes form when excess moisture buildup in your board tries to escape through thin copper plating.

You can avoid this problem by preheating the boards as this will ensure that the moisture contained in it escapes as vapor.

13. Solder Flags

Solder flag occurs when the solder drains too slow from the wave soldering machine. This problem is generally considered to be the abnormal height of the solder on the board. You should try to avoid applying flux inconsistently during PCB soldering to prevent solder flags.

14. Solder Balling

Solder balls are spherical soldering pieces that have separated from the main body forming the joint. It creates as a result of excessive oxides in the solder paste.

Solder balls form when air or water vapor, trapped in the solder paste, escapes and turns into liquid. When this process occurs fast, a small amount of liquid solder will take from the joint; hence, a solder ball is formed after it cools. Based on experience, avoid using balls with a diameter greater than 0.13 mm.

You should also avoid storing your PCB in humid environments, as this will ensure that the PCB doesn't contain water. As a rule of thumb, you should dry all PCBs before soldering or assembly and avoid applying too much flux in the solder paste.

15. Solder Discoloration

This issue is usually not caused by the engineer but by the manufacturers. It typically occurs from the use of different flux materials by the manufacturers. Higher temperatures during wave soldering could also cause it.

How to Make a Good PCB Solder Ball

What is a Solder Ball?

Solder balls are spherical soldering pieces that connect chip packages to PCBs. They are typically mounted on boards either manually or through a piece of equipment.

Even with the importance of solder balls in PCBs, they remain one of the most common defects in the SMT assembly process. Solder ball defects typically occur during reflow or printing processes.

Solder ball

When Solder Balls are Bad  News for PCB?

According to experience, avoid using solder balls with a diameter greater than 0.13 mm. Also, there is a stipulation by the IPC A 610 standard concerning solder balls. The rule suggests that solder balls less than or equal to 0.13 mm could cause defects.

You can mistakenly create solder balls during hand soldering or automated reflow processes. Once you realize that the ball is not covered in the conformal coating, you can be sure that it will cause worry.

IPC A 610 standard has also suggested a simple technique for determining if the solder ball entrapped. All you have to do is stroke the ball using a brush and observe if it remains firm on the surface. If it does, it will not cause any defect.

Solder Ball Valve

The primary function of a solder ball valve is to connect chip stacks in a circuit board. You will need a unique tool to make a solder ball valve.

How to Solder a Ball Valve

You will need a pick-up tool when soldering a ball valve. With the help of vacuum suction, the machine picks up solder balls from a fluidized ball reservoir. The device then shoots a stream of gas, which injects the solder balls into the substrate.

How to Make a Solder ball

Creating a solder ball is pretty straightforward as long as you have a lead-based or lead-free solder. You will have to fabricate the solder alloy into a sheet or wire, and cut them into tiny parts or specks. As a rule of thumb, you should reduce the coils and sheet such that they yield a solder ball of the desired diameter, preferably as 2 mm.

Your next step will be to place the specks in a column containing hot oil. This procedure will melt these pieces, leaving you with solder balls. Next, all you have to do is to allow the nuts to cool by placing them in a viscous liquid.

What Causes Solder balls during hand soldering?

A challenge plaguing most engineers during hand soldering is the creation of unwanted solder balls. Here is a list of the causes:

• Moisture: Moisturebeing present in solder paste could cause solder balls.
• Circuit Board: Air, moisture, and alcohol from cleaning procedures sometimes cause unwanted solder balls in circuit boards.
• Smeared stencil: Irregular deposition of solder paste by the stencil also causes solder balls
• Inappropriate solder paste formulation: Solder pastes that poorly formulated during heat reflow may lead to unwanted solder balls across the board.

The Best Troubleshooting Technique for solder balls that occur during hand soldering

Testing different products will help you determine if the solder balls are peculiar with specific PCBs. The next section will focus more on the reliability of solder balls and techniques for troubleshooting.

Solder Ball Joint Reliability

The Pd film thickness in the range of 0.02 – 0.05 microns would give the best solder joint reliability. Also, the shape and depth of intermetallic compounds would determine a solder ball's safety.

Problems and Defects

While solder balls with diameters less than or equal to 0.13 mm should avoid, they are not only the cause of worry. Many other activities or processes cause unwanted solder balls during soldering.

1. Wave Soldering

In the wave soldering process, unnecessary solder balls were generated due to spatter. It could also occur when you fail to apply the stipulated preheating temperature to the solder ball. It makes the moisture or solvent in the flux to escape, thereby causing solder balls.

A simple method of solving this problem involves the use of a plate made of glass. All you have to do is place the glass above the wave, and check the bottom to see if there are bubbles. If the bubbles are few or non-existent, then you are right.

2. Exploding Volatile Materials

It is not uncommon for volatile residues to be present in the flux. Unwanted solder balls are also created when these residues explode. Solving this is pretty simple: place a piece of thick white paper above the wave, and allow the stream to run.

3. Summary

Here is a list of everyday things that causes defects in solder balls:

• Low preheat temperature, which fails to activate the flux.
• Inadequate space between pads
• Excessive use of solder pastes
• Inorganized or wrong placement of components on the PCB

• The Best Troubleshooting methods

To solve or avoid these issues, here is a list of things you should do:

• Ensure to clean stencils regularly and properly
• Ensure to separate new flex from old flex
• Ensure to eliminate the gap between the circuit board and stencil

The Ultimate Guide to Wave Soldering

What is Wave Soldering?

Wave soldering is a soldering procedure that allows mass production of PCBs within a short time. Its operation mode is pretty simple: it showers a surge of solder over the PCB, thereby soldering components onto the board.

Technical Details about wave soldering

It is crucial to ensure that the temperatures adequately set during the wave soldering process. This step will help prevent mechanical stress on the circuit board.

When to use Wave soldering

Wave soldering is applicable for surface-mount circuit board assemblies. They are also suitable for welding through-hole electronic components.

Wave Soldering Process

1. Wave soldering machine

The first step in the wave soldering is choosing the correct wave soldering machine. There are generally two types of wave soldering machines: lead-free and lead wave solder machines.

Both machines contain some fundamental parts and operate similarly. A conveyor transports circuit boards via different zones. A pan, flux sprayer, preheating pad, and a pump incorporated in the device generates the wave.

The lead solder machines typically contain 49.5 % lead, 50 % tin, and 0.5 % antimony. Lead-free mechanisms remain the best option if you are concerned about the health risk associated with lead-based machines.

2. Wave soldering temperature

When using lead-based tin alloys for soldering, the operating temperature must always stay within the range of 255 ºC - 265 ºC.  It is also essential to consider the overall weight of the PCB and parts. Heavy components can withstand temperatures as high as 280 ºC, while lightweight components could be heated as low as 230 ºC.

3. Fluxing

Your next step would be the application of liquid flux on the PCB surface. This procedure helps to remove dirt and oxides from the metal surface, thereby enhancing the electronic components' soldering quality.

A standard method of applying flux to the PCB surface is via the spraying method. This method involves spraying the board through a nozzle as the board passes. Another technique consists of using foaming flux, which allows you to attach the flux to the board.

Both methods are useful but have their drawbacks. It is easy for poor soldering to occur in the foaming flux method when you fail to apply the flux uniformly. Whereas in the spraying method, flux can easily pass through the gaps.

4. Preheating

Preheating, before the primary welding process, helps to get the best soldering results. It helps the flux to get to every part of the PCB.

5. Cleaning

You should clean the PCB using deionized water to help you get rid of flux remains.

Types of Soldering

1. Dip soldering vs. wave soldering

Dip soldering is similar to wave soldering as it is commonly used for surface mount and through-hole board assemblies. However, its scope is somewhat limited as it is the manual process of the automatic soldering process. Nevertheless, it provides reliable mechanical and electrical connections.

2. Reflow soldering vs. wave soldering

Reflow soldering helps to fasten surface mount components onto a PCB. In reflow soldering, the solder paste, created out of flux, is used to attach components onto parts.

The reflow soldering process is much easier than the wave soldering process since environmental conditions are not significantly affected. You won't have to monitor how long the PCB stays in the solder wave. Also, you don't have to give too much attention to the PCB temperature.

Selective Wave Soldering

Selective Wave Soldering Machine

Selective soldering would be the best option when you fear that components might not withstand the high temperatures associated with reflow or wave soldering. The particular wave soldering process performed using the specific wave solder machine.

Several selective wave soldering machines in the market, such as solder pot machines and the standard machines with nitrogen inserted in them.

Selective Wave Soldering Guidelines

The selective wave soldering process is pretty simple. All you have to do before soldering is to apply the liquid flux and preheat the PCB.  After preheating, you will have to use a solder nozzle to solder the joints.

Selective Soldering Problems

Common problems such as solder bridging, excessive solder, and solder ball could occur during this process. These problems arise due to excessive weld or high temperatures associated with sticking the solder mask.

You should also avoid extremely high temperatures, causing the copper pad to dissolve into the molten solder.

Selective wave soldering machine cost

Although the selective wave soldering machine doesn't come off the line cheap, it is less expensive than the wave soldering machine. Besides, it is advantageous as it requires lesser flux and solder consumption.

Also, the Selective wave soldering machine is five times cheaper compared to the wave soldering machine. It also features less electricity requirement, less flux, and solder consumption.

Wave Soldering Defects and Problems

Issues arise during the wave soldering process when the soldering environment is not adequately controlled or when the temperature is excessively high. Here are some of the defects and problems you will notice:

• Consumption of flux and solder increases
• Consumption of electricity increases
• Formation of cracks
• Formation of cavities
• Irregular solder thickness

Wave Soldering Cost

Wave soldering machines are much more expensive than the selective soldering machine. This high cost is associated with the problems and complexity of the wave soldering process.

Conclusion

In this article, you have learned about choosing the best solder for circuit boards, avoiding common PCB soldering problems, making good solder balls, and wave soldering. By now, you should also know different types of PCB soldering and troubleshooting methods for various issues. As long as you follow the helpful tips outlined in this article, you will obtain the best results during PCB soldering.