Flexible PCB – How To Choose The Right Assembly Technology

Are you thinking about using a flexible PCB for your product?

These boards are an excellent fit for many applications, and they can serve you well. Before you order them, you may want to get to know the assembly technology of flexible boards.

Let’s look at various techniques used during the manufacturing process, and learn more about the technology companies use when designing a flex PCB!

1、Flexible PCB Structure

During the first several decades of the PCB industry, all we had were standard (rigid) printed circuit boards. The industry started designing flexible boards only recently. It was wearable products that inspired the experts to develop a PCB design that offers flexibility and secures a smaller size than rigid boards.

The advantages of a flexible board lie in thinness, reduced size, and improved performance and reliability.

The industry has made significant progress, and you can now choose between several different structures of flexible boards:

• Single-side boards – the main advantages are a simple fabrication, and the structure itself is not complex either.

• Double-side boards – these boards have a far more complex structure, which is why you need to take care of many factors during design and assembly.

• Multi-layer boards – these are the most complicated variants, and you need to pay attention to numerous details to ensure flawless performance.

The choice is up to you and the best suitability for your application. Keep in mind that you can also consider a flex-rigid PCB for your product.

Their manufacturing process is also rather complex, but they can be the best solution in some cases.

Image 1: Flexible PCB structure

2、Choosing the Right Material for a Flex PCB

The first step in the process of assembling your flexible PCB is choosing the right materials. We are talking about picking the right substrate material and choosing a suitable conductor, adhesive, and cover layer.

As for the primary material, you want to consider a film with solid insulating properties. It is also vital for it to have the required electrical and mechanical performance. It is why the manufacturers usually go with polyimide or polyester. Standard boards use FR4, but you can even consider using its thin variations, especially in flex-rigid boards.

If you compare the materials, you will discover that polyester has the lowest solderability temperature. However, it also has the lowest maximum constant operation temperature – 110C than 150 in FR4 and 220 in polyimide.

Polyimide an admirable tensile strength and elongation and seems to be a better choice when it comes to absorbing humidity. The dielectric loss factor is twice as low in polyimide when compared to polyester.

Overall, polyimide is a non-flammable compound that produces admirable electrical and physical performance. On the other hand, polyester offers affordable cost and adequate performance, but it is susceptible to damage during the soldering process.

Image 2: Printed circuit board assembly

3、Flexible PCB Manufacturing Process – Blanking and Drilling

The first thing to do in the assembly process is to prepare the materials. The approach is also called blanking, and you should identify all the materials you will need during the process. That also includes a separation film and a paste used for lamination, and anything else you might need from auxiliary materials.

The drilling process can be challenging because you will be dealing with thin and soft materials. It is not easy to drill them, which is why the experts recommend stacking the boards first. For example, take the cladding layer and stack-up all its pieces. Once you combine them, they might remind you of a book, but they will be thicker than using a single piece.

Image 3: A detail of the flexible printed circuit board manufacturing process

4、Desmear and Pitting

If you have completed the drilling process, the chances are you will have some residue left on the material. That is why you should first try to remove any dirt before moving to the next step. Since you are dealing with small and thin components, it is vital to be careful.

The next step in the process is PTH (plated-through hole) and pattern plating. Here is how that process looks like:

• Electroless plating – you will need a pre-treatment solution, and the experts recommend going with acidic colloid palladium. According to them, it will perform better than alkaline ionic palladium. You want to focus on optimal speed and reaction time because you don’t want the materials to swell or make unnecessary holes.

• Electrical plating – the layer with electroless copper plating has poor elasticity and other mechanical properties. That makes it susceptible to damage from thermal shock. The electrical plating objective is thickening the panel as that will secure hole wall coating integrity later in the process.

• Imaging – before this step, you should clean the board surface first. The assemblers apply a similar technology for standard PCBs. However, it would help if you used electrolytic and chemical cleaning for flexible boards because they might deform. Now, stick dry film and wait for it to develop. Polymerization will make it brittle, and you will notice a decrease in its copper foil binding force.

Image 4: Electronic circuit board device repair

5、From Etching to Lamination

We are progressing well when it comes to the manufacturing process of flexible PCBs. The next technology we use is etching.

You will notice that substrates have numerous parallel wires at bending locations. If you want to maximize etching results, focus on maintaining the etching solution direction during the process. Additionally, consider the position, pressure, and transmission direction.

The goal is to attach the rigid substrate to the flexible one. Make sure to do that in front of the flexible substrate because you want to pull it forward.

Once the etching phase finishes, you should treat the surface to boost the binding potential. Now, take the cladding layer and position it. Due to different absorbing humidity potential, you should bake both the flexible board and the cladding layer. Please put it in the oven for about 34 days, but make sure that the stack-up height isn’t more than 25mm.

5.1. Lamination

The important thing to know is that you need to choose the process's specifics, depending on the substrates you picked. The time needed for lamination and pressure and the heating rate will all depend on the materials.

The general tips are:

• Heating rate – you want the temperature to reach 173C in a timeframe between 10 and 20 minutes.

• Pressure – Use 150N-300N/cm2. You should be able to acquire full pressure within five to eight seconds.

• Time – the experts estimate the net pressure time to be around 60 minutes under full pressure.

If you are eager to design a PCB at home, you can even convert a regular laminator into a PCB one with this video's help. However, you will need professional lamination equipment for any advanced uses and professional applications.

You also want to ensure to pick the right gasket material. Consider low fluidity, excellent formability, and go with a material that won't be shrunk by the cooling process. Conclusion: gasket materials for flexible boards are usually a polyethylene or PVC film or any other material with thermoplastic characteristics.

Image 5: Blue circuit board

6、Flexible PCB Assembly – Finalizing the Process

The final steps of the flexible PCB assembly process involve baking, hot melting, and sharp processing. The experts apply these methods to eliminate the excess moisture, dry it, and keep the board from wetting.

Finally, it would help if you considered packaging, too. You cannot use only wrapping paper to separate the boards, so manufacturers use the vacuum packaging technology to pack multiple boards. The most reputable companies out there will do everything in there power to package the PCBs safely and ship them via a reliable service.

Image 6: Flexible printed circuit boards

7、What Makes OurPCB the Right Choice for Your Flexible PCB?

Here are the things that you expect from your flexible PCB manufacturer:

1. Reading your order carefully.

They might even offer advice if there are any better alternatives. The best companies out there can also help you choose the best materials and other details of your board.

2. Fast quotations.

You want to receive the quote quickly so that the company can start working on your PCBs as soon as possible. The most reliable manufacturers will offer affordable rates that are excellent value for money.

3. Reliable components.

The company should ensure to acquire components only from reputable distributors. Additionally, they should have all the necessary components to meet your order specifics.

4. Quick turnaround.

Whether you only need a couple or a massive number of flexible boards, the best companies will complete the assembly process in the shortest possible timeframe.

5. Fast delivery time.

Thanks to reliable shipping solutions, your board can arrive anywhere in the world in a matter of days.

In short, you need a reliable manufacturer, employs skilled engineers and professionals, and owns state-of-the-art equipment to meet the needs of even the most complex orders. OurPCB meets all those requirements, making it a reliable manufacturer you can trust for all your PCB needs.

8、Conclusion

We hope that you now know more about assembly technology and the manufacturing process of flexible PCBs. The assembly difficulty and technologies used might depend on the design and complexity of the boards. That is why you should always stick to the most reliable manufacturers because they can meet all your needs while ensuring optimal reliability and performance of your board.