Flexible PCBs – How to Choose the Right Material For Your Needs

We know that flexible PCBs can deliver consistent performance and be a reliable solution for many different applications.

What factors affect the performance of a flexible circuit board? While design, structure, and assembly are important, the critical factor lies in choosing the right materials.

That is why we are focusing on primary materials for a flexible PCB in this article. Keep reading to find out everything about the topic and learn how to pick your application materials!

1、What Are the Advantages of Choosing a Flexible PCB?

Here is something that probably won’t surprise you – the number of sold flexible PCBs has increased for several years now. That trend will probably continue in the next decade. It is partly because the new electronic products require flexibility and bendiness of the board.

It is also because flexible boards can be a better choice than standard PCBs. For starters, they are usually smaller and thinner than ordinary boards. As such, they can fit in restricted spaces, but also in corners or areas where you need bendiness.

If you want to keep your product as light as possible, flex PCBs can be the right choice. But it is not only about size and weight because you want the board to be both flexible and high-performing. That shouldn’t be a problem because bendy boards tend to be very versatile, and you can fit them into almost any application.

Image 1: A printed circuit board with letters flexible over it

2、Which Materials Should You Use as Flexible PCB Substrates?

As expected, the choice of materials for a flexible printed circuit board won’t be the same for the ordinary unit. You will need to consider other specific characteristics.

Let’s take a look at three material types that clients usually choose for their applications:

• Polyimide
• Polytetrafluoroethylene
• Polyether ether ketone
• You can always consult the company you trust in designing and assembling your PCB. But if you believe your application is "standard," you probably won't go wrong with polyimide.

Polyimide (PI) manages to combine a budget-friendly price with adequate reliability and performance. When it comes to chemical and heat resistance, it should meet your expectations in those areas, too. It also has average electrical characteristics, which is why people choose it for products throughout different industries.

You can find polyimide in flexible PCBs placed inside the system of smartphones, tablets, and other entertainment products. Also, you will notice them in calculators and even cameras.

The second option is polytetrafluoroethylene, but you probably know it as PTFE, which is its abbreviation. PTFE offers an excellent temperature range, a high level of stability, as well as a low dissipation.

PTFE can be suitable for different applications, including advanced uses in the aerospace and automotive industry. Guidance telemetry, driving assistance control, antennas, and other applications are an excellent fit for this material.

Finally, you have a material based on PEEK, which is polyether ether ketone. It is a substrate that offers amazing chemical resistance, and it also handles radiation well. PEEK’s heat range is impressive, and the material won’t melt even at extremely high temperatures.

It is the medical industry that primarily uses PEEK-based material in flexible printed circuit boards. The chances are that the X-ray machine in your local medical facility contains this type of PCB.

Image 2: Electronic flexible circuit board

3、Cladding Components

The cladding layer is the one that covers your flexible PCB. It is important to pick the right components for it as its objective is to deliver sufficient insulation to keep the board protected. By protecting the board, the product will also contribute to its reliability and consistency.

A copper-clad laminate (CCL) is a natural choice for flexible boards. You will find that professionals often choose a polyimide resin that has excellent thermal control. In other words, if the CCL faces a high temperature, there won’t be a risk of softening, which will reduce the chance of board failure. The ability to stay flexible and elastic is the reason why people choose this material.

Apart from thermal control and resistance, we should also mention the electrical properties of polyimide. You will need to choose a high-quality film to decrease the absorption of humidity and improve tear strength. But even a higher investment is worth the money if you get a reliable and durable solution.

Image 3: Flexible circuit board – blue pattern

3.1. Why Should You Go Adhesive-Free in CCLs?

The manufacturers used to design CCLs this way – they used a polyimide film, adhesive as a connector, and a copper foil. Although adhesive has some positives, the experts determined that the board could perform better without it.

The clients didn’t like limitations in electrical performance and dimensional stability caused by adhesion, which is why the industry turned to alternatives. The smartest choice turned out to be a CCL made of two layers. Skipping adhesive proved to be possible, and the results were more than satisfactory.

However, eliminating adhesive did change the assembly process, which increased the cost of the board production.

These are the process manufacturers use to assemble these flexible copper-clad laminates:

• Lamination – it might be a good choice if your board is double-sided.
• Film coating – the experts recommend it as the most suitable choice for production at a massive scale.
• Electroplating – is the most affordable option for the provided ones.

4、What Role Do Halogen-Free Substrate and Liquid Crystal Polymer Play?

We already discussed polyimide, and it can be a great choice for your flexible PCB. However, it is not perfect, and it is understandable why experts are trying to improve it. Liquid crystal polymer is one of the solutions that the industry considers.

You get a liquid crystal polymer film, and you place a copper foil on top of it. A hot pressing process is how we get CCL. The LCP has an envious absorption of humidity, which is often lower than 0.04%. The CCL you get this way can also be adequate to use in high-frequency circuits with digital characteristics.

Since we live in an era of eco-friendly solutions, you will be glad to know that LCP is recyclable. That makes this material environmentally-friendly.

While we are talking about eco-friendly solutions, let’s mention the regulations of the European Union. The EU changed the laws in 2003 and restricted the use of hazardous materials in electronic devices.

As for PCBs, they were most affected because of halogen. That material was often a part of printed circuit boards, but those that wanted to continue selling their products in the EU had to give up on using it.

Image 4: A flex PCB connected to LCD

5、What Other Flexible PCB Materials Should You Know?

Image 5: Flex PCB with connector pads

We will start by discussing materials with conductive properties. In most cases, you won’t go wrong with copper and copper foil. The only reason why you would experiment with other materials is that your application requires it. You may want to consider aluminum, nickel, gold, silver, or other alloys with high conductive properties.

Now, let’s take a look at the different copper foil types you can use:

Rolled and annealed (RA) has a column array shape, which implies that its structure is flat and even. It has incredible flexibility, which I the reason why it is often a part of flexible PCBs.

Electro deposit (ED) has a fish scale structure that can secure the required smoothness and toughness. It would help if you kept in mind that you can’t roughen the foil or put it through the etching process. High-density boards are the main application of ED foil.

You may also see a silver paste with conductive properties used as a printed layer in flexible boards. Both reliability and flexibility are the reasons why some use to apply it in their projects. Another consideration can be using photosensitive polyimide as an overlay. That way, you can improve both density and stability.

6、Can You Make a Flex-Rigid Board?

Image 6: Electronic flex circuit close up

We already mentioned that flex-rigid boards are also an option you can use in your products. These PCBs combine rigid and flexible materials. That means you will use polyimide or your preferred material, but you should also find a suitable rigid option.

The two most frequent choices are RO3000 and RO4000. The former is a PTFE variation infused with ceramics. That secures electronic and mechanical stability. You can use it for complex boards, including multi-layer units used in radars, satellites, etc.

RO4000 can be an affordable option if you need reliable performance in high frequencies. The RO4000 is a hydrocarbon ceramic laminate, which can contribute to dimensional stability.

7、What Factors Should You Consider When Choosing the Right Materials for Your PCB

Image 7: Blue circuit board close up

You want to choose the right materials and design an optimal structure. The final piece of the puzzle will be picking a suitable company.

The manufacturer needs to have the right equipment and experts to ensure every detail of the process is handled properly. You want a company that can assemble a board that will perform up to your expectations, and do that at the best possible cost. Finally, you also want to receive your order in the shortest possible timeframe, which is why you need a manufacturer that will handle the entire production swiftly.

If you need professional advice, don’t hesitate to contact OurPCB. The company has engineers with years of experience in the industry. They can help you to select the right materials for your application.

Additionally, the company is skilled in manufacturing flexible PCBs. Thanks to experienced staff and modern equipment, they can assemble even the most advanced flexible boards in a short timeframe.

8、Conclusion

The materials are the crucial factor in affecting the reliability and performance of your flexible PCB. While you don’t want to overspend, it is essential to ensure you get the best value for the money. The main consideration should be to find a solution that will fit your application and secure optimal durability and longevity. Please feel free to contact us if you have any questions or needs.