Introduction
Coverlay is a component of flexible printed circuit board (FPCB) manufacturing. It helps in protecting FPC from damage. Also, it strengthens the board for long-term usage. However, pasting an overlay on FPC is somewhat difficult. In this article, you will learn how to paste coverlay on FPC without any errors.
What is Coverlay in Flexible Printed Circuits?
In flex printed circuit boards, some of the circuit layers remain exposed. Coverlay is a material that is used to cover (encapsulate) these exposed circuit layers on FPC.
The FPC coverlay material is commonly made of a polyimide substrate. Additionally, an adhesive material is used to bond polyimide to FPC. It helps seal the exposed circuit layers.
After creating a bond with the polyimide substrate, you must align the coverlay with the circuit’s surface. During alignment, specific temperatures and pressures must be met. So, the overall role of coverlay in FPC is to encapsulate the surface of the circuits.
Here are the main benefits of pasting coverlay over FPCs:
- Protects the circuit
Coverlay works as an outer layer. It protects the circuit from dust, moisture, and chemicals. Also, it reduces damage during bending and handling.
- Provides insulation
Coverlay also stops unwanted current flow between circuit parts. As a result, it helps prevent short circuits. The board works in a safe and stable way.
- Keeps the board flexible
Coverlay materials are flexible by nature. So, the FPC can bend without breaking. This helps in small and tight spaces.
- Improves durability
Pasting the coverlay makes the circuit stronger over time. Because of this, the FPC lasts longer. It also performs well in different conditions.
Coverlay vs Flexible Solder Mask Options
Different types of coverlay films can be pasted on FPC. Each type has its own features. The choice depends on the design and need. The following are the four common types of coverlay films.
- PI coverlay film
This type uses epoxy or acrylic adhesive and offers strong insulation. It also provides good bending strength and works well in many applications. However, it needs special lamination equipment. Also, controlling adhesive flow can be hard in PI coverlay film.
- Photosensitive developing coverlay film
This film forms window openings through exposure and development. It allows fine and accurate openings. A photosensitive developing coverlay film suits precise designs. But it costs more and sometimes gives lower bending performance.
- UV heat curing ink
It utilizes screen printing and curing, resulting in a low processing cost. It does not require molds. However, it cannot create very fine openings. The bending strength of UV heat curing is also limited.
- Photosensitive ink
This type of material also uses screen printing, but with exposure steps. It forms a thin coverlay layer and supports fine openings. The material cost is low. However, it sometimes causes plug holes and delivers weaker bending performance.
Process of Pasting Coverlay onto FPC
Follow these steps to paste coverlay onto FPCs without errors while ensuring strong bonding.
1. Prepare Materials and Work Area
First, ensure that all the required materials are prepared properly. Ensure the work area is clean and clear. A clean work area with materials prepared will help you work without errors.
2. Inspect Coverlay Material and Film
Check the coverlay and film before use. You have to check for defects like burrs or wrong punching. Also, you need to ensure that the surface is smooth and clean. If you see dust, remove it with a proper cleaner. Doing this will help you avoid bonding issues.
3. Clean Your Worktable
Clean the worktable before starting. There should be no dust and dirt on the surface. Next, arrange all tools in place.
4. Inspect Flex Circuits
Check the flex circuit carefully. Look for:
- Oxidation
- Dust
- Small particles
All these can affect bonding quality.
5. Follow Alignment
Align the coverlay with the circuit marks, such as:
- C marking line
- Pads
- Counterpart holes
Follow the given process rules and manufacturing instructions carefully. This will help in preventing errors during lamination.
6. Peel Off the Release Film
Place the coverlay on a flat surface while keeping the release film facing up. Then, peel it off slowly and gently. Do not rush at this step. While peeling, you need to be very careful. Otherwise, damage can occur.
7. Fix Coverlay
After alignment, fix the coverlay temporarily on the FPC. This helps stop any movement during lamination. Use a low temperature hot bar to tack the edges. Do not apply too much heat. Otherwise, the flexible PCB can wrap or get damaged.
8. Perform Lamination
First, peel off the release film before lamination. Then, pass the FPC through the lamination process. Keep the temperature between 160°C and 165°C. After lamination, let the assembly cool down completely. Check the result to ensure proper bonding.
Necessary Precautions
While pasting coverlay on FPC, assembly facilities must take these necessary precautions to avoid any damage to the circuit.
- Check cleanliness before initiating the process.
- Inspect the coverlay and FPC surface for defects. There must be no scratches, wrinkles, or particles.
- Maintain proper alignment accuracy during placement. Misalignment can cause defects in lamination.
- Control process temperature and pressure carefully. This ensures stable bonding and avoids material damage.
FAQs
What Is the Difference Between FPC Coverlay and Solder Mask?
FPC coverlay and solder mask are different. Coverlay is a film layer used on flexible PCBs. It protects the circuit and allows bending. On the other hand, a solder mask is used on rigid PCBs. It is a liquid layer and is not flexible.
Why Is Coverlay Used in FPC?
Coverlay is used to protect the circuit. It keeps dust, moisture, and chemicals away. Also, it prevents short circuits by providing insulation and keeps the board flexible.
What Is the Structure of Coverlay?
Coverlay has three main layers. The first layer is the base material, usually polyimide. It provides heat resistance and flexibility. The second layer is the adhesive, which bonds the coverlay to the FPC. The third layer is the release liner, which protects the adhesive before use.
Conclusion
Follow the above process to paste the coverlay on FPC. Each step helps improve bonding and performance. So, keep the surface clean and work carefully. This reduces errors and damage. A simple and careful approach ensures strong protection and long life of the circuit.
As a professional manufacturer specializing in flexible PCBs and rigid-flex PCBs, MV Flex Circuit has more than 20 years of mature and reliable production experience in the FPC industry. To ensure stable production cycles and consistent board quality, we maintain sufficient inventory coverlay, like DuPont & Panasonic, with up to 8 layers for flexible PCBs and 20 layers for rigid-flex PCBs.
If you have upcoming flexible PCB or rigid-flex PCB projects, you are welcome to Send us your Gerber files today to start your project.
