What is FPC Coverlay & how is it different from Solder Mask?

Coverlay PCB

The FPC coverlay is the last layer of material applied on the top of the flex PCB to give it a final touch which protects the structure of copper used in the PCB. It is a thick film for covering made of materials like polyimide or acrylic and a layer of flexible adhesive. The adhesive layer is placed on the board by applying pressure and temperature. Coveraly is used to give protection to the substrate of flex PCB. Coverlay has high-heat resistance, which enables it to work fine even in ovens and heaters. 

A flex-printed circuit board coverlay works in the same way as the solder mask onto the surface of the rigid PCB. However, the flex coverlay of PCB makes the board more flexible and enhances its durability, which makes it ideal than the solder mask, which provides limited flexibility. 

What Is A Flexible PCB?

Flexible PCBs are also termed flexible printed circuit boards (Flex-PCBs), or Flex printed circuits (FPC). These are high-quality PCBs with the capability of bending, folding, and twisting the circuit board. Long ago, PCBs were developed in solid form made up of a solid base called the substrate. However, with the revolutionary advancement in technologies, technicians are able to design and create a PCB that can easily bend and fit into smaller spaces. 

The addition of flex PCBs is incredible in the advancement of the electronic industry. Flex PCBs are flexible, lightweight, and smaller in size, which means they can take up fewer spaces. Flexible PCBs are widely used in consumer devices like wearable and portable products, medical, financial, military, aerospace, telecommunication, automotive industries, and much more. 

Flexible printed circuit boards are quite expensive and mostly designed by specialized and skilled manufacturers for consumer products. Some incredible features of flex PCBs are listed below.

  1. High Impedance control.
  2. It can easily be customized in several sizes and shapes.
  3. Lightweight and takes less space.
  4. Gives incredible flexible features to the devices like smartphones
  5. Gives easy access to installation and power services like satellites, power generators, and solar panels.
  6. Enhanced the integrity and reliability of the device
  7. High thermal conduction ability.
  8. Eradicate connectors needed.
  9. Enhanced aesthetics of the device.
  10. Decreases the assembly and production cost
  11. Improves the expansibility of the circuit board to give space for more components to connect. 
  12. Reduces the noise and the disruption and enhances the quality of signals.
  13. Gives an equilibrium and safe passage to pass the current at high-speed.

What is FPC Coverlay?

Full PCB Manufacturing

The main feature of flex PCB coverlay is to give full protection to the circuit of the PCB. It works the same as the solder mask for the flex PCB. Traditional solder masks have limited flexibility and deductibility. As flexible PCBs require durable and long-lasting flexibility and protection that is why flex PCB overlay is the best option. It shields the whole circuit including copper and wirings. In the PCB industry, flex PCB coverlay is the most widely used solution for covering the layer of Flex PCB. It is because it provides high efficiency, integrity, flexibility, and dielectric strength than other conventional solder masks. 

Flexible printed circuit board coverlay is a solid covering film. It consists of two layers; one is of polyimide, and the other one is a strong, flexible adhesive layer. The adhesive tape ensured to intact the polyimide on the surface of the flex PCB and enclosed the circuit thoroughly. After applying the layer of both polyamide and adhesion, the board is then placed under specific pressure and temperature, which ensures the intactness of the material completely. 

The flex PCB coverlay comes in a variety of combinations which vary in the thickness of both the material and adhesion layer. However, the widely used combination of flex coverlay is a 1-by-1 ratio. Each combination of adhesive and film is dependent on the weight of the outer layers of copper, the resistance voltage of dielectric, bending needs, and the overall cost. 

The brown color of flex printed circuit board coverlay is mostly seen in the flex PCBs as the coverlay is in a solid form, which is why the voltage resistance of the dielectric is the same as the polyimide core. Moreover, it also comes in several thicknesses for different bending needs. 

Thickness Of FPC Coverlay

The typical flex PCB coverlay has a thickness of 1 mils polyimide and one mils adhesives. However, coverlays also come in thicknesses of 2 mils and five mils as they depend on their usage. 

Difference between a FPC Coverlay & Solder Mask Layer

The outer exposed layer of the circuit of flex PCBs can be enclosed either with a solder mask or with a coverlay of polyimide. Although both have the same goal of insulating, covering, and protecting the external layer of the flex circuit. But both are different in terms of abilities, features, and integrities, which is why both are distinct in terms of usage. The necessary difference between both materials is as follows:

  1. Solder mask is liquid form material while a flex circuit coverlay is a solution of polyimide and adhesive. 
  2. The minimum thickness of the solder mask dam must be four mils while the minimum thickness of the coverlay dam must be 10 mils. This ensures to eradicate moisture and protects the circuit to get liquefied from evaporation. 
  3. Solder mask is usually suitable for rigid PCBs while the coverlay is suitable for Fex PCBs.
  4. Solder mask can be utilized on the components directly while coverlay is not allowed to be used directly on the components. 
  5. There should be 3 mils of mask opening in coverlay while 3 mils could create non-recurring problems in solder masks. That is why 4 mils would be best for solder resists.


Flexible printed circuit boards provide highly efficient insulation and protection to the circuit of outer layers of PCB. It not only protects the copper layers but also acts as an insulator while eradicating evaporation so that no moisture could reach the circuit. 

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