What is a Coverlay PCB?

Coverlay PCB

Printed circuit boards, or PCBs, are crucial elements in the electronics industry. Their contribution to the industry has led technicians to develop powerful compact gadgets that perform impeccably. Moreover, these gadgets are available in varying types according to their design specification. However, the most common types of PCB are rigid PCBs, rigid-flex PCB, and flexible printed circuit boards.

Moreover, printed circuit boards combine various components, ranging from silk screens to coverlay and copper substrates. However, out of all the components that make up a printed circuit board, one crucial element is mainly overlooked, the coverlay. So what does a PCB coverlay entail? And why is it an essential part of a PCB’s design?

Printed Circuit Board Coverlays in a Nutshell

Coverlay flex PCB

Flexible printed circuit board coverlays carry out the same function provided by solder masks on rigid or inflexible printed circuit boards. However, on top of providing the functions of solder masks, these components also offer more flexibility and durability, making them more efficient compared to solder masks. Moreover, due to these crucial features, these components tend to be more efficient in terms of application in flexible PCBs compared to the traditional solder mask, which has limited flexibility.

Technicians typically apply printed circuit board coverlays during the production process of flex circuit boards to encapsulate and shield the flex circuit board’s copper (CU) structure.

Coverlays are typically covering films made from thick sheets of either polyimide or acrylic and flexible adhesive layers that are laminated under pretty high temperatures. After this process, the coverlays are efficiently pressurized onto the circuit board’s surface.

Coverlays have one primary function: shielding the FPC material’s base components. Moreover, this component bears impeccable temperature resistance. Therefore, it can be utilized in ovens and heaters.

Flexible Printed Circuit Boards

Full PCB Manufacturing

Flexible printed circuit boards, or FPCs, are advanced types of PCBs that can bend, twist or even fold into different shapes. Initially, PCBs were generated using solid base components known as substrates, making the resulting printed circuit board solid and inflexible. However, with more advancements in tech, manufacturers have found a way to generate flex PCBs that can bend according to the will of the user.

Flex PCB are impeccable additions to the electronics manufacturing industry. These printed circuit boards are:

  • Flexible
  • Lightweight
  • And they are also space efficient

Due to these features, FPCs have become pretty popular in the circuit board industry and have multiple applications in multiple industries. Some application areas of these circuit boards include:

  • Telecommunication
  • Automotive
  • Medical
  • Finance
  • Military
  • Wearable products

Flex-printed circuit boards are expensive. However, their advantages surpass their cost. Moreover, these circuit boards have a lot of features which include:

  • Impedance control
  • Easy customization
  • Elimination of connectors
  • Thermal management
  • Improved reproductivity and reliability
  • Easy installation and maintenance
  • Conformance with specific flexing requirements
  • Improved aesthetic

Flex PCB Coverlay Thickness

Coverlay flexible PCB

Flexible printed circuit board coverlays exist in varying thickness combinations of the film and the adhesive. However, the most common combination was in a simple ratio of 1:1 (i.e., one mil adhesive and one mil film). Moreover, the specific combo of these two components is primarily reliant upon the following items:

  • Cost
  • Dielectric resistance voltage
  • The external copper layer’s weight
  • Minimum bending specifications. Thinner coverlays may function better in applications that have tougher bending specifications.

Flex PCB Coverlay Coloration

If you carefully look at flex printed circuit board, you will notice a natural brown coloration on the top layer of the board. That is the color of a flex PCB coverlay. Moreover, since coverlays are solid sheets of polyimide or polymers, they have a dielectric voltage resistance that matches that of a flexible polyimide core (approximately 3KVs/inch).

The Approximate Thickness of Printed Circuit Board Coverlays

Standard flex PCB coverlays have a thickness of around 0.002″, which encompasses 0.001″ of adhesive and 0.001″ of polyimide. However, this is not the only thickness variation available. You can also find thickness variations of 0.0005″ and 0.002. However, the thickness variations come in handy when necessary according to specific design requirements.

The difference between solder masks and Flex Printed Circuit Board Coverlays

Coverlay FPC

The metal section of a flex-printed circuit board is usually on the outermost layer of the PCB stack-up. Therefore, technicians must utilize either a flex solder mask or polyimide to encapsulate and shield the circuitry from external damage. Even though these two components perform similar functions of insulating or protecting the board circuitry, they have pretty different characteristics, capabilities, and features.

The most crucial difference between solder masks and flex PCB coverlays include the following:

Their Composition

You can make flexible printed circuit board coverlays using a combo of Kapton (polyimide) and adhesive. On the other hand, solder masks are liquid-based components. Therefore, their structural composition is entirely different.

Dam Size

There is need to maintain the dam size of coverlays at a minimum of 10 mils. However, the dam sizes of solder masks should be at a minimum of 4 mils.

Dams stop the flow of liquefied, molten solder from one PCB pad to another.

Trace Mask Openings

Traces of mask openings for coverlays should be at least three mils apart. However, if you decide to go with the exact measurement when working with solder masks, you might face non-record and undercut issues. Therefore, you should keep the trace mask openings at least four mils apart when dealing with solder.

Applications

Generally, flex-printed circuit board coverlays are useful on flexible sections of flex-printed circuit boards. On the other hand, solder masks are good on rigid or stiff areas of printed circuit boards. However, in exceptional cases that require technicians to apply coverlay on the entire semi-flex printed circuit board, they leave out window openings or fang openings.

Unlike solder masks, we can’t use coverlays on pitch elements.

Conclusion

Flex printed circuit board coverlays provide impeccable insulation and protection to flex printed circuit board circuitries. Doing so helps technicians satisfy multiple design requirements. Moreover, coverlays offer mechanical support to flex printed circuit boards, improving their durability and efficiency. Moreover, when working with coverlays, you must understand solder masks since they go hand in hand. These two components perform the same function. However, due to their physical properties and characteristics, they have varying applications. But these two components play crucial roles in producing efficient PCBs. Therefore, we hope that this article has provided you with all the answers that you were looking for regarding coverlay PCB.

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