A flex PCB, made from a flexible substrate, is typically used in applications that require bending or are constrained by limited space. During the manufacturing process of flex PCBs, a material known as ‘coverlay’ is often utilized.

What is Coverlay?

Coverlay is a material used to encapsulate or cover the exposed circuit layers of a flex printed circuit board (Flex PCB). It typically consists of a polyimide substrate and an adhesive. The adhesive bonds the polyimide to the flexible circuit and seals the circuits. The coverlay is then carefully aligned with the circuit surface and applied under controlled temperature and pressure conditions. Its primary role is to protect and insulate the circuits on the flex PCB.

Wellcircuits’ coverlay options include the following:

Coverlay serves several key functions:

• Protection of Circuit Components: As an outer layer, coverlay shields the circuit components on the flex PCB from mechanical damage, dust, or chemicals. This is essential for ensuring the reliability of flexible circuit boards during bending and movement.
• Insulation: Coverlay has insulating properties that prevent electrical current from flowing between different layers of the circuit board. This is crucial for maintaining proper circuit function and avoiding short circuits.
• Maintaining Flexibility: Since coverlay itself is flexible, it helps maintain the overall flexibility and bending capabilities of the flex PCB. This is especially important for applications requiring complex shapes or confined spaces.

During the manufacturing process, coverlay is typically applied, laminated, or bonded onto the flex PCB over the circuit patterns. It can be bonded to the flexible substrate using various techniques such as lamination or coating. Choosing the right coverlay material and process is a critical step in ensuring the performance and reliability of flex PCBs.

How to Choose the Right Coverlay for Your Flex PCB?

The coverlay in a Flex PCB comes in various combinations of adhesive and film thicknesses. The most common combination is a 1:1 ratio (i.e., 1 mil of film and 1 mil of adhesive). The specific combination of film and adhesive depends on the following factors:

• Minimum Bending Radius Requirements: A thinner coverlay may be necessary to meet strict bending requirements.
• Outer Layer Finished Copper Thickness: At least 1 mil (25μm) of adhesive is required per ounce (35μm) of finished copper to ensure complete encapsulation. For example, 1 ounce finished copper requires at least 1 mil of adhesive.
• Dielectric Withstand Voltage: This may be a factor in some applications.
• Cost: Material costs and overall production costs also play a role in coverlay selection.

What are the Differences between Coverlay and Soldermask for Flexible PCBs?

The exposed outer circuits of a flexible printed circuit can be protected using either a polyimide coverlay or a flexible soldermask. While both materials serve the same primary function of protecting and insulating the outer circuits of a flex PCB, they have distinct characteristics, functionalities, and suitability depending on the design requirements.

The key differences between Flexible PCB Coverlay and Soldermask are as follows:

• Material Composition: Flexible PCB Coverlay consists of a combination of adhesive and Kapton (a type of polyimide), whereas soldermask (solder resist) is based on a liquid formulation.
• Dam Size: The dam size for coverlay should be a minimum of 10 mils, while the dam size for soldermask can be as small as 4 mils. The dam prevents molten solder from flowing between pads.
• Trace Width: Coverlay mask openings can approach 3 mils. However, using a 3 mil opening with soldermask can lead to issues like slivering or misregistration. It is advisable to maintain a minimum of 4 mils for soldermask openings.
• Application Areas: Coverlay is typically used on the flexible portions of flexible printed circuits, while soldermask is applied to the rigid or stiff portions of the PCB. If coverlay is used across the entire rigid-flex PCB, make sure to create tented or windowed openings where necessary.
• Component Spacing: Coverlay is not suitable for use on components with very small pin spacing, whereas soldermask can accommodate finer details.

If you have any questions about PCBs or PCBA, please feel free to contact us at info@wellcircuits.com.