A flex PCB, made from flexible substrate, is typically employed for applications requiring bending or constrained space. In the manufacturing process of flex PCBs, a material known as 'coverlay' is often involved.
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 adhesive. The adhesive bonds the polyimide to the flexible circuit and seals the circuits. The flex PCB coverlay is then aligned with the circuit surface and aligned under specific temperature and pressure conditions. Its role is to cover the surface of the flex PCB to protect and insulate the circuits.
PCBWay' s coverlay has the following options:
Specifically, coverlay serves several main functions:
- Protection of Circuit Components: As an outer layer, coverlay can protect the circuit components on the flex PCB from mechanical damage, dust, or chemical substances. This is crucial for the reliability of flexible circuit boards during bending and movement processes.
- Insulation: Coverlay possesses insulating properties, preventing current flow between surfaces of the circuit board. This is essential for ensuring the proper functioning of the circuits and preventing short circuits between different layers.
- Maintaining Flexibility: Since coverlay itself is flexible, it helps maintain the flexibility and bending performance of the entire flex PCB. This is particularly important for applications with complex shapes or restricted 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 through various techniques such as lamination or coating. Selecting the appropriate coverlay material and process is a crucial 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 film and 1 mil adhesive). The specific combination of film and adhesive depends on the following factors:
- Minimum Bending Radius Requirements: Thinner coverlay may be needed to meet stringent 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 (in certain cases).
- Cost.
What are the Differences between Coverlay and Soldermask for Flexible PCBs?
The exposed outer circuits of a flexible printed circuit can be encapsulated using either a polyimide coverlay or a flexible solder mask. Although both materials serve the same primary function of protecting or insulating the outer circuits of the flex PCB, each material has different characteristics, functionalities, and suitability for certain design requirements.
The differences between Flexible PCB Coverlay and Soldermask are:
- Flexible PCB Coverlay is a combination of adhesive and Kapton (a type of polyimide), whereas solder mask (solder resist) is based on a liquid.
- The dam size for coverlay should be kept at a minimum of 10 mils; however, the dam size for soldermask should be kept at a minimum of 4 mils. This dam prevents molten solder from one pad flowing to a nearby pad.
- The trace width for coverlay mask openings can approach 3 mils. However, using 3 mils for soldermask may result in slivering or non-registration issues (relative to solder mask imaging). It is advisable to maintain a minimum distance of 4 mils for soldermask.
- Coverlay is typically used on the flexible portions of flexible printed circuits. Soldermask is used on the rigid or stiff portions of the PCB. However, if you intend to use coverlay throughout the entire rigid-flex PCB, ensure there are tented or windowed openings.
- Unlike soldermask, coverlay cannot be used for components with smaller pin spacing.
