When a flexible circuit requires high dielectric or dynamic flexing, an adhesive coverlay film is often the best choice.
This coverlay film is traditionally a layer of adhesive bonded to a layer of polyimide. During processing, heat and pressure are applied to the stack up causing the adhesive to soften and flow. The adhesive will flow (squeeze-out) slightly beyond the coverlay openings.
This process is necessary for complete encapsulation of the coverlay and to protect the edges of the film from chemicals or abrasion which might cause delamination.
Although this is a desirable result of bonding the coverlay, this “adhesive squeeze-out” also reduces the solderable area of the coverlay opening, and must be accounted for in the design stage.
We are often asked what an acceptable amount of adhesive squeeze-out is. According to IPC-A-600, the coverlay coverage shall have the same requirements as the soldermask coverage in rigid printed circuit boards. The acceptability requirements for coverlay coverage include both the coverlay and the squeeze out of adhesive and are different based on which Class is being built to.
For example, Class 3 requires 0.05 mm (0.00197”) solderable annular ring for 360 degrees of the circumference. Class 2 requires this same solderable annular ring for 270 degrees of the circumference and Class 1 requires a solderable annular ring for 270 degrees of the circumference.
We always recommend involving your supplier in the early stages of the flexible circuit design. An experienced flex circuit engineering will be able to guide you to the correct material stack up and tolerances needed to ensure you receive the product you require.
Please contact us for additional information. Designing printed circuit boards should not be difficult!