Most flexible circuits require a dielectric layer that insulates and protects the conductive traces, while exposing pads used for component attachment or terminations. Dielectric openings may need to expose a group of pads when there is not enough room for individual openings.
There are a variety of methods and materials that can be used: Photo imageable soldermask, screen printed covercoat and pre-cut coverlay (aka coverfilm) are the three more common methods to provide dielectric, mechanical and chemical protection. The best choice depends on the application and construction of the flexible circuit.
Coverlay film is often the best choice in cases where there is dynamic flexing or high dielectric requirements. The coverlay is generally a polyimide film that is coated with a thermoset adhesive. Film thicknesses range from .0005” to .005” with .001” and .002” the most common. With single copper layer circuitry, the thicknesses of the coverfilm is generally equivalent to the base film so copper is neither in compression nor tension (the neutral axis), especially when frequent bending is a requirement.
The coverlay is pre-punched, pre-drilled, or cut with a laser so the openings align with the circuitry areas requiring exposed copper. This film is bonded to the circuit by heat and pressure. The coverlay is first aligned and tacked to the circuit, then placed in a platen press that applies heat and pressure. Temperature, pressure, heating and cooling cycles are precisely controlled. As the heat and pressure is applied, the adhesive softens and flows. The adhesive flowing is necessary as it helps assure complete surface contact and encapsulation. The adhesive will tend to ooze out slightly around the openings as shown in the diagram below. This oozing is commonly referred to as “adhesive squeezeout” and is actually a desirable phenomenon.
By flowing or oozing out slightly beyond the opening, the adhesive helps protect the edges of the coverlay film from chemicals or abrasion that might pull the film away and cause delaminating. After cooling, the thermoset adhesive is irreversibly hardened.
Slight adhesive squeezeout, while necessary and desirable, can cause issues if not accounted for during the design stage. The effective coverlay opening is slightly reduced by adhesive squeezeout. The degree of expected squeezeout depends on the type and thickness of the adhesive, the thickness of copper and coverlay film and several other factors. An experienced flexible circuit engineer will be able to design the circuit with the tolerances needed to meet the desired functionality.