A Flexible Circuit Coverlay
In the flexible printed circuit board manufacturing process, a flex circuit coverlay or coverfilm is used to protect the external circuitry of a flexible circuit board.
A flexible circuit coverlay serves the exact same function as soldermask that is used for rigid printed circuit boards. The difference with the flexible coverlay is the flexibility required for flexible circuit boards and the required components for durability.
The coverlay consists of a solid sheet of polyimide with a layer of flexible adhesive that is then laminated under heat and pressed to the circuit surfaces. The required component feature openings are mechanically created using drilling, routing, or laser cutting.
Typical coverlay thickness is 0.001" polyimide with 0.001" of adhesive. 0.0005" & 0.002" thicknesses are available, but only used as needed to meet specific design requirements.
Polyimide coverlay and flexible liquid photoimageable (LPI) solder mask are two primary options for encapsulating the external circuit layers of a flex circuit board.
Of the two, polyimide coverlay is the most commonly used and preferred solution throughout the industry. It provides a much more robust and durable solution with very good flexibility and a high dielectric.
Flexible LPI solder mask is essentially the same formula that has the same capabilities and is applied in the same manner as on rigid circuit boards.