PCB Polyimide Coverlay and LPI Solder Mask
Polyimide coverlay and flexible liquid photoimageable (LPI) solder mask are two primary options for encapsulating external circuit layers of a flexible PCB board. However, the two materials have very different capabilities and requirements.
Of the two, polyimide coverlay is the most commonly used and preferred solution in the whole PCB industry. It offers a much more robust and durable solution with very good flexibility and high dielectric.
Flexible LPI solder mask is essentially the same formula with the same capability and is applied in the same manner as that on a rigid circuit board.
Coverlay Materials and Construction
Coverlay is a two-part sheet material consisting of a polyimide layer and an epoxy or acrylic-based flexible adhesive layer. The function of the adhesive is to bond the polyimide to the flexible circuit board and encapsulate the circuit. The coverlay is then aligned and laminated to the circuit surface under heat and pressure.
Coverlay is available in various combinations of film and adhesive thickness, the most common one is combined 1 mil film with 1 mil adhesive.
SMT & PTH Feature Openings
As a sheet material, plated through holes (PTH) and surface mount technology (SMT) are processed into the coverlay by using one or more of the following methods: drilling, routing, laser cutting, knife cutting or die set. The exact method depends on the shape, size, complexity and quantity of the parts to be manufactured.
This in-turn imposes additional considerations beyond that of an LPI solder mask:
a. Larger min. annular ring to exposed feature requirements.
Allows for material and manufacturing tolerances and potential adhesive squeeze out during lamination.
b. Larger min. web thickness between adjacent features.
Prevents easily damaged thin sections/webs
Allows for sufficient adhesive to ensure proper lamination and circuit encapsulation
c. Isolated “island” type features not available.
Would fallout of layer after machining
In flexible PCB designs with higher density SMT and PTH features, if design allows, multiple feature openings need to be combined into larger ganged openings to accommodate the above items.