As FPC supplier knows that, the circuitry of the exposed external layers of flexible printed circuits can be encapsulated with either a polyimide coverlay or a flexible solder resist or solder mask. Although both materials perform the same primary function of protecting or insulating the circuitry of the outer layer of a flexible PCB, each material has different features, capabilities, and characteristics that are suitable for certain particular design requirements.

The differences between flexible PCB coverlay and solder mask are:

• A flexible PCB coverlay is a combination of adhesive and Kapton (a polyimide) but a solder mask (solder resist) is liquid-based.
• Coverlay dam sizes should be kept at 10 mils minimum; however, solder mask dam sizes should be kept at 4 mils minimum. The dam stops the flowing of molten, liquefied solder from one pad to another pad nearby
• The trace to mask opening for coverlay can be as close as 3 mils. However, using 3 mils for solder masks could cause undercut or non-recording issues (with respect to the imaging of solder masks). It is advisable you maintain a minimum distance of 4 mils for a solder mask.
• Coverlay is generally used on the flexible sections of flexible printed circuits. A solder mask is made use of on the hard or rigid section of the PCB. However, if you want to make use of coverlay on an entire rigid-flexible PCB, ensure fang opening or window opening is present.
• Unlike a solder mask, coverlay cannot be used on pitch components.