The coverlay opening is used to expose the unsupported (no PTH) pads with diameter at least 0.25mm smaller than the diameter of PAD to ensure that the coverlay is over the pads and stop PAD peeling off from the substrate. It is recommended that the coverlay overlaid on the pads at least in two locations.
1. Coverlay opening space (no supported hole)
If the distance between two coverlay openings is bigger than 0.3mm, it’ll be okay to punch the opening by making two-step jumping mould tooling, otherwise, it have to be done by CNC or laser cutting, but the part between will be easily broken after CNC or laser cutting, and become the burrs left on the coverlay and finally become to the poor coverlay on flex circuits. Under that situation, it will be better to have the whole area between two openings been cutout.
2. Coverlay opening design of the supported holes
The opening on the coverlay is used to expose and support the Plating Through Hole (PTH) pads. The diameter should be at least 0.25mm larger than the size of pad for coincidence tolerance and adhesive extrusion purpose.
3. It is not practical to use single coverlay opening in the high density soldering area. Taking the structure of the connector as an example, it can be made as below. For the unsupported hole you can add toes on the copper pad.
a. The cost of combined and single coverlay opening is the highest.
b. The single coverlay opening should be used for the Flex PCB with low density pad.
c. The ribbon or combined coverlay opening should be used for Flex PCB with high density pad.
4. Gold finger coverlay opening
Try to make gold finger coverlay opening in the area where trace changed from big to small. In order to avoid breaking, the gold finger should be extended to coverlay opening for more than 0.3mm. A disadvantage of coverlay opening is that the gold finger are easily broken at the connection area, therefore, the shape of usually designed to be wavy.
5. It is not suitable to use coverlay to cover the surface finishing such as ENIG, because the metal coating is easily melted when soldering, and after that, the coverlay will be lapped or bubbled.
Surface Mount Access
BEST TECH can provide flex circuits with areas that are specifically designed for surface mount components. Because covers are drilled, not silk-screened, round access holes are easier to be provided. Square access holes will increase the cost because the pad access area would have to be punched out with a punch-and-die tooling. Square pads with round access holes are a good compromise. Below are some ideas for configuring pads for surface mount
Photoimageable coverlay materials are also available, and can provide intricate, irregular shaped openings for dense surface mount patterns.