Capping Vias’ Holes with Screened Resist
Via capping is a process by which holes can be capped. Vias may be screened using solder mask with the creation of an epoxy cap for PCBs that have a liquid photo-imageable mask. As a task that is accomplished during the initial tooling, changes to artwork that are critical to processing should be made. The client must provide the vias that must be capped by detailing them in a unique design file. In addition, master pad solder mask and via files, such as solder mask and via pads which are identical to the outer layer pads, must also be provided by the client.

Design Limitations of Via Hole Capping
The largest via hole caps should have a diameter of 0.50 mm (0.020 inches). The best drill diameter should be 0.53 mm (0.021 inches).

Typically, the board’s up side is where non test vias are capped. This is particularly true beneath BGA parts. Avoid capping vias on the top and bottom sides of the board because it can cause either broken or raised caps.

A raised surface of about 0.06 mm ±0.05 mm (0.0025 in. +/- 0.002 in) above the outer layer of the copper pad will occur with via caps. This specification can include the solder and / or permanent solder-mask thickness.

For random locations of open holes, a board fabrication facility guarantees a 98 percent plugged-hole minimum.

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