Via Filling / Plugging

Silver filling is a process by which through hole vias are transformed into surface mount pads in order to recapture valuable board real estate. The process consists of drilling holes through the board as usual, plating copper in those holes then filling the holes with a silver epoxy. The silver epoxy is forced through the holes in one continuous motion via a pressurized vessel. This continuous motion produces a near void free filled hole.

At this point the silver epoxy is ground flat as to make it planar to the surface of the board. The epoxy is then thermally cured and the boards are pattern plated with copper. This copper plating provides a copper-to-copper electrical connection from the top of the board to the bottom. It is important to note that silver is not relied on to provided the electrical connection through the board. In fact the only reason the epoxy fill contains silver at all is to make it compatible with the copper-plating bath without requiring additional sensitizing steps.

Via Plugging is normally used to either prevent solder from wicking through the holes or to provide a good seal for a vacuum application. The following picture is an example of a solder mask plugged via. Note the bump of solder mask over the vias. The approximate height of the bump is .0005" +-.00003". Please use the following guide when designing for Solder mask via plug:

Hole size to be plugged must be <= .020". (refer to the third image on the left)
The solder mask plug is preferred to be .012" larger than the hole to be filled. Premiums apply for any plug size smaller, down to a minimum of .008". (refer to the third image on the left)
Plug distance from adjacent smt pad. Minimum .010". (refer to the 4th image on the left)