Tenting of vias can be a very baaaaaad thing. Tenting on only one side of
the board guarantees you get a "cup" to entrap material in. Tenting on both
sides is equally bad. All you need is a small pin hole opening and the via
is filled with undesirable material. This material can literally "explode"
out of the via during subsequent soldering processes. The risk is higher as
the via gets smaller since the final rinses are less effective.

Plugging ( a secondary solder mask process) is much safer but our vendors
charge more for it because it adds another process step.

I am currently working a nasty issue where liquid from an immersion tin
bath was entrapped in 10mil non-relieved vias tented on both sides (under a
BGA). I now have small volcanoes of tin material on my boards. I can send
pictures/details if you're interested.

Cheryl Tulkoff
Phone:(512) 683-8586
Fax: (512) 683-8847
National Instruments
11500 N. Mopac Expressway
Building A
Austin, TX 78759-3504

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