"Tenting" is covering the pad and hole with solder mask.  "Plugging" the 
via is a much more expensive operation, and is not generally done unless 
one is worried about wicking solder away from joints such as BGA balls, or 
the via is plugged and surface plated to accommodate a spring loaded 
contact pin that might otherwise ream away the plating at the edge of the hole.

Tenting has some small risk of chemical trapping, but most cleaning 
processes today are sufficient to reduce the risk to negligible.  I have 
been tenting vias with solder mask for over three decades and never had a 
failure that could be traced to trapped chemical contamination.  NASA / 
MILSPEC may have some limitation on tenting, but I am not familiar enough 
with those requirements to comment.


At 06:10 PM 9/12/01 -0400, you wrote:
>Hello, all:
>
>I am wondering what to do about vias.  I have a design for an 8-layer 0.090
>in. PCB with about 700-800 vias 32mil pad, 18mil hole (no BGAs).  Since the
>vias can be very close (6 mil) to other vias and pads, I want to cover the
>vias with either soldermask or tent them.  What is the difference between
>just covering them with soldermask, or tenting?  I guess tenting is where
>the via holes are filled with an inert material to plug them, right?
>
>I have read in trade journals (such as PCB Design) that trapping residual
>PCB etching chemicals inside vias can "eat them out" over time and result in
>boards going bad after X years of service.  If this is so, wouldn't
>soldermask AND tenting both trap these chemicals inside the via holes?
>
>Best regards,
>Ivan Baggett
>Bagotronix Inc.
>website:  www.bagotronix.com

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