At 08:58 PM 6/26/00 -0400, the prolific Mr. Gregory wrote:

>This is a weird one...

<Twilight Zone theme music "on">

>We've got some boards here that we build for a DOD customer. This board has
>some LCCC's on it (small ones, 32-28-pins). We've been building them and
>masking off certain test via's that are un-masked that wind-up beneath the
>LCCC's because of the concerns (justifably) of cleaning. Now they want to
>expand the test coverage that they've had, and want to un-mask all the vias!
>
>I know from reading posts from Werner and others that if you have un-masked
>vias, that you need full-fill after wave for reliabilty concerns. I also know
>that cleaning flux residues from vias beneath an LCCC is a lost cause.
>
>I have heard that the fab vendors can fill vias for you...that to me would
>alleviate the concerns about cleaning. Vias that are filled already would not
>allow flux to get beneath the part.
>
>My question is; is that very expensive to get done? Getting the via's filled
>at the fab vendor? I've never asked that be done before...

<Rod Serling voice over>

That must be of the few board related experiences you have missed, Steve.  ;-)

I believe what you are referring to is what I call "via plugging", which we have done at our fabrication suppliers. This is a thermal epoxy applied after HASL by silkscreening thermal epoxy soldermask. We normally do this only on the "solder" side of the panel.

Our vendors create the necessary artwork to do this from the drill chart, and plug only the vias (typically 0.010" dia PTH's with 0.025" pads). This is done because the "tenting" of the via with LPI mask would not be reliable, and because the LPI "plugs" might rupture in the thermal excursion of HASL, entrapping HASL flux or other nasty stuff.

The cost is just a few dollars per panel, so on a unit basis, should be slight.

<back to "normal" comm mode>


regards,

Jerry Cupples
Interphase Corporation
Dallas, TX USA

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