In a message dated 3/23/99 4:43:25 PM Pacific Standard Time, [log in to unmask]
writes:

> I'm not sure if either method is better then the other, but from my
perspective, having tenting on the secondary side presented more of a risk
then the primary side as far as providing a place for getting "stuff" trapped
under the BGA was concerned. I suspect Steve has his perspective as well, and
I have no data to say one way or the other, but we tent the primary side, to
give you another datapoint.

Regards......DT

Hi Jim, it's me again,

Thinking about what Darrel said, he may have a point. I think a lot of it may
depend on how big the via's are. You said that the vias in question are
.012"...that's pretty small. So even if the opening isn't completely closed
off by the mask,  it sure does make the opening smaller than .012''...how much
smaller I don't know.

What I'm thinking, is that when you think about the the physics of surface
tension of the flux and solder, that's what's preventing "stuff" from getting
in there, it can't get through openings that small.....I guess I'm thinking
out loud trying to understand why Darrel doesn't have any problems when vias
are open on the wave side. Maybe when I had the problems I spoke of, maybe the
vias were a larger diameter...

One last issue with vias open on the wave side (and this may be size related
too...) is re-reflow of connections that have short trace lengths to a topside
pad...there's a bunch of us that have seen that before...

-Steve Gregory-

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