Jim,
We had some similar difficulty with a BGA pattern. I prefer to have vias
either completely filled/tented or completely open. We wanted the back side
accessible for test but wanted the component side tented so the solder from
the BGA dog bone wouldn't run down the via. We ended up adding a dog bone
copper pattern on the back side to support test and plugged the entire via
with thermally cured soldermask.
Steph
Stephan Meschter [log in to unmask]
Lockheed Martin Control Systems Phone :(607)770-2332
600 Main Street, MD R52F FAX :(607)770-2056
Johnson City, NY 13790-1888 MARCALL: 8 * 255-2332
> ----------
> From: Jim Kittel[SMTP:[log in to unmask]]
> Reply To: TechNet E-Mail Forum.;[log in to unmask]
> Sent: Wednesday, March 24, 1999 8:40 AM
> To: [log in to unmask]
> Subject: Re: [TN] FW: LPI Tented Vias for BGA's...Part II
>
> Steve & Darrel,
>
> I appreciate both your inputs. Sounds like my vendor is correct in
> saying 'some people do tent only one side'. Steve, I have to agree with
> you about getting contamination into a 12 mil via which has been reduce
> in size by solder mask. In fact, I am not so sure I can even clean a 12
> mil via (0.062 thick PWB) if it didn't have solder mask tenting. (I am
> using a water/saponifier in-line cleaning system.) Maybe my concern
> about trapping additional contamination is not valid.
>
> Jim Kittel
>
> >-----Original Message-----
> >From: Stephen R. Gregory [SMTP:[log in to unmask]]
> >Sent: Tuesday, March 23, 1999 10:21 PM
> >To: [log in to unmask]
> >Subject: Re: [TN] FW: LPI Tented Vias for BGA's...Part II
> >
> >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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