We have done boards with BGA's on both sides, but not in mirrored
positions, for the very reason you point out - the difficulty in X-raying
them. You will probably get a result on one side only, by X-raying one side
before populating the other.
Or you can point out the problem to your potential customer and see if they
can re-route the card to avoid this. Either that or find out if the
inspection problem is a concern to them (it should be, even though I've
found most X-rays to be pretty inconclusive as a means of determining how
well assembled the BGA's are).
As for applying epoxy to the components, unless you use a no-flow type
underfill that also acts as a fluxing agent in the assembly, you have no
other means of bonding unless you bond the corners of the BGA's fitted and
x-rayed after the first soldering pass. How big are the BGA's? It's
possible that they're light enough for surface tension to hold them in
place while up-side down during the second solder pass without them falling
off. Steve Gregory, I think it was, once gave a figure of how much weight
per contact can be held in place by surface tension of molten solder - was
it 5 grams per contact? But I don't know if this figure can be applied to
BGA's with their all-solder contacts. The effects of gravity and other
forces may distort the solder joints' form enough to reduce their
reliability.
My tuppence worth (UK value), but hope it helps a little.
Peter Duncan
Larry Koens
<lwkoens@EIMI To: [log in to unmask]
CRO.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: Aero/ST Group)
TechNet Subject: [TN] double sided BGAs
<[log in to unmask]
ORG>
10/18/01
12:32 AM
Please
respond to
"TechNet
E-Mail
Forum."
Dear TechNet,
My company has been asked to bid on assembling a board that would have
two BGAs on the topside and two more BGAs mirrored on the bottomside.
That is, they will be directly across from each other on both sides. I
never done them on both sides. So far, everything that I have done has
been on one side only. Has anyone done them on both sides before?
I am figuring xray will be a problem as after the second side is
placed, because the pads/balls will mirror each other on both sides.
Would I be able to tell the difference between the two BGA connections?
Suggestions?
Will I need to epoxy the first side BGAs so they would not drop off
during my second reflow? Wouldn't I need to epoxy them after first
reflow so that the balls could collapse?
Thanks,
Larry Koens
SMT Manufacturing Engineer
E.I. Microcircuits
Mankato, MN
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