I've done up to 35 mm packages (PBGA) with 492 balls, 1.27 mm pitch... the
surface tension was plenty to hold them in place.

Bill...


At 10:51 AM 10/18/2001 +0800, you wrote:
>..........................................................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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