Scott, Ioan, et al,

I think it was about 10 years ago this point was subject of  a good TechNet discussion and references were presented.  Louis Hart

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Post, Scott E
Sent: Friday, April 01, 2011 2:37 PM
To: [log in to unmask]
Subject: Re: [TN] BGA on Solder Side

Many years ago I ran across a paper that said you're OK as long as the ratio of part weight to total land area is less than 0.81 g/mm^2.  This was in the SnPb days, but since the surface tension of SAC is higher than SnPb the rule should still be valid.  The paper is long gone so I can't give a citation.  Let's say the pad diameter for your BGA is 0.3 mm. The ratio works out to 0.039 g/mm^2 so you'll be fine.  I can't imagine a BGA that wouldn't have enough surface tension to hang on during upside down reflow.

Scott Post
Staff Manufacturing Engineer
P.O.U. 0000-001E-0CTC
2151 E. Lincoln Road
Kokomo, Indiana    46904-9005
765-451-2983 (Phone)
765-451-0287 (FAX)

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
Sent: Friday, April 01, 2011 2:11 PM
To: [log in to unmask]
Subject: [TN] BGA on Solder Side

  Technetters,

We are faced with a design that puts 2 BGAs on the solder side of a
board. It will therefore go through reflow on the bottom when building
the other side. The BGA is 19 x 19 mm 484 balls .8mm pitch. Av Weight
1.3302 g.  This is a lead free design. I seem to have seen a paper on
the subject discussed on technet, but the archives do not work for me
right now.

We have done thousands of boards with BGAs on both sides, but always
small ones with max ball count at 84. We have never one this size. Any
experiences out there that you can share? Do's and don'ts?  I can tack
the corners of the  BGA with SMT adhesive, but I would rather not if I
can avoid it.

--
Leif Erik Laerum




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