Hello,


I have a copy of the 'Head-in-pillow' document by Alpha Metals.  Is there
any way I could send Steve a copy and he could post it on his site?


Thank you,

David


-----Mensaje original-----
De: TechNet [mailto:[log in to unmask]] En nombre de Kane, Joseph E (US SSA)
Enviado el: Friday, April 20, 2007 1:07 PM
Para: [log in to unmask]
Asunto: [TN] BGA Head In Pillow

Steve was good enough to post some pictures of one of our recent
failures:

http://stevezeva.homestead.com/files/solder_ball_fracture_site_115x.jpg
http://stevezeva.homestead.com/files/solder_ball_SEM_photos_4-13-07_003.
jpg

These are pictures of the underside of the same failed BGA connection,
and
I believe this illustrates a phenomenon discussed here a while back,
called
variously "foot in mud" or "head in pillow" or "ball in socket".
Looking back 
through the archives I couldn't figure out why sometimes the divot
appears on 
the underside of the ball, and sometimes the depression is in the solder

remaining on the land.

I take it that sometimes the BGA ball doesn't melt as fast as the solder
paste, and 
the intact ball may be pulled upward out of the paste when the BGA
warps.  When 
things begin to cool, the ball descends into the pasty solder on the
pad, resulting 
in the "ball in socket".

But if everything does melt together, and the corner warps upward, most
of the 
solder can be pulled up and away by surface tension, leaving a smaller
solder
deposit on the pad.  When things begin to cool, this smaller mass
freezes first, 
and the pasty mass attached to the underside of the BGA settles onto
this hard 
mound, creating "head in pillow".  Like in this case.

Is this fair?

I also saw a number of citations of this link:

http://alphametals.com/products/solder_solutions/pdf/pillowhead.pdf

..which might have led to some good information, but the link appears 
to be broken.  Anyone have a copy of this file, or know a valid
shortcut?

Joe Kane
BAE Systems
Johnson City, NY

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