Hi Joe - one other "divot" root cause to consider is that as the
solderpaste and solderball melt, the boiling off of the flux carrier
materials and the remaining flux material may form a flux "pillow or
buffer" that produces the divot. Many of the pictures of divots show
smooth, rounded contours which means the divots formed when the solder was
molten. Good luck on making this go away!
Dave
"Kane, Joseph E
(US SSA)"
<joseph.kane@BAES To
YSTEMS.COM> [log in to unmask]
Sent by: TechNet cc
<[log in to unmask]>
Subject
[TN] BGA Head In Pillow
04/20/2007 01:07
PM
Please respond to
TechNet E-Mail
Forum
<[log in to unmask]>
; Please respond
to
"Kane, Joseph E
(US SSA)"
<joseph.kane@BAES
YSTEMS.COM>
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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