Hi Werner - we are both on the same page (I should have typed out a more
detailed response). As you detailed, the actual metallurgical reactions of
shrinkage are the same, the focus should be on what is staying molten
last. We have observed on some BGA solderballs that were part of the
circuit connection to ground planes some slightly different shrinkage void
differences due to the solidification reactions. Interesting from a
technical viewpoint but didn't make a difference from a solder joint
integrity perspective.
Dave
Werner Engelmaier <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
04/20/2010 10:55 AM
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Re: [TN] SAC Shrinkage Voids in BGA Balls
Hi Dave,
A shrinkage void in an ingot occurs where it solidifies last---in its
center.
The same is true for SJs, except that they do not necessarily cool from
the outside in. but a connected to various heatsinks and reservoirs---the
component is typically a heat sink [coolest] and the PCB is a reservoir
[hottest], with various heat transfer conditions on the surface due to
forced air/N2.
Overall, SJs tend to solidify first at their surface; hence, most
shrinkage voids are to be found there.
Werner
-----Original Message-----
From: David D. Hillman <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, Apr 20, 2010 10:13 am
Subject: Re: [TN] SAC Shrinkage Voids in BGA Balls
Hi Bev - in theory, a shrinkage void in a solderball is the same physics
as a shrinkage void in a aluminum or steel casting so no, the shrinkage
void would not have to necessarily always reach a surface. However, after
looking at several hundred SAC BGA/CSP cross-sections, I have nearly
always (99.999%) found a path of a shrinkage void to the solderball
surface. Because we have a very limited and very small molten solder
"volume", I think you will find a surface connection. And I guess I'll
bite - why the question?
Dave Hillman
Rockwell Collins
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Bev Christian <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
04/20/2010 08:16 AM
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[TN] SAC Shrinkage Voids in BGA Balls
TechNetters,
Do all SAC shrinkage voids in BGA balls reach/start at the surface of
the ball? I realize I may be stumped by a 2D cross-section of a 3D
phenomenon, but I thought I would ask this august body.
Bev
RIM
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