Thanks Werner and Shean,
This adds to my concern about the missmatch. This is a large device (40mm)
so these issues are fairly important.
I was worried about the pad pulling all of the solder from the joint and creating an open, but it seems that even an otherwise good, but assymettric joint can also cause problems.
I can't really do anything about the sphere diameter unfortunately.
I had thought of using SMD pads, but I'm convinced that that will cause a greater problem with the sort of stress concentrating geometry created in the resultant joint.
regards,
Phil.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Shean Dalton
Sent: Thursday, 29 July 2004 4:34 AM
To: [log in to unmask]
Subject: Re: [TN] BGA pad missmatch
Hi Phil,
I have been hoping for a response from someone who has practical
experience with this, as it is interesting to me. That said, I suspect
areas of concern would be for TCE. The larger pad size takes away
solder from the bulk joint, effectively reducing the standoff height.
Again, I suspect the reduced standoff height might reduce the "flexure"
ability of the solder joint to "absorb" the TCE mismatch. And, using
a larger sphere diameter may compensate for the large pad size,
effectively raising the standoff height. Sure would be nice to do a
finite elemental analysis.
Have fun!,
Shean Dalton
Austin American Technology
www.aat-corp.com <http://www.aat-corp.com>
-----Original Message-----
From: Werner Engelmaier
Sent: 7/28/2004 1:46 PM
To: [log in to unmask]
Subject: Re: [TN] BGA pad missmatch
Hi Phil,
Land area is not the issue, solder joint geometry is. If the unequal
soldering areas create solder joint geometries with stress
concentrations, earlier
failures [everyting else can be anticipated]. This is similar to NSMD
vs. SMD
solder pads.
Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com
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