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April 22, 2009

SAC lead Free Solder Tear

I believe it is the nature of SAC lead free solder to experence "solder tear" or shrinkage cracks on the surface of PTH solder joints during cool down after the wave soldering process. I believe the phenomenon has been accpeted as stated in IPC 610 Rev D.

Is there any lab analysis to show this will not cause reliability issues in the future if the assembly is exposed to harsh environment applications?

S. L. Lee

Experts Comments

I would say that we don't know whether they are a long term reliability issue if exposed to harsh environment applications. The shrinkage cavities are formed in SAC alloys such as SAC305 in part due to the alloy not being a eutectic.

SAC305 has a mushy zone thus the differences in solidification behavior. I do not know of any lab results or test data that proves a shrinkage crack will never cause a failure under severe conditions.

There is some evidence that a cavity in the maximum shear plane can initiate a crack under vibration conditions (Blattau, SMTAI Conference 2005). Others tests have shown a shrinkage cavity to expand within a 1000 cycles under thermal cycling conditions of -40 to +125C.

These tests show that under certain conditions, shrinkage cavities may be a reliability concern but do not prove that a shrinkage cavity will always cause a failure. Shrinkage cavities are a concern thus the guidelines on the depth and contact with the lead or land stated in IPC-610-D.

You might want to look at some of the other lead free alloys that don't have this issue, i.e. SN100c.

Mike Scimeca
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.
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