Hi Ken,
You write:
>I've read an article regarding reflow-soldering a stand-up surface mount
pins on
>a pcb. It says that a thin solder layer between the bottom of the pin and the
>solder land is much stronger than a thick joint. Do anyone know the rationale
>behind this?
There is a good reason for this. As solder joints get thinner--less than 2
mils or 50 um--the solder joint changes its composition considerably. You are
forming intermetallic layers a both wetted surfaces which consume Sn in their
formation. You also have some base metal migrationg into the SJ interior.
So what you are left with is: IMC layers (for the most part much stronger
than solder, but brittle), a thin solder layer with a high Pb content (higher
melting temperature) that may now also contain Cu or Ni.
However, it should be noted that the strength of a solder joint is no
indication of its reliability. Properly wetted solder joints have adequate
strength for most anything they are likely to see on normal processing,
handling and use. Thus the increased strength is not likely to improve your
product and the thinner SJ experiences higher strains for the same thermal
expansion mismatch actually reducing reliability.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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