Hi Andrea,
Why are you interested in solder joint strength? A solder joint is basically
in one of three possible conditions: 
(1) the solder joint is properly formed and all soldering surfaces are
properly wetted and have formed a metallurgical bond between one of the
solder constituents (typically tin) and a constituent of the soldering
surface base metal (typically copper or nickel)--this solder joint requires
great force to separate it (other than in fatigue), because metal has to be
ripped apart and this solder joint has significantly more than adequate
strength for even the most abusive handling and transport conditions;
(2) the solder joint does not have a metallurgical bond to the base metal of
one or both soldering surfaces (even though the solder joint may give a
wetted visual appearance), either because those surfaces are not solderable
(wrong material or contamination) or the soldering process was inadequate in
terms of temperature and/or time)--this solder joint is held together only by
weak mechanical adhesion and normal manufacturing, handling or transport
conditions are adequate to cause separation and typically cause early
(in-house) catastrophic failure;
(3) the solder joint is only partially properly wetted--this solder joint
will have adequate strength for normal manufacturing, handling or transport
conditions, however vibration and/or mechanical shock, particularly at cold
temperatures may cause failures; however, the solder joint strength will be
significantly above case (2), because metal still will have to be fractured,
but stress concentrations exist at the boundaries between wetted and
non-wetted areas. 

The differences in solder joint strength in cases (1) and (3) do however have
no influence on long-term solder joint reliability for the most prevalent
cause of solder joint failure, thermal cyclic expansion mismatch fatigue.
That damage mechanism is driven only by the loading conditions causing the
fatigue and solder joint strength is not an important consideration.
Thus the question: Why are you interested in solder joint strength? Also,
because of the wide differences in the metals and geometries involved, there
can not be any standards-- all measurements of pull strength will have to be
relative.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]
 

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