Dear all,

Back from holidays I find a long list from the forum, discussing the solder lifting effect with Sn AgCu solder on plated holes that is  assigned to the presence of Pb and Bi in the solder joint. As a member of the IDEALS project, and did the wave 
soldering part of the project, I can give a direction of thinking to prevent solder lifting.
We found that solder lifting can be prevented using a super-eutectic composition, e.g. >3.5% Ag in the alloy composition.
While solidifying, the primary solidification fraction is a skeleton of Sn dendrites filled with a liquid eutectic residu. During solidification the board thickness shrinks (TCE ~ 300 ppm/dC). The volume fraction of the liquid eutectic part of the 
solidifying joint has to bridge the gap that is caused by this shrinkage and the TCE of the residual (liquid) eutectic. The longer the solidification trajectory, the greater risk for solder lifting. The phase diagrams can indicate the permissible 
contamination of Pb and/or Bi in the solder joints.
In IDEALS we found that the liquid eutectic fraction is likely more than about 40% by volume to prevent solder lifting. AND with a Pb content of about 2 w% in the solder joint on a 1.6 mm thick SnPb-electroplated hole and SnPb finished wires, NO solder 
lifting was found. A Pb-rich zone is found only at the component side of the hole. In reflowed holes the Pb will be more homogeneously dispersed.

The IDEALS project used the SnAg3.8Cu0.7Sb0.25 alloy in their wave soldering verification.
Results will be presented in the Berlin EGG 2000 Conference (Sept. 11-13, 2000) as well as in the Miami IPCWorks 2000 Conference (Sept. 11-15, 2000).

____________
P. Langeveld
Philips CFT
P.O. Box 218   SFK-028
5600 MD Eindhoven   NL
Phone/Fax: (+31 40 27) 37010 / 36815
E-Mail: [log in to unmask]
_______________________________

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