Pb-free solders based on SnAgCu, SnAgBi, SnAg and SnCu, fair wave soldering of electronic assemblies

Abstract

Pb-free wave soldering technology, using SnAgCu, SnAgBiSb and SnAgalloys, has been observed to be technically viable. These Pb-freesolders are compatible with Cu-OSP, immersion Sri, immersion Ag andelectroless Ni/Au board finishes. Mechanical and physical properties(expansion coefficient, E-modulus, strength, ductility, and meltingtemperature/range) were measured using TMA, DMTA tensile tests and DSC.CTE is slightly lower than that of Pb-containing solders. E-modulus andyield stress are larger than those of SnPb40 solder. The most importantprocess parameters, solder bath temperature, contact time between boardand solder wave and pre-heating temperature have been thoroughlyinvestigated, and required values are presented. Extended processtrials have been executed to verify that the process can run with thesame yield as SnPb wave soldering. Thermomechanical fatigue life ofPb-free soldered joints has been investigated with temperature cyclingtests. It was shown that the fatigue life of most lead-free solderedjoints is at least equal to that of SnPb40. SnBiAgSb joints have aneven longer fatigue life by a factor of 1.5-2, depending on componenttype. SnCu joints have a fatigue life of only 0.5-0.6 times that ofSnPb joints.