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

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