Abstract
Copper plates were soldered with tin foil of different thicknesses to examine the influence of the solder thickness on formation of secondary phases at the interface and fatigue properties. In the case of an initial solder thickness of 60 μm, the thickness of the η (Cu6 Sn5) phase and the ɛ (Cu3 Sn) phase linearly increased with the square root of the bonding time. The fatigue strength was 3 MPa and the fatigue life decreased with increasing stress amplitude and had a low scatter. In the fatigue process, fine cracks appeared in the η phase and propagated in the solder layer. This process was different from the case of static shear fracture. In the case of a 5 μm solder thickness, the solder was replaced by secondary phases in a short time, and only the ɛ phase remained at the interface after a bonding time of more than 300 min. The fatigue strength was 13 MPa and was independent of the interfacial structure, but the fatigue life showed a large scatter. In the fatigue test, unstable fracture occurred along the interfaces, similar to that observed in static shear fracture. From these results, it was concluded that solder joints become brittle with decreasing solder thickness.
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