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Alternative Design Concept to High Temperature Lead-Free Solders
Keywords: high temperature, lead-free, die attach
Zn-Sn based lead-free solders were successfully developed by designing and controlling the liquid fraction at reflow temperature. The properties were compared with Bi based high temperature lead-free solder. The coefficients of thermal expansion of the Zn-Sn alloys increased with decreasing Sn content, i.e., 29.2 x 10-6 K-1 to 33.2 x 10-6 K-1 in the temperature range of -50 C to 200 C for Zn-Sn alloys. With increasing temperature above eutectic temperature, all alloys began to deform, indicating the formation of a liquid phase. The deformation of the Zn-Sn alloys decreased with increasing Sn content. In the tensile test, the ultimate tensile strength (UTS) and 0.2% proof stress of the Zn-Sn alloys were almost same, but the elongation of the Zn-Sn alloys decreased with increasing Sn content. The phase stability of the Zn-Sn alloys under thermal and humidity exposure was examined. After thermal and humidity exposure for 1000 h (85C/85%RH), only the outer surface of the Zn-Sn alloys oxidized. The UTS and 0.2% proof stress of the Zn-Sn alloy slightly decreased with increasing exposure time. The elongation of the Zn-Sn alloys decreased with decreasing Sn content for 100 h exposure. However, the elongation of the Zn-Sn alloys showed no further degradation beyond 100 h exposure, which remained a ductile deformation. The elongation of the Zn-Sn alloys exposed for 1000 h is still larger than that of the as-cast Bi-Ag alloys.
K. Suganuma, Professor
Osaka University
Osaka 567-0047,
Japan


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