| 摘要 |
PROBLEM TO BE SOLVED: To obtain solder which has a melting point as low as the melting point of a PbSb system and has excellent tensile strength and impact strength by composing the solder of a composition containing specific ratios of silver, bismuth and copper and the balance tin. SOLUTION: The lead-free solder having the melting point of 187 to 220 deg.C is obtained by specifying its composition to the range containing 0.1 to 3 wt.% silver, 3 to 7 wt.% bismuth and 0.5 to 1 wt.% copper and consisting of the balance the tin. The silver is effectual in improving the thermal fatigue resistance of the solder and the bismuth has an excellent effect of lowering the melting point of the solder. The copper is added to the composition in order to improve the thermal fatigue resistance of the solder like the silver. The tensile strength and impact strength are preferably balanced by not incorporating the materials exclusive of the materials intruding as impurities into the solder. As the form of using the lead-free solder, the lead-free solder is preferably used in the form of cream solder, etc., by kneading the lead-free solder classified to, for example, about 10 to 75μm in average grain size with an RMA flux or residue-less flux.
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