Abstract
The relationship between microstructure and fatigue crack growth behavior was examined at Sn-Ag solder interfaces on copper and electroless-nickel metallizations. On copper metallization, the solder interface was lined with a coarse Ag3Sn intermetallic phase in addition to the Cu6Sn5 intermetallic and the adjacent solder alloy contained nodular Ag3Sn phase. This interfacial microstructure was shown to result in inferior fatigue resistance, with the fatigue crack path following the interfacial Ag3Sn intermetallic phase. In contrast, the solder interface on the electroless-nickel metallization was covered with a thin layer of Ni3Sn4 intermetallic phase, and the solder microstructure was composed of fine needles of Ag3Sn phase dispersed in the Sn-rich matrix. This solder interface was found to be significantly more resistant to fatigue than the copper/Sn-Ag solder interface.
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