Influence of microstructure on fatigue crack growth behavior of Sn-Ag solder interfaces

The relationship between microstructure and fatigue crack growth behavior w as examined at Sn-Ag solder interfaces on copper and electroless-nickel met allizations. On copper metallization, the solder interface was lined with a coarse Ag3Sn intermetallic phase in addition to the Cu6Sn5 intermetallic a nd the adjacent solder alloy contained nodular Ag3Sn phase. This interfacia l microstructure was shown to result in inferior fatigue resistance, with t he fatigue crack path following the interfacial Ag3Sn intermetallic phase. In contrast, the solder interface on the electroless-nickel metallization w as 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 r esistant to fatigue than the copper/Sn-Ag solder interface.