Abstract
This paper presents a detailed analysis of sagging phenomenon happened in right-angled solder joints formed by laser reflow soldering. When the solder joints were subjected to 125 °C isothermal aging or thermal cycle process, in which the temperature varied from −40 to 125 °C, sink steps formed near the interface of solder and pad with 4.0, 3.0 or 0.9 μm thickness of Au surface finish, and this phenomenon was called as sagging. The sink steps were larger for pads with thicker Au surface finish, and sagging phenomenon was more serious for Sn–37Pb solder joints than that of Sn–2.0Ag–0.75Cu–3.0Bi and Sn–3.5Ag–0.75Cu solder joints. Moreover, evolution of the sink steps were faster in thermal cycle process than that in aging porcess for Sn–37Pb solder joints. Through SEM and EDX analysis, it was supposed that the emergence of sagging phenomenon was caused by the formation and evolution of AuSn x intermetallic components (IMCs) and/or CuSnAu phases at the interface of solder and pad, and the stress cumulated in the solder joints during thermal cycle process may have great effects on the evolution speed of the IMCs.
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