This paper describes a new lead-free SMT rework process which avoids component overheating, reduces board warpage and cratering, and prevents adjacent components from thermal damage. In the replacement of a BGA, on an organic substrate printed circuit board assembly, a solder alloy with the melting range lower than Sn-Ag-Cu solder is used. The lead-free solder pastes analyzed in this work include two Indium containing alloys, melting range 181-187 °C and 138 °C respectively, and a Bismuth containing alloy, melting range 195°-209 °C. When the solder alloy melts, dissolution of the Sn-Ag-Cu balls occurs. The resultant composition and microstructure of the solder joints provide good mechanical and electrical properties with high fatigue life, while avoiding high temperature stresses on component bodies. In this work CBGA937, PBGA196, and CSP46 components with SAC405 balls were assembled using SAC405 solder. Then rework was performed with three compositions of low melt solders using different profiles. The microstructures of mixed solder joints were analyzed using optical and scanning electron microscopy methods. Two solder pastes and rework profiles were chosen for thermal cycling. A limited ATC study was done in a temperature range of 0 -100 °C. It was observed that fatigue life was component-dependant and when fully mixed, solder joints had better or equal reliability to that of the pure SAC405 assemblies.
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