`Low Melting Alloys: A New Solution to Lead Free Rework Process Issues

ABSTRACT Different BGA packages with SAC (Sn-Ag-Cu) 405 balls were assembled on several Printed Circuit Board (PCB) test vehicle designs using no clean SAC 405 paste. These components were then reworked using a new low temperature lead free rework process. In this new process, a solder alloy with a melting range lower than that of SAC solder was used. The lower rework process temperature prevents components from overheating, reduces board warpage and cratering, and avoids thermal damage to adjacent components. The microstructure of the resulting mixed solder joints, SAC 405 ball / low melt paste, was examined “as assembled” and also after “accelerated thermal cycling” (ATC) using optical and scanning electron microscopy methods. The ATC study is being performed in a temperature range of 0-100°C following the IPC-9701 specification [1]. Interim ATC results are reported in this paper. ATC is ongoing to 6000 cycles. This evaluation included different surface finishes on assembly test vehicles, namely Immersion Silver (ImmAg) and Electroless Nickel/Immersion Gold (ENIG). The low melt rework process was performed on 0.093” and 0.125” thick boards and was done in a nitrogen atmosphere. It has been observed that fatigue life is component-dependant. When the component SAC ball is fully mixed with the solder paste, the formed solder joints have better thermal fatigue reliability compared to that of the pure SAC405 assemblies.