IMPLEMENTATION OF INCREASED Cu LEVELS (1) IN SAC ALLOYS FOR PBGA APPLICATIONS

摘要

With Pb-free solder implementation and the higher Sn composition of the solder and higher reflow temperatures, Cu pad consumption is a concern and is often closely monitored. Certain applications require relatively thin Cu pad structures. Therefore, Cu pad consumption during soldering and reflow may become critical. BGA applications may be of particular concern, since the ratio of the solder volume to the Cu pad surface area is large, and the Cu consumption effect cannot be neglected. In this context, work was undertaken to characterize the effect of using SAC alloy solders with higher levels of Cu to mitigate the Cu consumption effect. SAC alloy solder joints with 3.0 wt % Ag content and with Cu composition ranging from 0.5% to 1.5 weight % were characterized, as a function of reflow conditions. Important reductions in dissolution and consumption of the Cu pad structures were observed, as initial solder sphere Cu content was increased. In the compositional range of Cu initially present in the solder spheres , it was found that the Cu content of BGA solder joints on Cu pad structures reached identical Cu concentrations within approximately 2 reflow cycles. Saturation values of approximately 1.2 wt % Cu were found, at a 250 °C, peak reflow temperature.The resulting solder joints were evaluated, using ball pull testing, metallurgical cross sectioning, polarized light microscopy, SEM examination and hardness testing of the solder. Different PBGA module types were prepared with SAC310 solder alloys and were tested in thermal cycling and shock and vibration testing and thermal aging, to resolve any potentially important differences in behavior and these results were compared to general application requirements using the SAC305 as a control.It was found that increasing the initial levels of Cu in solder had little impact on basic solder joint properties, but had the significant benefit of diminishing the Cu pad dissolution during solder reflow, with no impact to reliability testing results normally required to qualify this type of change in solder composition for field applications. Implementation of the Cu rich SAC alloys enables more flexibility for thethermal treatment of the solder joints and dramatically improves solder joint reworkability in the context of thin Cu pad structures.