Effect of SAC Alloy Composition on Drop and Temp cycle Reliability of BGA with NiAu Pad Finish

The search for an optimized Sn/Ag/Cu (SAC) solder alloy with drop performance as good as SAC105 and temperature cycling (TC) performance as good as SAC305 is ongoing to reduce solder ball inventory and to avoid assembly problems, such as mixing of different solder balls. This study focused on Nickel-Gold (NiAu) pad finish using five different SAC solder alloy compositions (Sn0.3Ag0.7CuBi, Sn1.0Ag0. 5Cu, Sn1.0Ag1.0Cu, Sn1.2Ag1.0CuNi, Sn3.0Ag0.5Cu) with small additive elements. The JEDEC drop test (JESD22-B111) was performed on 84 ball foot prints, 0.8 mm pitch Ball Grid Array package (BGA). The temperature cycling condition of 40/125degC, 10 minutes dwell time, and 2 cycles/hr was performed on 48 ball foot 0.8 mm pitch BGA. All the test samples were mounted on Cu-OSP finished printed circuit board (PCB). Failure data from the experiments were analyzed using Weibull statistics. First failure and characteristic life of each SAC composition was determined and compared. The results show that solder alloys with Ag content between 0.3% and 1.2% have similar failure distribution, but have significant improvement on drop performance compared to SAC305 solder. On the contrary, the solder alloys with excellent drop test performance have negative behavior to temperature cycling loading. SAC305 solder has the best TC performance. The effect of Cu% alone in solder alloy, in the range of 0.50% to 1.0%, is not found to have significant impact on both the drop and TC performance improvement if the pad finish on the print circuit board (PCB) is Cu-OSP.