Lead-free solder ball attach improvement on FCPBGA with SOP pad finishing

In the recent years, lead-free solder material have been increasingly applied to the green semiconductor products for RoHS compliance. For Flip Chip Plastic Ball Grid Array (FCPBGA), recent industry trend is changing from ENIG pad finishing to Solder-on-pad (SOP) pad finishing using Sn3.0Ag0.5Cu to improve board level reliability due to black pad issue on ENIG pads. However, SOP has posted a challenge in ball attachment process due to higher oxide level on SOP pads. The use of Sn3.8AgO. 7Cu solder ball on SOP pad finishing has been facing slanted ball and wrinkled ball issues after ball attach reflow. Slanted ball is a defect as it fails the solder ball radius-trueposition and coplanarity specification. In this paper, Sn3.5Ag solder ball on 33 × 33 FCPBGA with SOP was being compared to conventional Sn3.8AgO.7Cu solder ball. After assembly, samples were subjected to laser scanning for slanted ball inspection. Visual inspection under low power scope was done to check for wrinkled balls. Cold ball pull (CBP) was used to evaluate the solder joint strength at 3 conditions, namely after assembly (TO), after six time reflow and after 168 hours high temperature storage (HTS). In addition, tray drop test and packing drop test were conducted to assess solder joint integrity due to handling and impact force. Solderability test was also performed per Jedec standard to assess board mounting reliability. Finally, Sn3.5Ag is recommended to replace Sn3.8AgO.7Cu ball for 33 × 33mm FCPBGA package to resolve the wrinkled and slanted ball issue and improve the overall solder joint reliability.