Solder joint reliability performance of electroplated SnAg mini-bumps for WLCSP applications

As microelectronic packages migrate to smaller and thinner form factors, WLCSP packaging is becoming more prevalent in the industry. In certain applications, overall WLCSP package heights are restricted to a maximum of 300um. In these situations, electroplated solder mini-bumps can be used to reduce the solder bump height to the sub-100um range. This in turn allows the use of manageable Si wafer thickness for processing. The downside of the electroplated mini-bump structures is the reduced package standoff after surface mounting, which can reduce solder joint reliability. When electroplating Pb-free solder, the composition is typically limited to a binary alloy composition. This reduces the flexibility a vendor has over the control of mechanical properties through solder alloy selection. In addition, the use of these bump structures at high temperatures can be detrimental since any intermetallic compound growth that may occur will represent a much larger volume percentage of the overall solder joint when compared to a larger bump size. This study focuses on the solder joint reliability of WLCSP devices with electroplated Sn-2.5Ag bumps. The bump size is 80um height × 120um diameter on a Cu/Ni UBM stack. Drop testing and thermal cycle testing was performed on 4×4 ball array daisy chain devices following JEDEC testing specifications, and Weibull lifetime estimation plots were created. In addition, Sn-Ni intermetallic compound (IMC) growth was characterized. Board mounted samples were aged at 125°C and 150°C for over 1000hrs. Cross sections were performed in roughly 168hr intervals in order to measure the resulting IMC layer thickness. IMC growth over time was fitted using a power relationship, and the diffusion rate constant and activation energy was calculated. It was found that thermal aging at 125°C resulted in very little IMC growth, while increasing the aging temperature to 150°C resulted in severe IMC growth. The high stress applied to the solder joint from the rapid IMC growth in the small bump area caused cracking of the UBM layer shortly after 500hrs of aging, which resulted in catastrophic failure after roughly 1000hrs. The results show that the Sn-2.5Ag mini-bump structure is quite robust through mechanical stressing of the solder joints. Depending on the final application temperature, they are an adequate interconnect structure to obtain ultra-low package thicknesses.