Risk Assessment of Hybrid Low Temperature Solder on Surface Mount Technology and Board Level Reliability for BGA Packages

Low temperature solder (LTS) enables surface mount technology (SMT) at a lower reflow temperature than the traditional Sn-Ag-Cu (SAC) alloy. This allows lower energy cost and C0
2
emission during assembly, and its mass production adoption is spreading to various applications. In this study, SMT and board level reliability (BLR) risks of LTS hybrid solder joint system, where package with SAC solder balls are joined with LTS paste are evaluated. Results show that LTS hybrid system is very sensitive to reflow profile. When the reflow peak temperature is not high enough, some solders did not fully collapsed, which increased the Head-in-Pillow (HiP) risk and also showed early PCB side solder cracks during thermal cycling (TC). Higher peak reflow temperature resulted in more stable solder joint shape; however, Bi element with brittle characteristics spreading to package side actually decreased the TC performance. Amount of Bi Mixed Ratio (BMR), which is related to reflow temperature, seems to explain the TC performance and finite element analysis (FEA) also confirm this behavior. FEA showed highest strain energy
$(\Delta SED)$
density per test cycle as BMR reached 80 % and 30 % at the package side and PCB side respectively. The results suggest optimal BMR range as between 50 % and 70 %. Solder paste volume and package surface finish also showed significant impact on TC performance. As solder paste-to-ball volume ratio increased from 35 % to 50 %, more stable wetting occurred on PCB pad and TC lifetime improved by 40 %. Also electrolytic Ni/Au finish showed 30 % higher TC lifetime compared to Cu/OSP as Ni/Au increased solder joint hardness.