On the Incorporation of Fine Pitch Lead Free CSPs in High Reliability SnPb Based Microelectronics Assemblies

The thermal cycling performance of lead free soldered ball grid arrays (BGAs) and chip size packages (CSPs) soldered with eutectic SnPb solder paste, so-called backward compatible assemblies, tends to drop with increasing Pb concentration in the joints. This is a particular concern for fine pitch CSPs where concentrations are invariably rather large. Flip chip assembly life is almost always improved by underfilling, although lead free soldered assemblies are more sensitive to the underfill material properties. In the case of BGAs and CSPs, however, underfilling with the wrong material may actually reduce life in thermal cycling substantially. In general, underfills with low coefficients of thermal expansion (CTE) and high glass transition temperatures (T_g) are preferred, especially for lead free solder joints. However, recent work showed a reworkable underfill with a high CTE and a very low T_g to still improve the life of SnAgCu joints with 3% Pb mixed in. A variety of 0.4- and 0.5-mm pitch CSPs with SnPb, SAC305, or SAC405 solder bumps were assembled onto a six layer PCB with a SnPb solder paste. Assemblies were tested in 0/100 °C thermal cycling both with and without a reworkable underfill with a reasonably high modulus and a T_g of 105°C. Results deviated systematically from trends previously observed for larger pitch BGAs.