Effect of underfill materials on Pb-free flip chip package reliability

In this study, Sn/Ag/Cu Pb-free solder paste was printed and reflowed onto a thin film Al/Ni(V)/Cu under bump metallization (UBM) on a 4mm × 5mm die and, packaged in a 5mm × 6mm LGA (land grid array) package using four different underfill materials with a range of different material properties. The packages were then examined after 1, 3 and 6 reflow cycles in addition to various standard package reliability tests. Package level Finite Element Analysis (FEA) was also conducted to simulate the solder creep strain under various strain rates of temperature cycling (TC) or thermal shock (TS) at. In addition, SEM and concurrent Elementary Mapping were used to characterize the integrity of the UBM. The FEA results show that underfill materials with a high Tg and a low coefficient of thermal expansion (CTE) could substantially minimize the solder's maximum equivalent creep strain under cyclical stress excursions. This conclusion was in agreement with actual package level reliability results. By selecting the appropriate underfill properties, this Pb-free flip chip package has passed moisture sensitivity level (MSL-3) with 3 reflow cycles at peak temperature of 260°C, followed by TC 1500 cycles, low temperature thermal shock (LTS) 800 × and unbiased highly accelerated stress test (UHAST) 168h. Additional observations noted include that the Cu was nearly completely consumed after these package assembly reflows. It was also found that Ni diffused into bulk solder after the aforementioned multiple reflow cycles.