Advanced Materials 1

Electroless nickel immersion gold (ENIG) has emerged as a leading surface finish for under bump metallization (UBM). An ENIG surface typically exhibits excellent planarity, good electrical characteristics, wettability for solder, and Al-wire bondability. Reliability and performance of ENIG systems have been extensively reported at temperatures below 250°C, however information is not readily available about its reliability at higher temperatures. In this paper, thermal behavior of electroless Ni/Au and electroless Ni/Pd/Au during solder reflow processing at temperatures of up to 350°C was characterized. Wafer level evaluations were completed using optical microscopy and FIB cross-sectional SEMs, while packaged devices were subjected to standard reliability tests. Electroless Ni began to exhibit signs of cracking when exposed to temperatures of 350°C under certain conditions. Based on these findings, we propose ENIG material structures and maximum processing temperatures to eliminate cracking. Power devices manufactured with ENIG metallization can be capable of passing JEDEC qualification when implemented with these design parameters.