Characterization of Electroless Nickel Plated Materials During High Temperature Solder Processing

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 wire bond-ability. 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, Ni/Pd and Ni/Pd/Au during solder reflow processing at temperatures of up to 350°C is 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 electroless Ni material structures and maximum processing temperatures to minimize stress cracking at high reflow temperatures. This paper builds on our previous work by expanding the tested material structures to a gold-free process of Ni/Pd as well as 5um Ni/Pd/Au. Power devices manufactured with electroless Ni metallization can be capable of passing JEDEC qualification when implemented with these design parameters.