Critical new issues relating to interfacial reactions arising from low solder volume in 3D IC packaging

The present study aims to reveal new issues relating to interfacial reactions arising from low solder volume in 3D IC packaging. Sandwich structures of Ni/Sn/Ni and Ni/SnAg/Ni were prepared by a general electroplating process. A high-temperature storage test was conducted by isothermal aging at 150, 180 and 200°C. Microstructure characterizations revealed that initially a whisker-like Ni-Sn phase was located at the Ni/Sn interface in the as-plated condition. According to previous research, these whiskers are believed to be NiSn
₄
. However, most of these NiSn
₄
whiskers were no longer visible after 24 h of aging. Throughout the aging process, the main interfacial IMC was Ni
₃
Sn
₄
. Grains of Ni
₃
Sn
₄
growing from opposite interfaces started to impinge on each other after aging at 150°C for merely 72 h. When the aging time reached 240 h, the Ni
₃
Sn
₄
occupied most of the interfacial area. Consequently, residual Sn formed islands located in between Ni
₃
Sn
₄
grains. The growth of Ni
₃
Sn
₄
followed parabolic kinetics during reactions. The parabolic constants are different from cases where the solder volumes are large but contain the same order of magnitude. IMC growth curves fitted a power-law relationship with exponents between 0.3 and 0.5, which suggests the growth of Ni
₃
Sn
₄
was controlled by both volume diffusion and grain ripening. Regarding the Ni/SnAg/Ni reaction, NiSn
₄
whickers that formed during the electroplating process also shrank and disappeared after 24 h of aging. Afterward, the only IMCs were Ni
₃
Sn
₄
and Ag
₃
Sn. The parabolic constant of Ni
₃
Sn
₄
growth is similar to values in the literature, regardless of the solder volume in reactions. The power-law exponent of Ni
₃
Sn
₄
growth is calculated as 0.27, which may be greatly a response to a process controlled by grain-ripening. The Ag
₃
Sn IMC coarsened with prolonged aging and was ultimately located in the middle of the interface. Explanations of the microstructure and growth kinetics of the IMCs are presented and discussed.