In the flip chip interconnection using solder bump, the Under Bump Metallurgy (UBM) is required to perform multiple functions in its conversion of an aluminum bond pad to a solderable surface. In this study, various UBM systems such as Al 1㎛/ Ti 0.2㎛/Cu 5㎛, Al 1㎛/Ti 0.2 ㎛/Cu 1㎛, Al 1㎛/Ni 0.2㎛/Cu 1㎛ and Al 1㎛/Pd 0.2㎛/Cu 1㎛ for flip chip packaging system using the low melting point eutectic 63Sn-37Pb solder were investigated and compared in their mechanical and metallurgical properties. 100㎛ size bumps were prepared using an electroplating process. After sequential deposition of the UBM on Si substrate thick photoresist was patterned, and then 63Sn-37Pb alloy was electroplated in an organic sulfonate bath.
The effects of the number of reflow which simulated commercial processes -solder ball reflow, flip chip bonding reflow, organic carrier bonding reflow, and PCB board assembly relow- and aging time on the growth of intermetallic compounds (IMC) and mechanical bond strength were investigated. Solder reflowing was performed in an infrared reflow oven by peak temperature of 210℃ and dwell time of 90 seconds under ambient atmospheric condition, and aging temperature was 150℃.
Cu$_6$Sn$_5$ and Cu$_3$Sn IMC phase were observed after aging treatment in the UBM system with thick copper (Al 1㎛/Ti 0.2㎛/Cu 5㎛), however only the Cu$_6$Sn$_5$ was detected in the UBM systems with thin copper even after 2 reflow and 7 day aging at 150℃. On the solder side, the compounds grow so rapidly with big scallops shapes that the interface become extremely rough comparing to Cu side interface. This is induced by dissolving of Cu into liquid solder which causes the coarsening of the Cu6Sn5 IMC by ripening. Complete Cu consumption by Cu-Sn IMC growth gives rise to a direct contact of Sn and inner layer such as Ti, Ni and Pd, and hence to possibly cause reactions between two of them. In this study, however, only for the Pd case, IMC of $PdSn_4$ was observed by Cu consumption.
The effects of Cu-Sn and Pd-Sn IMC growth on mechanical bond strength were investigated using solder ball shear test. According to those results, the Cu-Sn IMC growth was not directly associated with the decrease of shear strength with heat treatments including the reflow and aging. The decrease of the shear strength was mainly caused by the delamination between solder and nonwettable metal such as Ti and Al, which was contacted to the solder after the complete Cu consumption by Cu-Sn or Pd-Sn IMC growth.
