Soldering technology plays a key role in various levels of electronic packaging. such as flip-chip connection, solder ball connection in ball grid arrays, or IC package assembly to a printed circuit board (PCB). Among the Pb-free solders which was investigated and developed as alternative of Pb-Sn solder, eutectic Sn-Bi alloy are already used in some applications because of advantages such as lower process temperature and compatibility with particular substrates.
In the electropackaging process, the solder bump and coating are produced by electroplating process having merits on the reliability, accuracy and economics. So, it is necessary to develop the electroplating process and reflow process of eutectic Sn-Bi alloy, and to study on the properties of solder bumps which were electroplated and cooled with different cooling rate after reflow process.
In this work, effects of plating condition on composition, morphology, and solder properties of eutectic Sn-Bi solder electroplated from methane sulfonate bath were studied. And, effect of cooling rate in reflow process on microstructure characteristic of eutectic Sn-Bi solder bumps fabricated by electroplating was studied.
Sn-Bi alloy were electroplated from an methene sulfonate bath with the additive using DC current, and reflowed at 160℃ to form spherical solder bumps on the Cu pad of Si wafers.
Addition of grain refiner to the bath increased the cathodic polarization and reduced the difference of deposit potential between Sn and Bi. So, Sn-Bi can be electroplated with fine and rounded grains in bath containing the additive. The Sn content in the deposit was increased according to a parabolic shaped curve with increasing Sn content in the bath. the preferred deposition trend of Sn was stronger than that of Bi, because the deposit potential of Sn was more positive than that of Bi in the bath with the additive. The Sn content in the deposits was decreased with increasing the current density and the grains of deposit became finer because of the increased cathodic overpotential. The grains of deposits became finer with increasing bath temperature and agitation because of the decreased viscosity of the bath solution.
Large Bi phases were found on the surface of the eutectic Sn-Bi solder bumps except quenched bumps. Increasing cooling rate in the reflow process, large Bi phases were decreased on the surface of the solder bumps. Any large Bi phases were found in the cross-section of the bumps with all kinds of cooling rate, 2.5 ℃/min, 10℃/min, 17℃/min, quenching. Bumps from electroplating method had no faceted Bi phases and no Bi precipitates in the Sn phase. They had more uniform structure than bumps from evaporation method,
The degree of sphere was decreased with decreased the bumps size and solder bumps size had nothing with the microstructure of Sn-Bi solder bumps. The shear strength of Sn-Bi bumps increased with increased cooling rate because of the grain of solder bumps became finer with increasing cooling rate.
