Fluxless Sn-Bi-Au bonding process using multilayer design

For many years lead-free solders have been a topic of interest in the packaging industry. One possible substitute material among the lead-free solders is eutectic Sn-Bi alloy (42Sn-58Bi) which has a melting temperature of 139°C. Earlier studies have shown that the conventional 63Sn-37Pb solder can be replaced by eutectic Sn-Bi in some low temperature applications. A fluxless alternative to the conventional soldering method is successfully developed using Sn-Bi-Au multilayer composite. The essence of this fluxless bonding technique comes from the spontaneous in situ formation of a thin AuSn₂ layer on the outer surface of the multilayer composite. This stable layer acts as a natural barrier against oxidation of the inner solder metals. The Sn-Bi-Au multilayer for bonding is prepared using a thermal evaporating vacuum chamber. The bonding temperature of this process ranges between 170°C and 180°C. The resulting as-solidified joint is made up of eutectic Sn-Bi alloy with small grains of AuSn₂ imbedded inside the solder. Sn-Bi joint cross-sections are examined to determine the quality of the joints. The process is found to be effective. SEM and EDX systems are used to examine the joint cross-section. The SEM images shows a uniform joint thickness of approximately 5 μm. They also show the joint microstructure, exhibiting the usual lamellar structure of eutectic Sn-Bi alloy. EDX analysis also indicates the separation of the darker Sn-rich regions and the brighter Bi-rich regions. The fluxless feature, lead-free nature and processing temperature range of this bonding technique are particularly valuable for packaging photonic, optoelectronic, fiber optic, and MEMS devices and components