Fluxless non-eutectic joints fabricated using gold-tin multilayer composite

Fluxless bonding processes using two different noneutectic gold-tin (Au-Sn) multilayer composites to fabricate high quality solder joints have been successfully achieved. In contrast to the well-known eutectic solders of 80 wt. % Au and 20 wt. % Sn commonly selected by the packaging industry, we have adopted a substantially cost-effective strategy by purposely designing and constructing our solder joints to be predominantly tin-rich instead, rather than relying extensively on gold as a major ingredient. In research two designs were implemented; one with compositions of 80 at. % Sn (70.54 wt. % Sn) and 20 at. % Au (29.46 wt. % Au), and another with 95 at. % Sn (91.82 wt. % Sn) and 5 at. % Au (8.18 wt. % Au). The bonding process temperatures chosen for constructing 80Sn-20Au and 95Sn-5Au joints are 285°C and 225°C, respectively. Once produced, both types of joints were examined using the combination of X-ray microfocus imaging and scanning acoustic microscopy (SAM) to confirm the bonding quality and the results obtained are nearly void-free. To study the microstructure and composition of the samples the scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) detector were conducted on the joint cross sections and the solder thickness of 3.9 and 2.1 μm were uniformly identified throughout from 80Sn-20Au and 95Sn-5Au joints, respectively. Furthermore, the EDX data obtained have consistently shown that a mixture of AuSn, AuSn₂, and AuSn₄ intermetallics were spotted from the 80Sn-20Au sample joints, while AuSn₂ and AuSn₄ embedded in β-Sn matrix were discovered from 95Sn-5Au specimens. In addition, the shear tests conducted on the samples unequivocally suggest the shear strength of each joint measured is actually greater than the die itself. Finally, the re-melting temperatures of 80Sn-20Au and 95Sn-5Au solder specimens ranging from 275 to 281°C and 214 to 220°C, respectively, were also experimentally determined.