A fluxless Au-Sn process achieved in air ambient
Materials Science and Engineering
University of California, Irvine, CA 92697-2625
A new alternative fluxless bonding process conducted in ambient air is reported based on two different Au-Sn multilayer composite designs that are substantially tin-rich; namely, one with compositions of 80 at. % Sn and 20 at. % Au, and another with 95 at. % Sn and 5 at. % Au. We believe that this is the first time that Au-Sn bonding is achieved in air without the use of flux. The bonding process temperatures chosen for constructing 80Sn-20Au and 95Sn-5Au joints are 285oC and 225oC, respectively. Once produced, both types of joints were examined using the 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 across the 95Sn-5Au joint cross-section and a thickness of 3.78mm was uniformly identified throughout the joint. Furthermore, the EDX data obtained have consistently shown that many AuSn2 grains were identified and found to be embedded in a matrix with a mixture of AuSn2 and AuSn4, as obtained 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 281oC and 214 to 220oC, respectively, were also experimentally determined.