Room temperature interfacial reactions in electrodeposited Au/Sn couples

Two Au/Sn couples electrodeposited on metallized Si wafers, a Au/Sn couple (Au deposited first) and a Sn/Au couple (Sn deposited first), were employed to study the room temperature interfacial reactions between Au and Sn. The electroplating sequence has a significant effect on the phases and the microstructures of the reaction regions. AuSn4 and AuSn are formed in the as-deposited Au/Sn couples, while Au5Sn and AuSn are formed in the as-deposited Sn/Au couples. Upon ageing, AuSn2 formation depends on the Sn (Au) content (i.e. the Sn (Au) layer thickness) in the Au/Sn (Sn/Au) couples. In aged Au/Sn couples, with 1.5 μm of Au and 3 μm of Sn, AuSn, AuSn2 and AuSn4 are sequentially distributed in the reaction region with fine Kirkendall voids at the AuSn/Au interface. In the aged Sn/Au couples, with 15 μm of Sn and 1.5 μm of Au, AuSn is distributed on either side of the original Sn/Au interface, while Au5Sn only exists on the Au side. The presence of a thin interfacial SnO2 film significantly affects phase formation and reaction kinetics.