Mechanical properties of electroless Ni/Au wire bonded to an Al pad with the effects of chemical activation

Abstract

The mechanical properties of electroless nickel (EN) deposited onto pure gold (Au) wire used for the semiconductor industry have been investigated with the effects of the chemical activation, using the electrochemical quartz crystal microbalance (EQCM) technique, tensile, torsion and ball shear (BS) tests, and optical microscopy (OM). The combined results of in situ electrogravimetric curves with ex situ cross-sectional views obtained from the EN/Au composite wire clearly showed that EN deposits with the presence of pre-immersion Au were more rapidly formed on Au wire compared to the absence of the pre-immersion Au. This is presumably due to the surface roughness increased by the hydrogen evolution as validated from OM. The load–displacement curves for the pre-immersion Au-coated and uncoated Au wires prior to EN revealed that the elastic modulus value of the EN outer layer, E Ni, calculated on the basis of a linear elastic deformation model, was higher for the former than the latter. Moreover, it was found from the torsion and BS tests that the shear modulus of the EN, G Ni, and the shear strength of the composite wire bonded to the aluminum (Al) pad with the zincate pretreatment were greater than for wires without this pretreatment; the values increased with increasing zincate immersion time. Consequently, it can be stated that the tensile strength of the EN/Au composite wire and the bonding strength between the wire and the Al pad are effectively increased by the existence of the pre-immersion Au layer and the zincate pretreatment, respectively.