Interfacial reaction and thermal ageing of ball and stitch bonds

In wire bonding, reliability of ball and stitch bonds is the upmost challenge. Thermal ageing (high temperature storage) is the first reliability test conducted to understand interfacial reaction, either on the growth of intermetallic phases between dissimilar metal wire bonds, or on the integrity of diffusion bond interfaces without intermetallic phase formation. The present paper discusses bonding silver (Ag), copper (Cu) and gold (Au) base wires on different plated surfaces of Ag, Au, palladium (Pd) and aluminum (Al) metallization and on thermal ageing at 175°C until 1000 hours. Thus, explaining the diffusion based binary reaction at the ball and stitch bond interfaces. Bonding similar metals of 4N Au wire to Au plated surface (Au-Au) exhibited excellent integrity and interface bond strength without any degradation. The behavior of Ag-Ag bond is alike to Au-Au. On thermal ageing Ag, Cu and Au base wire bonds to Ag and Pd plated surfaces; ball shear, ball/stitch pull revealed stable bond without degradation. Unlike Au ball bonds, Cu or Ag ball bonds showed absence of Kirkendall type interfacial voids when bonded to Al-0.5wt% Cu metallization. Thermal ageing of Cu wire bonds under atmosphere of the unmolded device showed wire surface oxidation. Surprisingly, bonding dissimilar metals of 4N Cu, 2NCu, CuPd and Ag to Au plated surfaces revealed inferior bond integrity and predominant mass diffusion of Cu and moderate diffusion of Ag from wire on to the Au plated surfaces.