AuAg Alloy wire bonding process capability and reliability assessment

Discrete products are greatly driven by the never-ending pressure to reduce costs. 20mum AuAg wire was evaluated with full reliability assessment to replace existing 22mum wire diameter 4N gold wire as the direct drop in interconnect material for better packaging cost position. AuAg alloy wire is ~ 20% to 30% harder than 4N gold thus needs greater ultrasonic power and bond force to bond it onto aluminum bond pads as such its has a risk of bond pad cratering due to harder wire properties in nature despite the benefits of lower cost ~ 20% lower than 4N Au with the similar wire diameter. Since AuAg alloy wire is harder than Au wire it is robust to mold wire sweep despite robust for sagging wire. DOE were run to bond Alloy wire Au-15%Ag and Au-16%Ag with an older generation bonder like Shinkawa ACB25 series, output responses are pad cratering, non-stick on pad, lifted bond, broken wire, ball shear test, wire pull test, ball thickness, ball diameter, % IMC underneath the ball bond, cross section for IMC thickness, voids and yield were monitored. All the output responses result were comparable to 4N Au wire except the IMC growth rate is slower in the AuAg-Al interface compare to Au-Al interface, from the cross section it was found that no Kerkindall voiding between AuAg alloy interface IMC were observed and its passed HTS 1000times @ 150degC but fail PCT @ 96hrs. AuAg alloy wire was found robust to thermal and mechanical stresses but it has one major weakness to humidity. Two types of hypothesis were raised to explain the root cause of this PCT failure