Free air ball and bondability behaviours of Au-coated Ag alloy wire at various electro flame-off and bonding parameters for CMOS iBGA

Abstract

Au-coated Ag Alloys (ACA) have emerged as a new bonding wire in semiconductor industries with lower costs for CMOS iBGA applications. However, a wide gap of study, especially in terms of free air ball (FAB) formation, ball shear, pull test and intermetallic compound (IMC) phase, has left these areas unclear. This paper extensively studied FAB and morphology behaviours for 4N Au (AA) and ACA wire bonds at different EFO currents and EFO times. The bondability performance, ball shear, and wire pull tests were also conducted at the optimum setting of the EFO process and various bonding parameters. The results showed that the EFO current of 45 mA and EFO time of 0.37 ms (#ACA5) provided the best FAB size and morphology formed as predicted by the JMP Prediction profiler with maximum shear force and pulled test up to 0.34 N and 0.20 N at S1 and S2 Force of 0.74 N and 0.15 N, respectively as well as S2 Power of 60 mA and 70 mA. The IMC formed is recognised as Ag–Al (Ag₂Al) and not Au-Al phase since the Au layer coated onto the Ag alloys is 2.5 ± 0.05 to 5 ± 0.07 μm only. The Vickers hardness obtained is almost similar to AA wires that descended the ball. No obvious ripples formed, unlike AA wires, which signified that the ACA wires may reach a good bondability as conventional wires.