The optimization of Free Air Ball for Ag alloy wire bonding (IMPACT 2014)

For cost reduction, assembly factory has intention to use cheaper wire material in wire bonding process. As a result, currently Cu wire with many efforts has become one popular wire material instead of Au wire. However, some customers may hesitate to use Cu wire due to it is much harder than Au wire. For this impact, silver (Ag) alloy wire becomes another selection in marketing. Ag alloy wire product's performance can be comparable to Au or Cu wire and it has compatible price as Pd coated Cu wire. For the incentive, here the evaluation is conducted to develop these techniques on how to make Ag alloy wire bonding well. In one wire bonding cycle, first step is sparking to make round FAB (Free Air Ball), then wire bonding tool (capillary) holds FAB to do ball bonding to joint Al bondpad on die, then make looping formation per wiring layout and later to perform stitch/ tail bonding to connect wire to finger of leadframe or pad of substrate and finally to break the wire for next FAB sparking. Round and no oxidized FAB are crucial important for ball bonding quality for Ag alloy wire as the same as for Cu wire. The same Cu wire kit with inserted protective forming gas (N2+H2 mixed) is adopted with wire bonder for Ag alloy wire bonding evaluation. This paper focuses on sparking (or called EFO, Electric Flame-Off) step/process because there are 2 issues observed in the beginning of our trials which are based on recommend sparking settings from the supplier. First issue is off-centered FAB and another is rather large variation in as-bonded ball thickness. Till now there is no reference paper available to address FAB optimization for Ag wire via EFO sparking settings as well as protecting gas flow rate in detail. Here, we arrange/ perform the study in order to overcome above 2 issues as well as considered this optimization of wire sparking setting as first wire bond development step for Ag alloy wire. In this paper, key sparking settings (EFO current, gap and tail extension, etc.) including forming gas (N2H2) flow rate are examined through DOE (Design of Experiment) to verify if they can help overcome off-centered FAB and get better precision of as-bonded ball thickness. Positive result is gained and shown in this report for no serious off-center FAB, irregular ball size observed and better as-bonded thickness variation came out. In the end, the sparking parameter range/window is defined per sparking settings which can be implemented for further mass production. In general, this study documented the related working experiences and key sparking parameters about how to make a good FAB (or bonded ball shape) for Ag alloy wire.