The Bonding Performance of Au-coated Ag Alloy wires (IMPACT 2019)

Gold wires have been widely used as bonding materials in the packages of integrated circuit and semiconductor discrete devices due to their high chemical stability and electrical conductivity. However, the high price of gold restricts its application, which triggered extensive study on seeking new bonding materials to substitute gold. Silver has been considered for use in wire bonding because of its best electrical and thermal conductivity among all elements. However, certain drawbacks of pure silver such as insufficient tensile strength, unsatisfactory oxidation resistance, low bonding reliability, prohibit the application of pure Ag wire in electronic packages. Herein, Au-plated Ag-Au alloy (99%Ag-1%Au) wires with diameter of 25 μm and 95 nm-thick plating layers were fabricated by cyanide-free Au plating technique and precise drawing process. Bonding process for the Au-plated Ag-Au alloy wires was conducted by wire bonder, and the microstructure and performances were investigated by scanning electron microscope, wire pull test and ball shear test, etc. The results show that the diameter of the free air ball (FAB) was twice of that for wire, and the FAB possessed regular spherical morphology with good symmetry, no deviation or golf ball phenomenon could be observed. What's more, uniform coating element distribution along the periphery of FAB could be obtained after carefully adjusting the electronic flame off (EFO) parameters. The length of heat affected zone (HAZ) is 50 μm, and the grain coarsening of HAZ was slight. The bonding arc height could be as low as 120.7 μm. The Au-plated Ag-Au alloy wire not only exhibited high pull strength (15.2 ~16.4 g) and ball shear strength (38.4 ~ 45 g), but also show a high wire-bonding reliability, the failure rate was only 0.5% after 300 thermal shock cycles (-40 - 120°C). Based on the above results, this work show that the Au-plated Ag-Au alloy with high bonding reliability ascribe to the firm bonding between the wire and the Al pad meets the requirements of packages, and has considerable market prospect.