Comparison of Au/Al, Cu/Al and Ag/Al in wirebonding assembly and IMC growth behavior

Currently, semiconductor packaging is using Gold (Au) wirebonding as the main connection from chip to the lead frame or substrate. Increases in Au price, causes suppliers to look at another alternative wire alloy as such Copper (Cu) wire. Cu wire is another option rather than Au wire due to Cu has higher tensile strength, better elongation and electrical and thermal conductivities. Cu wire has outstanding ball neck strength after the ball formation process [1], high-loop stability and high stiffness, which results in lower wire sweeping during encapsulation. These will lead to Cu wire able to have longer, lower loop profiles [2] and minimize wire sagging for fine and ultra-fine pitch wirebonding application. Another proposed alloy for wire bonding is Silver (Ag) wire. Ag has the best electrical and thermal conductivities compare to the three kinds of wires material. Ag wire has low Young's modulus compare to Cu and it is another option for replacing Au wire in wirebonding. In this paper, we shall discuss the three types of alloy wire in term of comparing the bonding quality. These will include the disadvantages and advantages of individual by comparing ball shear measurement. The silicon chip surface after bonding will be checked to confirm any crack observed on the bond pad surface occurred after ball shear for each type of wire. IMC growth for the three types of wire will be carried out at 175°C for 0hr to 504hrs will be discuss and the bond interface will be observed using the high power optical microscopy and scanning electron microscopy (SEM).