Bondability and reliability of Ag Alloy wire (92 and 95% Ag Alloy) on thin aluminum bonding pad

Abstract

There are several types of Ag Alloy wires being introduced for discrete IC manufacturing, a low (92%) and high purity level (95%) Ag Alloy wires are the most common one. The IC industries have been shifting to this type of wires due mainly to its lower cost and softer property as compared to gold and copper wire. Copper wire even with the improvements done such as alloying the material with palladium for improved reliability has limitations of usage for very thin top metallizations. At most 60-70% conversions have only been implemented to the usage of copper wire because of this main obstacle for BOAC (Bond Over Active Circuit) and devices with thin top metallization of at least 2um only are difficult to convert to copper wirebonding. Due to its relative hardness, higher parameter settings are required to make IMC (Intermetallic), and thin bonding pad metallization of AlSi or AlCu cannot withstand the compressive effect of this wire. This is specially observed for BSOB (Bond Stitch on Ball) wirebonding, where aside from the ball bonding, a stitch bond is placed on top of this ball bond. The impact coming from this additional bonding is adding severe compressive effect, for copper wires (usually Pd coated copper wire is use for BSOB) require high bond parameter to create an alloy effect or interconnection of hard copper ball and the bonding pad.