Effect of Pd addition on ultra-fine pitch Au wire/Al pad interface

Due to increasing cost of gold and technological demand for finer pitch application, Au bonding wires with finer diameter are needed. In such demands, Au bonding wire manufacturing companies produced ultra-fine Au wires with diameter of 12 μm (0.5 mil). However, interfacial reactions between ultra-fine Au wires and Al pads may cause serious reliability problems as wire diameter get smaller. Decrease in bond reliabilities may be caused by interfacial failures such as Kirkendall voids and oxidation of Au4Al IMCs. For the application of ultra-fine Au wires in real products, studies on interfacial reactions of ultra-fine Au wires/Al pads and effects of Pd addition on Au wire/Al pad interface which is previously proven to be enhancing bond reliabilities are necessary. In this study, interfacial reactions and failures of two types of 4N ultra-fine Au wires(12 μm)/Al pads with different bonding types were analyzed, and effects of Pd addition on interfacial reactions and failures are also investigated by using 3000, 6000, and 10000 ppm of Pd added Au wires. High temperature storage test (HTST) at 175D under air-ambient was performed to simulate an accelerated thermal exposure characteristic of device operation conditions. Intermetallic compound (IMC) phases and Pd behavior at the Au wire/Al pad interface were identified using various analytical tools. Ball pull test (BPT) was conducted to evaluate bond reliabilities depending on bonding types, wire compositions, and aging times. According to experimental results, two types of ultra-fine Au wires/Al pads showed different bonding reliabilities depending on initial inter-metallic coverage. It was found that initial intermetallic coverage determined the interface bond reliability because interfacial oxidation problem depended on initial intermetallic coverage. Addition of Pd on Au wires/Al pads significantly affected bond reliabilities depending on Pd contents. 3000 ppm of Pd added Au wires/Al pads re- - sulted the best bond reliabilities and strongest resistance against the interfacial oxidation due to the Pd accumulation at Au4Al grain boundaries. However, 10000 ppm of Pd added Au wires/Al pads showed low bond reliabilities because separated Pd-rich island grains formed among Au4Al grains resulting in less protection of Au4Al from oxidation.