(A) study on interfacial reactions of Pd added ultra-fine Au bonding wires and Al pads

Abstract

Au wire-bonding have been playing dominant role as interconnection in electronic packaging area due to its high reliability and convenience. However, as the dimensions of packages decrease and material cost of Au abruptly increases, use of ultra-fine pitch wires become inevitable. Ultra-fine pitch wires, less than 0.7 mil ($17.8 \um$) show lower reliability compared to fine pitch Au wires with diameter of 1.0 mil ($25.4 \mum$). In order to enhance poor reliability of ultra-fine pitch Au wire, studies on new Au wires and their interfacial reactions are needed. Therefore, interfacial reactions of ultra-fine pitch Au wires with diameter of 0.5 mil ($12 \mum$) were analyzed, and compared with fine pitch Au wires with diameter of 1.0 mil. Pd was added as alloying element on 0.5 mil Au wires to enhance interfacial reactions, and mechanisms of the reliability enhancement by Pd addition were investigated. Pd has been known to reduce Au/Al diffusion rate by forming diffusion barrier at the interface. For the analysis and the comparison of the interfacial reactions, both 1.0 mil and 0.5 mil Au wires were bonded on Al pad, and subsequently aged under high temperature storage test (HTST) at $175 \degC$ up to 1000 hrs in air-ambient condition. 1.0 mil Au wires showed high inter-metallic coverage about 60% between bonded Au wires and Al pads, whereas 0.5 mil Au wires showed low inter-metallic coverage about 40%. Due to difference in inter-metallic coverage 1.0 mil Au wires formed regular inter-metallic compounds (IMCs) of $Au_8Al_3$ over entire bonded area; however, for 0.5 mil Au wires irregular IMCs of $Au_8Al_3$ grew only on small bonded areas due to lower inter-metallic coverage. At 200 hours of aging, for both of the samples Au4Al layer started to form above $Au_8Al_3$ IMCs. Abrupt increase in ball-lift during ball pull test (BPT) and oxidation of $Au_4Al$ layer were observed at 500 hours of aging for 0.5 mil Au wires, whereas 750 hours of aging made observable oxidat...