Study of intermetallic compound growth and failure mechanisms in long term reliability of silver bonding wire

Silver wires have become a novel bonding material in recent years. But users & field engineers are still divided over the issue of reliability performance including failure mechanism and intermetallic compounds (IMCs) formation. In this study, new type of high purity silver wire with 96Ag-3Pd-1Au alloy (96% Ag) is introduced, and the bonding properties on Al bond pad are evaluated through bondability & reliability test. Reliability tests for wire characterization are high-temperature-storage lifetime test (HTST) and unbiased highly-accelerated stress test (uHAST) with temperature and humidity. Two types of mold compounds are used which have different Cl- ion content. Below 10 ppm of Cl- ion for the green compound and 27 ppm for the normal one. Bondability, IMC formation(Ag₂Al, Ag₃Al) & growth rate are measured for both HTST150´C & 175´C for 2000h and the possible failure mechanism is defined based on the micro-structural characterization from uHAST in which the repetitive oxidation and reduction reactions occur due to the galvanic reaction and the Cl- ions with sufficient moisture and thermal energy, and the reduction reaction of Ag-Al IMCs and Al pad causes to form the micro-crack failure.