The bondability and reliability of a ternary Ag alloy wire on an Al bonding pad under N 2 -free conditions

Abstract

Generally, high-purity 99.99% Au wires are mainly used for electrical connection to transmit and receive data between an Al bonding pad and a printed circuit board (PCB) substrate in semiconductor devices. However, there have been several studies to reduce the cost of expensive Au wires due to the sharp increase in the price of Au. Thus, Ag and Cu alloy wires have been mainly considered as alternative materials. This paper focused on a ternary Ag alloy wire (Au: 1.5%, Pd: 2.5%) and explained free air ball (FAB) shape effects with various electronic flame-off (EFO) conditions in more detail and experimentally showed the susceptibility to bonding failure of the Ag alloy wire after exposure to the atmosphere at room temperature. In particular, a feasible Ag wire bonding process and grain structure effects under N 2 -free conditions were studied in depth for the first time. With this N 2 -free condition, weak surface tension of a melting wire led to move FAB quickly to the interface between FAB and heat affected zone (HAZ). In addition, a slender grain structure in the center of FAB area still remained because of the short time current stressing. It made good conductivity and no significant resistance change of a Ag alloy wire. For a Ag alloy wire exposed to the atmosphere for 19 days, the defective percentage during bonding process suddenly started to rise, and reached more than 0.7% in 24 days. According to our results, less than 18 days are highly recommended as a control standard of a Ag alloy wire when it is exposed to the atmosphere. The bonding between a Ag alloy wire and an Al bonding pad was formed without any crack in the lower part of an Al bonding pad. Higher intermetallic compound coverage and minimum Al splash were chosen as key bonding parameters, and KOH cratering tests were performed to secure good bonding quality. Finally, robust Ag wire reliability has been successfully achieved and demonstrated.