Novel coated silver (Ag) bonding wire: Bondability and reliability

For the last few years there is a demand in wire bonding industry for an alternate wire to gold as well to bond similar to gold without purging inert or reactive gas during free air ball (FAB) formation. Currently, alloyed/coated copper and alloyed silver wires are in use as a substitute for gold wires, but need purging of forming gas to form FAB. Whereas, the novel coated silver wire can form spherical FAB under atmosphere similar to gold wire. A feasible FAB process window is observed with good distribution of coating element on the FAB periphery when formed under atmosphere. The soft nature of the silver FAB is an advantage for easier deformation and bonding with lower ultrasonic energy (USG) and compressive force compared to copper base FAB. Design of wire core and coated layer revealed substantial benefits on stitch (2
^nd
) bond process window as wide as gold wire. Interestingly, for both ball and stitch bond, the process parameters of coated silver wire is similar to gold wire as well performing better than copper base wires. The paper discusses on the benefit of the coated silver wire with low electrical resistivity equal to gold wire. As expected, the reliability (thermal ageing (HTS), uHAST)) of the coated silver ball bond depends on the interface intermetallic coverage (IMC) at time zero. Usually, 75 to 90 % IMC uniformly spread in the center and periphery of the ball bond are practiced. Another key observation is the mean time between assist (MTBA) of the coated silver wire meets the minimum industrial requirement of 7h bonding without any stoppage.