Role of impact ultrasound on bond strength and Al pad splash in Cu wire bonding

Cu wire is replacing Au wire in the microelectronic industry due to its lower cost. However, during Cu ball bonding one of the main challenges is the increased stress that can damage the pad and underpad layers. Past work showed that using ultrasound superimposed together with impact force (pre-ultrasound) results not only in a softer bonded ball, but also in a flatter ball/pad interface. In this study, Cu ball bonding processes are optimized with five levels of pre-ultrasound. The wire material is 99.99% pure Cu wire, 25.4 lm in diameter. It is shown that by using pre-ultrasound of 37.5% bonds with adequately high shear strength (120 MPa) are achieved and the amount of splash is reduced by 31%. Using pre-ultrasound allows for lower bonding ultrasound levels that result in less stress on the pad and underpad materials. Thermosonic Au ball bonding is the main method for making interconnections between the semiconductor chip and the package in microelectronics packaging [1–3]. In this process, a thin metal wire loop is welded to a metallic bond pad, by a combination of heat, normal pressure and ultrasonic energy. With the increasing trend of the price of Au, less expensive materials such as Cu are being considered for wire bonding as an alternative to the Au wire [4]. In addition to the lower cost, Cu possesses better electrical, thermal and mechanical properties than Au. However, using Cu wire has disadvantages [5–8]. Among them are the increased tendency to oxidize, narrow process window for second bond, and increased bonding stress transferred to underpad materials [1,9,10]. Because of the higher hardness of Cu wire, higher levels of bonding force and ultrasound are normally required to make good quality bonds compared to processes with Au wire which is softer. The higher levels cause an increase in the underpad stresses, which might consequently lead to pad/chip damage. The damage can occur as pad peeling [11,12], silicon cratering [13–15], and pad metal splash [16–18]. Pad metal splash is the pad metal extruded from the edges of the bonded ball. It is accompanied by pad thinning , and occurs mostly in the ultrasonic direction. The amount of splash increases with higher bond force and ultrasonic power. It is not a failure by itself but can reduce bond reliability because of pad material thinning. Pad metal splash is of particular concern in today's ultra fine pitch applications where it may cause shorting …