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106 SMT007 MAGAZINE I JULY 2019 are made on PCB bonding pads as well as on the die bonding pads—unlike the standard gold ball bonding process shown in Figures 12 and 13 [2] . For the standard gold ball bonding pro- cess, ball bonds are expected on die bonding pads, and stitch bonds are expected on PCB bonding pads. For the standard gold ball bond- ing process, the wire-bonding tool is directly in contact with PCB bonding pads during the stitch bonding process. One can see the circu- lar tool mark on the bonding pad in Figure 13. For the SSB process, the bonding tool is not directly in contact with PCB bonding pads but sits on top of ball bonds on PCB bonding pads. The height (or thickness) and the diameter of ball bonds made using the SSB process on PCB bonding pads are around 13 mm and 63 mm for Product I, and 15 mm and 62 mm for Product II. The dimension of ball bonds on PCB bond - ing pads would be considered a critical factor for a wire-bonding experiment on nodules and scratches. The height of ball bonds directly indi- cates the height of the bonding tool from the bonding pad at the time of wire bonding since the bonding tool sits on the ball bond. In the- ory, ball bonds might not be squashed as much as programmed if the wire-bonding tool touches a tall nodule before the ball bond is squashed as much as programmed. Therefore, it is likely that the acceptable nodule height might be related to the height of the bonding tool from the bond - ing surface at the time of wire bonding. In addi- tion, ball bonding on an uneven surface due to nodules would be a factor for the integrity of wire bonds, and the height (or thickness) of ball bonds would be the amount of buffer that can conform with the shape of the nod - ules. In this aspect, the SSB process compared to the standard ball bonding process would be much more beneficial on wire bonding on nod- ules. The diameter of ball bonds is also directly related to the wire-bond contact area on PCB bonding pads. Therefore, when wire bonds are made on surface imperfections such as nodules and scratches, the percentage of the wire-bond area made intact on the bonding pad without any surface imperfections is determined by the ball bond diameter (i.e., area). The integrity of wire bonds was measured by wire pull tests before and after reliability test- ing. Wire bonds were made directly on nod- ules and scratches even though the alignment of wire bonds on nodules and scratches var- ied slightly so that some bonds were aligned slightly better than the others. There were only a couple of PCBs rejected by the manufacturer due to scratches, and therefore, there were not enough scratches with varying widths to make wire bonds to determine the maximum allow- able scratch width. For this reason, scratches with various widths were artificially created on PCB bonding pads by using a sharp knife blade for PCB Product I. However, the depth of artificially created scratches, as well as that Figure 12: Bonds on die for standard ball bonding process. Figure 13: Bonds on PCB for standard ball bonding process.