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102 SMT007 MAGAZINE I JULY 2019 the new tank. This test was repeated a couple more times with the same results. As an interim action, all wire-bond technology boards were plated in the new tank, which limited the num - ber of nodules to a consistent value for each lot. Subsequently, the remaining two plating tanks underwent the annual preventative main- tenance procedure, and all tanks were able to plate wire-bond boards consistently. Although the process improvements signif- icantly reduced the number of nodules and made the process more consistent, a percent- age of boards were still being rejected at final inspection for small nodules. It was important to understand the impact that these small nod- ules have on wire bonding—in particular, at which size does a nodule begin to negatively impact the wire-bonding process. Scratches Scratches refer to a surface imperfection with any depth or a discernible non-uniform contrasting marking. Scratches can originate over the base copper, over the plated copper, or over the final finish plating. When scratches are over the base copper, the plated copper is capable of filling in minor scratches to a certain degree, but significant depths will have plated copper conforming to the surface topography. Likewise, the final fin- ish—which, in this case, is typically ENIG or ENEPIG—also conforms to the surface topog- raphy. Although there is a depth at the scratch, the conductive feature is fully plated with a solderable and wire-bondable surface finish. This is the same situation when scratches are over the plated copper. When scratches are over the final finish, the results can vary depending on the magnitude of the scratch. Deep scratches could potentially break through the final finish and expose cop- per, which would oxidize and would not be sol- derable or wire-bondable at that location. Sur- face scratches could expose the nickel, which would also oxidize rapidly and cause solder- ability and wire-bonding challenges. Light scratches that only abrade the gold, but do not remove the gold entirely, would still function as a normal solderable and wire-bondable fea ture. In general, it was found that the majority of boards being rejected for scratches were being scratched after the final finish had been applied. An audit of the process flow of the panels after the final finish was conducted, which discovered that the scratches were occurring after depanelization of the boards in the routing process. After routing, the boards are washed off and stacked in trays in prepara- tion for electrical testing. Boards were making direct contact with each other resulting in light scratches and surface scratches. Corrective action was taken to produce foam trays with slots for individual boards so that the boards would not make contact with each other after depanelization. After the cor- rective action was implemented, the number of scratches on the boards was significantly reduced—but there was still a small number of boards being rejected for minor scratches. As was the case with nodules, it was appar- ent that scratches needed to be characterized to determine their effect on the wire-bonding process. Internal Corrective Actions Wire Bonding on Nodules and Scratches Despite significantly reducing the occur- rences of nodules and scratches, they were not completely eliminated. Even if there was only one small nodule or scratch on any wire- bonding pad, the board was still considered a reject, and therefore, the target yield remained unachievable. This was the reason for fur- ther investigation on the effect of nodules and scratches on the wire-bonding process. The type of plating applied to PCBs used for this study was electroless nickel/electro- less palladium/immersion gold (ENEPIG) with plating thickness specifications as follows; 3.0–6.0-mm Ni, 0.15–0.3-mm Pd, and 0.03– 0.06-mm Au. PCBs rejected due to nodules and scratches were received from the PCB manu- facturer. Nodules and scratches on rejected PCBs were inspected under a high magnifi- cation optical microscope with image analy- sis software that was capable of creating 3D images and profiles. The height of the nodules was measured on 3D profile images.