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JULY 2019 I SMT007 MAGAZINE 103 PCBs rejected due to nodules were from two different PCB products (i.e., two different PCB designs with different part numbers, called Product I and Product II hereafter) while PCBs rejected due to scratches were from only one of products—Product I. Product I had three dif- ferent types of nodules that could be formed by different mechanisms or at different PCB manufacturing process steps based on the appearance of the nodules while Product II had only one type of nodule. Nodules observed from Product I and Product II were arbitrarily divided into three types based on the appear- ance in this study, and designated as Type A, B, and C. In this study, the effect of nodules on the wire-bonding process was only focused on isolated nodules or small nodules formed along a small mechanical defect so that a wire bond was only interrupted by a single nodule rather than by multiple nodules. The PCB sam- ples rejected by the manufacturer due to nod- ules and scratches are summarized in Table 1. The three different types of nodules observed from Product I are shown in Figures 4–6. Type A nodules, as shown in Figure 4, were most commonly observed. Type A nodules are rela - tively circular in shape when it is viewed from the top and has a grain structure on the nodule surface. The cross-sectioned view in Figure 3 appears to be this type of nodule. The diameter of nodules is mostly in the range of 30–50 mm. The 3D optical microscope image is on the left, and the matching 3D profile view with nodule height is on the right of the figure. Type B nod - ules, as shown in Figure 5, are not circular (or not symmetrical) in shape, and the surface of the nodule looks very smooth without any grain structure. Type C nodules, as shown in Figure 6, are formed along with any mechanical defect such as a nick. Any protrusions on the surface in the plating steps seems to turn into a nodule. An example of a Type A nodule observed from Product II is shown in Figure 7. Only Type A nodules were observed from Product II. Table 1: PCBs rejected due to nodules and scratches. Figure 4: Example of Type A nodule observed from Product I. Figure 5: Example of Type B nodule observed from Product I. Figure 6: Example of Type C nodule observed from Product I. Figure 7: Example of Type A nodule observed from Product II.