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JULY 2019 I SMT007 MAGAZINE 99 average yield loss per month for wire-bond- ing boards due to nodules and scratches. The average yield losses from multiple PCB prod- ucts due to nodules and scratches for wire bonding boards were in the range of 4–41% and 2–30%, respectively, during the period. It is worth noting that the data in Figure 1 may not necessarily provide the picture of actual average yield loss due to nodules and scratches. When any defect—either nodule or scratch—was first found during an inspec - tion of a board, the reason for rejection was recorded and further inspection of that board was not pursued since the board was already determined to be a reject. For example, the manufacturer recorded a 39% yield loss for nodules and none for scratches in Period 3. However, it may not mean that there were o scratches on those rejected boards since those boards had not been inspected fur- ther for scratches after nodules were first observed. Figure 2 shows the maximum yield loss for nodules and scratches. The maximum yield loss was recorded from one of the PCB prod- ucts that showed the lowest yield at the per- iod or in the month. Unacceptable yield loss over the period of many months initiated a dis- cussion with the manufacturer to identify the root cause and conduct corrective actions to improve on nodules and scratches. Corrective Actions at the Manufacturer The PCB Manufacturing Process One of the PCB manufacturers who had experience producing bare boards had con- siderable data that suggested that nodules and scratches were likely the main reasons for yield loss. The specification initially defined by the user required inspection at a 20X start- ing magnification, which was beyond the IPC specification of 1.75X magnification. Since there were inspection escapes, the manufac- turer decided to start at 40X magnification as a preventative measure. This, combined with the criteria for absolutely no nodules or any surface imperfection, resulted in many rejected boards that may have actually been wire-bond- able and reliable. The manufacturer initiated root cause analysis to determine the source of the nodules and scratches to define an action plan to reduce or eliminate the defects. Nodules Nodules are surface imperfections on the plated surface that can originate and grow in the copper plating process or originate from processes preceding copper plating (i.e., elec- troless copper) and grow in the copper plating process. Electroless copper plating is used as a metallization process that deposits a thin layer of copper onto non-conductive surfaces, such Figure 2: Maximum yield loss from a PCB product with the lowest yield.