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54 PCB007 MAGAZINE I JULY 2019 transport limitations. The inability to move fresh plating electrolyte into blind vias will re- sult in thin plating near the junction of the side wall and capture pad. Thin plating thickness presents an area of weakness that can lead to separation or cracking within the copper. In addition, laser via formation of blind vias can lead to another issue shown in Figure 3. The section shown in the image points to an area where there are voids in the blind via. It is recommended that both front light (FL) and back light (BL) testing be included in the fabricator's routine quality control protocol. The FL technique is particularly useful because this test helps to determine the location of the void - ing. Figure 4 shows an example without voids. Further analysis of the nonconformance found in Figure 3 is shown in Figure 5. The gap shown in Figure 5 exacerbates the difficulty in getting plating into this gap and eliminating the voided or very thin cop- per plating. Fluid dynamics do play a signifi- cant role in achieving uniform plated depos- its. However, when there are these gaps or wedges existing between the copper and the resin, increased solution convection may be insufficient. Regardless of the defect shown in Figure 5, what are the potential causes of the gap or wedge seen in the previous image? There could be several causes. However, let's understand what it is not. In the real-life situation seen previously, the fabricator laid the blame on the micro-etch and the electroless copper process. There was no attempt to look either upstream or downstream in the manufacturing process. Figure 2: Glass fiber bundles protruding into the drilled via (the arrows show areas where there are apparent voids). Figure 4: Front light evaluation, showing no copper voiding issue. Figure 3: Dark area indicates no copper plating. This is a front light of the blind via.

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