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46 PCB007 MAGAZINE I JUNE 2018 Cross-Section Analysis Cross section analysis was started with the sample preparation process by punching or routing sections from a desired area on the board or test panel. Pre-grinding of the cou- pon was done to get a flat surface closer to the through holes. Plastic index pins were used to align the coupon vertically to the grinding surface. A fast-cure acrylic resin was used to mount the coupons. A ratio of 1-to-1, harden- er-to-resin, was used to provide optimum pen- etration and a quick cure rate (10–15 minutes). After the section hardened they were subjected to grinding, polishing, and microscopic inspec- tion. Figure 2 shows a cross-section of a via in- dicating the points of measurements. Process Flow The process flow included the following op- erations: • Acid cleaner: wets the hole and remove any organic contaminants • DI water rinse • Micro-etch: further smooths the surface and ensures excellent copper to copper adhesion • DI water rinse • Acid dip: acidifies copper surface prior to plating • Electroplating of copper in acid copper bath Results and Discussion Via Fill and Bath Plating Performance Initial plating results showed that excellent filling can be obtained with different surface copper amounts. A 125 x 75 µm via can be filled even with only 10 µm copper on the sur- face as shown in Figure 3. Flow rate variation showed that the formulation has a wide range of operation from 0.4 LPM to 0.8 LPM with re- sults shown in Figure 4. A larger dimple was Figure 2: Via fill ratio. Figure 3: Via fill performance of 125 x 75 µm with different surface copper. Figure 4: Via fill performance of 125 x 75 µm with different solution flow rates of 0.4, 0.6, 0.8, 1.2 liters per minute (LPM).