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AUGUST 2021 I PCB007 MAGAZINE 97 copper thickness of 43 µm. Void-free fill was achieved in the through-holes as shown in the cross-section picture (Figure 6) and confirmed by X-ray analysis. 400 µm thick core with 150 and 200 µm diameter through-holes Mechanically drilled panels 400 µm thick by 150 and 200 µm diameter were used for eval- uation. For 150 µm diameter through-holes, the plating was carried out for a total time of 174 minutes while the bridging was carried out for 38 minutes and fill was performed for 136 minutes. e bridging was carried out for 9 µm and fill for 23 µm with a resulting total sur- face copper thickness of 32 µm. 200 µm diame- ter through-holes were filled for a total surface copper thickness of 45 µm. Void-free fill was achieved in the through-holes as shown in the cross-section picture (Figure 7) and confirmed by X-ray analysis. 800 µm thick core with 150 and 200 µm diameter through-holes Mechanically drilled panels 800 µm thick by 150 and 200 µm diameter were used for eval- uation. For 150 µm diameter through-holes, the plating was carried out for a total time of 331 minutes while the bridging was carried out for 43 minutes and fill was performed for 288 minutes. e bridging was carried out for 10 µm and fill for 25 µm with a resulting total sur- face copper thickness of 35 µm. 200 µm diame- ter through-holes were filled for a total surface copper thickness of 67 µm. Void-free fill was achieved in the through-holes as shown in the cross-section picture (Figure 8) and confirmed by X-ray analysis. Process Capability e process of bridging and filling the through-holes using phase shi pulse plating and DC plating can be applied to a wide range of panel thicknesses and through-hole diam- eters. e time for bridging and filling were optimized to achieve void-free fill for a giv- en through-hole diameter and panel thick- ness. e cross-section images of through- holes of various dimensions and the surface thicknesses required to fill them are given in Figure 9. X-Ray Void Evaluation All the plated coupons were evaluated by X- ray analysis to confirm that void-free fill was achieved in the through-holes. e X-ray im- age for 400 µm thick and 200 µm diameter through-holes is shown in Figure 10. Void-free fill was seen in the through-holes. e coupons were then cross-sectioned and evaluated with a microscope to take other measurements such as surface thickness. Figure 6: 250 µm thick panel: (A) 150 µm and (B) 200 µm diameter through-holes. Figure 8: 800 µm thick panel: (A) 150 µm and (B) 200 µm diameter through-holes. Figure 7: 400 µm thick panel: (A) 150 µm and (B) 200 µm diameter through-holes.