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80 PCB007 MAGAZINE I APRIL 2019 Bridge plating technology offers a further advantage in terms of thermal conductivity compared to filling materials. Copper delivers one of the best thermal conductivity proper- ties compared to lower thermal conductivity properties of any plugging paste on the market (Tables 2 and 3). This advantage enables PCB designers to reduce the number of thermal vias by about 70% while focusing on thermal conducting and cooling efficiency at hotspots. As illustrated in Figure 7, there are three re- liability concerns with plugged vias that are negated by copper plating. Whereas the void (1) and dimple (2) are related to process con- trol (lower risk), the blistering (3) is related to material and dimension. The CTE mismatch of copper (16.5 ppm) compared to plugging paste (30–50 ppm) may result in a blister. These three phenomena on plugged vias have been investigated [4] . Whereas plugging with paste may create all three phenomena (Figure 7, right picture), the blistering phenomenon (3) is negated by us- ing copper as the filling material. The forma- tion void (1) can also be avoided fully using modern copper through-hole filling processes, which operate void-free. This is achieved using automated and sophisticated equipment con- trols during the copper bridging and filling pro- cess. Furthermore, the dimple (2) is also con- trolled below 5 µm with the right settings on the copper process. These avoidance techniques are simply not possible when plugging a through-hole via by a high-viscosity plugging paste enabled using screen print methods. Bridge-plating technology works as follows. During the first copper electroplating plating process, a copper bridge is formed (Figure 8a). This bridge plating is the key to fill the hole. After this step, only two microvias on the top and the bottom are left, which will be filled with copper by one or more subsequent micro- via filling steps (Figure 8b–c). The copper bridge plating process works best with a horizontal copper plating process because the horizontal system offers key ad- vantages in terms of fluid dynamics inside the hole. This promotes the generation of void-free plating results, very low plated-copper thick- ness on the surface, a much better copper uni- formity, and higher throughput compared to vertical plating units. The dimensions of through-holes that are achievable with filled copper are illustrated in Table 2: Thermal conductivity of copper versus plugging paste. Table 3: Thermal resistance of copper versus plugging paste (e.g., 1-mm board thickness, 0.3-mm diameter, 25-µm copper barrel). Figure 7: Plugging paste versus copper plating through-hole filling [4 & 5] .