Issue link: https://iconnect007.uberflip.com/i/1024460
70 PCB007 MAGAZINE I SEPTEMBER 2018 The Macro PCB Section: Through-hole Bridge and Fill A novel bridge-and-fill process has been de- veloped consisting first of an electroplating step to bridge through-holes in substrates/ PCBs followed by a second step of filling the generated "virtual" blind vias (the latter pro- cess like the one being presented beforehand). Figure 19 shows the demarcation line between the two coppers (a 0.25/0.20 mm hole in a 0.25 mm thick substrate). The overall process not only fills through- holes with extremely thermally conductive copper (l=390 W/mK), thus providing the seam- less copper path from die through the environ- ment, but also offers the chance to cost effec- tively replace some staggered blind via layups by (simple) through-drilled multilayers. Designing in a strong levelling performance of the bridging electrolyte, the crystalline ma- jority of preferrable 1,1,1 unit cell has been traded off against a required minimum copper thickness at the substrate´s surface (Table 7). Accordingly, the related SEM micrograph shows lesser discernible grain boundaries, even some "pico"-voids, which do not im- pact the total hole IMC reliability because of the bridge copper being covered by the second step fill copper not being in contact with an IMC layer on soldering (Figure 20). The achieved inherent reliability of the com- posite bridge & fill copper has been demons- trated by exhaustive elongation and tensile strength testing (Table 8). Bridged and filled through-holes pass all re- lated IPC class 3 thermal tests, including solde- rability, thermal shock, thermal cycling, solder dip/solder float, IST, HAST, etc. Representative for the reliability tests being Table 7: Copper crystallography of a bridging copper plating bath. Figure 19: Two-step bridge and fill of through-holes. Figure 20: SEM picture of bridge copper only (FIB ion milled specimen).