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74 PCB007 MAGAZINE I JULY 2023 flush process of 1 ounce or half-ounce Cu foil on outer layers or inner layers. Many times, based on density, the flush circuits do not require solder mask to prevent bridging. Conclusion As illustrated in Figure 8, the design poten- tial by WUS for RF and high-speed digital cir- cuitry application are apparent in that: 1. ere is improved reliability. a. Stronger foil-to-dielectric adhesion b. BGA ball adhesion to copper pad 2. Improved signal integrity (PIM). a. Trace near-rectangular geometry b. Smooth copper surface c. Minimize crosstalk 3. Other application possibilities. a. Selective copper thickness for power or heat dissipation purpose b. Very high-resolution outer layer circuit density using conventional chemistry (2.0-3.0 mil or 50-75 mm trace W/S c. Same net over two types of materials (patch antenna) 6 PCB007 References 1. Karl Dietz devoted two columns (#125 and #184) to flush circuit technology, but only one went into details about recessed circuit processes (TT#184), The PCB Magazine, May 2011. 2. "Selective Copper Metallization for Advanced Packaging," by Rashid Mavliev and Robert Rhoades, Proceedings of the International Wafer-Level Pack- aging Conference 2020, San Diego, Calif. Figure 7: Illustration of three different applications of flush circuits: a) Traditional subtractive circuits as reference; b) Flush circuits as outer layers or inner layer routings; c) Finer-line HDI circuit applications. (Source: WUS-Taiwan 6 )