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44 DESIGN007 MAGAZINE I APRIL 2022 PCB designers are continually challenged with demands for reduced product size. How- ever, form factor-driven design pressures have been relieved, in part, by the increased use of high-density interconnects (HDIs), which enable more functionality per unit area than conventional PCBs. Leveraging finer lines, thinner materials, and laser-drilled vias, HDIs have played a crucial role in device miniaturiza- tion. However, the traditional PCB subtractive etch processing becomes very difficult for fea- ture sizes below 3-mil trace/space. is forces PCB designs to become more complex as elec- tronics packages shrink—adding extra routing layers and microvia layers, and increasing the number of lamination cycles required, which impact yield, reliability, and thus cost. As smartphone technology evolves to 5 Gbps, the PCB industry's approach to HDI manufacturing has also evolved. Vast multi- ple input/output antenna configurations and increasingly complex RF front-ends expand the RF content footprint. e higher band- width inherent to 5G requires much stricter impedance control. If not formed with extreme precision, the thinner traces of HDIs can intro- duce an increased risk of signal degradation. To fulfill these demands, the amount of available space for HDI PCBs, within 5G smartphones, needs to be significantly reduced. e semi-additive process (SAP) is a pro- duction-proven method used on low dissipa- tion loss (Df ) build-up materials that enable the manufacture of ultra-fine-line circuitry. is technique utilizes additive process steps, adding copper to the base dielectric, rather than subtractive processes to create the cir- cuit pattern. Fabricators previously only able to offer 3-mil trace/space can now reliably produce 1-mil trace/space and below. Further reductions required the adoption of various SAP manufacturing processes commonly used in the IC substrate industry, including laser direct imaging (LDI), improved laser drill- ing, extremely thin copper foils, via fill pattern plat- ing, and flash etching, into a process called modified SAP (mSAP). SAP enables the printed circuit board fabricator to employ an additive screen process instead of an edge- removal, etched process— the end result is a PCB design that can dramati- cally reduce area, layer Designing for the SAP Fabrication Process Beyond Design Feature Column by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Trace width vs. process technology.