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94 DESIGN007 MAGAZINE I APRIL 2021 e really exciting news is that there are now a handful of U.S.-based fabricators offering this technology for low volume/high mix applica- tions. is opens so many new opportunities previously unavailable for PCB designers and at the same time generates a myriad of ques- tions from the design community as well as the fabrication community. Before we dive in and start addressing these questions, let's step back and discuss the semi-additive PCB pro- cess flow and how that is being integrated into existing subtractive etch PCB processing lines. Semi-Additive PCB Process is process flow from Averatek (Figure 1) gives a simplistic view of the semi-additive PCB process flow. I do not want to assume that all are familiar with the subtractive etch process and want to point out that with steps two and three, these are the same process steps used to manufacture with traditional processes. Rather than etching away the cop- per that is not required for the circuitry, with this process, all copper is etched from the manufacturing panel. From there, a palladium deposition using LMI™ technology is coated on the panel. e next step is electroless cop- per, typically the same electroless chemistry being used by the fabricator for the subtrac- tive process. is step is critical to understanding the benefits of the semi-additive process. e thin LMI coating enables a very thin, conformal layer of electroless copper, 4–20 µin, which is much thinner than typical electroless copper plating. More to come on the significance of this. Following electroless copper, panels move to dry film resist imaging, followed by electro- lytic copper and dry film resist strip, all stan- dard manufacturing processes. e final step in the circuit creation process is to flash-etch the electroless copper layer. Circling back to the thickness (or actually, thinness) of that electroless copper layer, it is important to point out that the flash-etch of the very thin electroless copper layer has very little impact on the trace itself. Etching of thicker copper results in the trapezoidal effect that we are all aware of. With semi-additive processing, the trace sidewalls remain straight, and the line width tolerance is tighter. In future columns we will discuss the impact of this on imped- ance control. Figure 1: A-SAP™ process flow diagram.