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102 PCB007 MAGAZINE I MARCH 2020 The Df or loss tangent of the material is a measure of the percentage of total transmitted power that will be lost as power dissipated into the laminate material. High-frequency signals switch backward and forward rapidly. There is the switch between positive and negative, causing the molecules within the resin (dielec- tric) materials to polarize with the electromag- netic field of the signal. During this situation, there is heat loss. Thus, there are signal loss- es that become greater at higher frequencies. These losses are proportional to frequency and become more pronounced at frequencies high- er than 10 GHz. However, today's technologies are requiring signal speeds in excess of 35 GHz for selected applications. Skin effect is real and must be considered as higher frequencies are used as part of the main- stream circuit designs going forward. What does all this mean? From a sim- plicity standpoint, these higher-perfor- mance materials are much more difficult to process for the PCB fabricator. Basi- cally, it is not your father's FR-4 any- more! From a fabrication standpoint, processing a 140°C Tg material through the desmear/etchback process as an ex- ample is much easier to etch the resin, remove drill smear, and create sufficient topography to enhance the adhesion of the plated copper. As one moves up the laminate material technology curve (lower Dk, lower Df, engineered mate- rials, including ceramic-filled), resin re- moval, or simply etching the resin be- comes more difficult. For the fabricator, this necessitates a change in mindset related to chemical desmear. If the end-user requires a significant amount of etch- back, this may not be practical with current mechanical and chemical set-ups. The fabrica- tor must resort to radical departures from the status quo. This includes extended dwell times in the desmear process, increasing the aggres- sivity of the chemistry and the use of plasma etchback as either a stand-alone or in combi- nation with chemical processing. Satisfactorily, bareboard fabrication now in- cludes a variety of materials. Glass-reinforced laminate for rigid printed boards—and unre- inforced laminates, used primarily for flexible printed boards—present significant challenges for desmear/metalization, as well as in lamina- tion. Further, these higher-performance, thin- core materials are being combined in a variety of applications to achieve a thinner profile of the end-product. Thus, lamination of the com- bined materials (including adhesives) requires changes in the lamination cycle parameters, including time, pressure, and heat rise rate. There are several additional material proper- ties that the PCB fabricator must consider to optimize the process: drillability and conduc- tive anode filament (CAF) formation. Skin effect is real and must be considered as higher frequencies are used as part of the main-stream circuit designs going forward. Figure 1: Rough hole walls after drilling.