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26 PCB007 MAGAZINE I JUNE 2024 As one moves up the laminate material tech- nology curve (lower Dk, lower Df engineered materials including ceramic-filled), resin removal or simply etching the resin becomes more difficult. For the fabricator, this necessi- tates a change in the mindset related to chem- ical desmear. If the end user is requiring a sig- nificant amount of etchback, this may not be practical with current mechanical and chem- ical set-ups. e fabricator must resort to radical departures from the status quo. is includes extended dwell times in the desmear process, increasing the aggressiveness of the chemistry, and use of plasma etch- back as either a stand-alone or in combination with chem- ical processing. Satisfac- torily, bare board fabrica- tion now includes a variety of materials. Glass-reinforced laminate for rigid printed boards and unreinforced lam- inates, used primarily for flex- ible printed boards, present significant challenges for desmear/metallization as well as in lamination. Further, these higher-per- formance, thin-core materials are being com- bined in a variety of applications to achieve a thinner profile of the end product. us, lam- ination of the combined materials (including adhesives) requires changes in the lamination cycle parameters, including time, pressure, and heat rise rate. ere are several additional material proper- ties that the PCB fabricator must consider to optimize the process. For consideration: Drillability and Metallization • Drillability is an important consideration in selection of a laminate for a backplane, particularly with high plated through-hole aspect ratios and the increase in backdrill- ing. Constant diameter and clean finish are essential. e grade of glass fiber, fillers, and dielectric material hardness all contribute to the hole quality. in lower signal propagation speed, i.e., the lower the Dk, the less impedance there is to the signal propagation. e characteristic imped- ance of a printed circuit board is determined by the thickness of the laminate and its Dk. Impedance control and impedance matching of critical linked functional modules become especially important in very high-speed, i.e., high-frequency designs. Dk also tends to shi with temperature so heat generation by such designs is another important factor. e Df (loss tangent) of the material is a measure of the percentage of total trans- mitted power that will be lost as power dissipated into the lami- nate material. High frequency signals switch backward and for- ward rapidly. ere is the switch between positive and negative, causing the molecules within the resin (dielectric) materials to polarize with the electromagnetic field of the signal. During this situation, there is heat loss. us, there are signal losses that become greater at higher frequencies. ese losses are proportional to frequency and become more pronounced at frequencies higher than 10 GHz. However, today's technologies require signal speeds in excess of 35 GHz for selected applications; at 35 GHz and above, everything matters. Skin effect is real and must be consid- ered as higher frequencies are used as part of the main-stream circuit designs going forward. So, what does all this mean? From a sim- plicity standpoint, these higher performance materials are much more difficult to process for the PWB fabricator. Basically, it is not your father's FR-4 anymore. From a fabrica- tion standpoint, processing a 140°C Tg mate- rial through the desmear/etchback process, as an example, is much easier to etch the resin, remove drill smear, and create sufficient topog- raphy to enhance the adhesion of the plated copper. Higher performance materials are not easy to desmear and metallize. The fabricator must resort to radical departures from the status quo.