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34 DESIGN007 MAGAZINE I JUNE 2021 Feature Article by Alun Morgan VENTEC INTERNATIONAL GROUP Whilst Gene Roddenberry was writing Star Trek in one corner of the U.S., another group of engineers was working on a less daring proj- ect, but one which was nonetheless ground- breaking and continues to influence our lan- guage to this day. at group was the National Electrical Manu- facturers Association (NEMA), and their mag- num opus, at least as far as the PCB industry was concerned, was the "L1" specification. As Star Trek was responsible for adding terms such as photon torpedo, dilithium crystal, and warp drive to our language, so NEMA was respon- sible for the terms G-10, FR-4, and FR-5. "FR" was the NEMA abbreviation for "flame retar- dant" and materials so classified were marked with a red manufacturer's logo. e terms also marked a change from the early 1960s where materials produced solely from paper rein- forcement were used to produce PCBs. NEMA defined the terms used during the transition to glass-reinforced PCB materials that were to become the mainstay of our industry. e evolution of printed circuit board sub- strates has necessitated a change of view in how performance is classified. e NEMA classifications are based on the physical prop- erties of the basic chemical makeup of the materials used. We are, for example, used to classifying substrates according to their glass transition temperature, Tg. Accordingly, we refer to "standard," "mid-range" or "high" Tg substrates. It is no surprise, therefore, that many designers and users of PCBs have assumed that the Tg value has a direct relation- ship to the thermal endurance of the substrate and have specified a "high Tg" product for an application requiring high thermal resistance. I am reminded of Spock's words, "Insufficient facts always invite danger." e Tg value merely marks a phase change in the resin chemistry whereby enough energy (temperature) is available to increase the rota- tional freedom of the polymer to allow it to transition from a glassy to a rubbery state. If energy is continued to be applied, then the next transition that would be expected is the Tm, or melting temperature. Here is where thermoset plastics spring a surprise. In theory, It's FR-4, Jim, But Not as We Know It! Alun Morgan

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