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Design007-June2021

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36 DESIGN007 MAGAZINE I JUNE 2021 there is indeed a Tm; however, the temperature required to melt a thermoset exceeds the ther- mal decomposition temperature, Td. e con- sequence of this is that a thermosetting mate- rial never reaches the Tm—it decomposes into carbon and a variety of gases long before it can ever get there. e Tg has no direct correlation with the thermal decomposition temperature. To specify thermal performance, we need to set our phasers to stun and to consider all per- formance aspects relevant to the end applica- tion. is marks a shi in the classification. It is not sufficient to specify a "high Tg" substrate; instead, performance should be specified according to the application. is may include the Tg, Tm, time to delamination, coefficient of thermal expansion in the Z-axis, and thermal conductivity to fully characterize a substrate suitable for applications requiring a focus on thermal management. Since the days of Star Trek, we have also moved from impulse power, and believe it or not, my first job aer leaving university was in a research laboratory where a space impulse engine was developed to "warp speed." I am, of course, referring to the continuing journey of increasing signal speeds and data rates. James Montgomery Scott (Scotty) famously said, "I canna' change the laws of physics." So far as we know, he was correct; however, we can cer- tainly exploit them. e PCB world of the 1960s was a much simpler place. Our reference frequency was 1 MHz and circuits were largely comprised of analogue devices, with very few of the nasty square waveforms with theoretical bandwidths tending toward infinity to deal with. e high- frequency spectrum in the past was divided into two bands—one where FR-4 worked and one where it didn't. Where it didn't, many designers faced a no-win scenario where they couldn't use FR-4 and the only alternative was PTFE which could push the project into an insurmountable position of unaffordable cost and limited materials availability. When Captain James T. Kirk faced his no-win scenario in the simulated encounter with the Klingons in "e Kobayashi Maru," his solution was to cheat. Circuit designers today are indeed fortunate that they have no need to cheat as the gap between FR-4 and PTFE performance and cost has been filled by entirely new ranges of substrate materials based on their performance at high frequency and high data rates for digi- tal circuity. ese new generations of material allow the designer the flexibility to select the appropriate substrate for the application with- out over-engineering performance that is nei- ther required nor can be afforded. FR-4 itself has also evolved. In the 1960s, it was a single grade, but today it encompasses a range of materials with diverse properties and chemistries. e recent split into FR-4.0 and FR-4.1 (characterizing brominated and non- brominated flame retardants, respectively) came about in recognition of the fact that all FR-4s do not perform the same and test results of one variant could no longer be considered representative of another. "It's FR-4 Jim, but not as we know it!" Despite the recent years of tremendous upheaval in our industry, I hope that you share my enthusiasm and excitement for our con- tinuing mission to aggressively seek out and develop the material solutions of tomorrow. May we all live long and prosper as we boldly go where no one has gone before. DESIGN007 Alun Morgan is technology ambassador for the Ventec International Group. To specify thermal performance, we need to set our phasers to stun and to consider all perfor- mance aspects relevant to the end application.

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