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68 DESIGN007 MAGAZINE I APRIL 2018 Polytetrafluoroethylene (PTFE) laminates are the original high-frequency laminate, and they have been around for many decades. I have heard people talk about PTFE laminates as if they are all the same and have the same capa- bilities and limits, but that is very misleading. I'll give a few examples of the many things to consider when choos- ing a PTFE laminate. A couple of issues to consider are circuit board fabrication and high-frequ ency perfor- mance issues. If a board is made with pure PTFE, it could have exceptional high-fre - quency performance. However, making a PCB out of a pure PTFE laminate is not a trivial matter, for a variety of reasons. It is very soft, which means dimensional stability is highly questionable and drill- ing can be difficult. If smear occurs during the drilling process for a PTFE laminate, the only option is to throw away the board because nothing can desmear PTFE. It has extremely high coef- ficient of thermal expansion (CTE) so PTH via reliability and soldering can be a concern. Nothing likes to adhere to PTFE, which means having electroless copper adhere to a drilled hole-wall can be challenging. The thermal coefficient of Dk (TCDk) is very high, which means the Dk will change significantly with a change in tempera- ture. Also, the thermal conductivity is very low, which for some high-power applica- tions can be detri- mental to thermal management. And there are more con- siderations. However, if you mix the right stuff with PTFE, you can overcome most of these obstacles. The first attempt to make PTFE friendlier to the PCB fabrication process was to have it reinforced with lay- ers of woven-glass fabric, fiberglass. The woven-glass rein- forcement certainly helps the mechanical aspect of dimensional stability, and it sig- nificantly improves the CTE. Addition- ally, the glass may Not All PTFE Laminates are the Same Lightning Speed Laminates by John Coorod, ROGERS CORPORATION

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