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May 2015 • The PCB Design Magazine 57 (1.6%) over the same range. Polyimides with high Tg and hence lower overall expansion, will result in lower strain on the copper barrel during thermal expansion and less work hard- ening due to repetitive expansion/contraction cycles. Laminates with high loadings of ceramic fillers that have reduced Z-direction expansion will also better match the copper and improve PTH reliability. In the X,Y plane of the board, copper will drive the CTE higher in many instances. Typical epoxy and polyimide boards made with high resin contents will have naturally occurring composite CTEs in the 15–17 ppm/ o C range and this will not be a substantial issue. On the other hand, boards made with nonwoven ara- mid or other materials designed to control CTE for SMT and chip attach applications, need to be designed with as little copper as possible, since copper will drive the CTE of the overall system up in proportion to how much etched copper remains, especially if there are a lot of power/ground planes. VI. Reading the Data Sheet Finally, we would offer this advice: "Do not be deceived by the data sheet." This is an old concern of ours, and frequent discussions with customers confirm that it is a widespread issue. Many reported physical properties are based on rigid 0.062" lams (37–40% RC) in accordance with IPC slash sheet requirements. If your application is for a high-layer count multilayer board and the product does not have a specified value across various resin contents, be sure to ask your supplier what the expected dielectric constant or CTE will be for the range of material you actually plan to use. PCBDESIGN CliCk To View Video Interview editor Joe Fjelstad sat down with al Wasserzug, senior business development man- ager for Cirexx, to discuss his white paper describing a low-cost alternative to co-fired ceramics. eclips involves using metalized layers inside a PCB to evacuate heat efficiently, with a low CTe. Solution for High-Temperature Applications by Real Time with... IPC APEX EXPO 2015 Chet Guiles is a consultant for arlon Materials for electronics. article THE COMPOSITE PROPERTIES OF RIGID VS. MuLTILAyER PCBS continues

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