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38 The PCB Design Magazine • August 2014 PCBs can be subjected to a variety of envi- ronmental conditions, which can cause chang- es in the material and alter how a PCB operates. For those who are less familiar with circuit ma- terial properties, there is often an unrealistic ex- pectation that material shouldn't change elec- trical performance when subjected to different environments. Actually, all circuit materials will change some properties when evaluated within a changing environment. Some properties may change more than others and some materials may have more change than others, but they all do change. The materials formulated for use in high- frequency PCB applications are formulated so that critical electrical properties have minimal change when subjected to a changing environ- ment. In the material development process, it is always a juggling act to allow some properties to change more so other properties will change less. All engineers typically struggle with diffi- cult tradeoffs on just about any complex engi- neering task, and it is no different when formu- lating circuit materials. One material property which is often over- looked until a field unit failure demands atten- tion is TCDk (thermal coefficient of dielectric constant). This property is innate to all circuit materials; however, materials not formulated for high-frequency applications often have an extremely poor TCDk. Conversely, high-fre- quency laminates are formulated to have good LIGHTNING SPEED LAMINATES feature column by John Coonrod ROgeRs CORPORATiOn Environmental Effects on High-Frequency Material Properties

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