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PCBs can be subjected to a variety of environmental conditions, which can cause changes in the material and alter how a PCB operates. For those who are less familiar with circuit material properties, there is often an unrealistic expectation that material shouldn’t change electrical 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 environment. 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 difficult tradeoffs on just about any complex engineering task, and it is no different when formulating circuit materials.
One material property which is often overlooked until a field unit failure demands attention 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-frequency laminates are formulated to have good TCDk properties and as a general statement, a good value would be 50 ppm/°C or less and this value would be an absolute value in the mathematical sense. Of course, the closer the material is to zero for TCDk the better.
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Editor's Note: This column originally appeared in the August 2014 issue of The PCB Design Magazine.