Deciphering datasheets for high-frequency circuit materials: Page 3 of 7

January 07, 2013 //By John Coonrod, Rogers Corporation
Data sheets for printed-circuit-board (PCB) materials carry a great deal of information. Understandably, these materials are the foundations for many circuits, and they are characterized by many different parameters, some related to applications, some to fabrication issues, some to environmental and mechanical concerns.
length of material, such as KV/mm or V/mil, with values based on an IPC measurement method that assumes a set thickness of the circuit material being tested. The test method measures for the dielectric breakdown point of a material with voltage applied as DC. Although this property may not be critical for lower-power circuits, it can be important for high-power applications or RF/microwave circuits employing very high DC bias.

Some of the properties listed on a PCB material data sheet are mechanical in nature, such as tensile modulus and tensile strength. Tensile modulus indicates the stiffness of the material, with higher modulus values denoting stiffer materials. Tensile strength provides a measure of how much a material can be stretched before suffering permanent damage. Both of these mechanical properties can be of concern for circuit fabrication and end-user applications. For example, non-glass-woven PTFE circuit substrates are sometimes formed into different shapes as part of an end-product; knowing more about a circuit material’s mechanical properties can help a circuit designer decide on a suitable material and just how much strain can be applied without causing damage to the final circuit.

Dimensional stability, given in terms of the width and length (x and y) dimensions of the PCB material, can be a concern in some cases of circuit fabrication. This parameter gauges the tendency of a circuit material to change x and y dimensions when copper is etched away from the dielectric material (such as when forming a circuit) or in some cases when the circuit material is exposed to temperature extremes. Copper tends to add rigidity and stability to a dielectric-based PCB material but, due to how copper-clad laminates are made, some stresses are built into the material. As copper is etched away, these stresses can relax and result in dimensional changes in a circuit laminate. This is a concern for any application where circuit dimensions must be precisely controlled. Testing for

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