Dielectric constants
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What is the dielectric Constant
The following table is to be used as a starting point only for determining impedances using calculators. Dielectric constant will vary depending on such factors as material thickness, number of ply’s and resin content. Always consult  us for feedback on your proposed stack-up and controlled Impedance trace width and space BEFORE layout. Every PCB Manufacturer has a different process and will modify trace widths to achieve desired impedances.
The dielectric constant (DK) is an important factor when designing capacitors and in other circumstances where a material might be expected to introduce capacitance into a circuit. The layers beneath etched conductors in pcbs also act as dielectrics. Dielectrics are used in RF transmission lines
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Electrical Fields are generated
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The dielectric Constant of the material is very important, as this is then used to calculate the required dielectric spacing and track width
Single Ended Impedance tracks looks like the following
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So watch out for customers stating FR4 with a DK of 3.5, as if they have used this in their design, they have used the wrong DK, Fr4 is more like a DK of 4.