How to Control your Controlled Impedance | Sierra Circuits
Channel: Sierra Circuits
Date: 2018-02-08

Trust us, you do not want to get your controlled impedance wrong. And as a PCB manufacturer, we can help you get it right.

1. How To Specify Controlled Impedance
You can get controlled impedance in two ways: first is to specify the dielectric thicknesses that you’re looking for on your fabrication drawing and not specify any controlled impedance at all. Second, which I think is the better way, is to specify the layers in which you’d like your impedance lines on and the target ohms. Standard tolerance is plus or minus 10% ohms. If tighter tolerance is needed, we can deliver plus or minus 5% ohms. Tighter tolerance would require a different game plan ahead of time. 

2. Press-Out Thickness
Remember, we also plan based on the press-out thicknesses that we expect from the prepreg. This is based on the amount of resin in the prepreg and the amount of copper area and thickness of copper on the opposing layers. We get the copper area from the data you send us to create a final model. So, we don’t follow the thickness on the data sheet, we follow our press-out thicknesses in our modeling, which varies slightly from design to design. That’s why after an initial stack-up and target line widths, we come back to you for approval on small adjustments to the trace widths and spacing.

3. The Difference Between Core and Prepreg
There is a difference between the core and the prepreg. The core dielectric thickness doesn’t really vary after lamination because the copper is on the outside. If you use prepreg for the dielectric thickness, the height does vary based on the copper area and height of the copper and the glass cloth styles that we’ve chosen at the time of planning the build. All of our lamination cycles have computer-controlled profiles to achieve consistency because this is what happens during lamination.

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