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July 17, 2013
Connecting the Dots: Nothing's Perfect - Understanding Tolerance
by Jack Olson
Yes, nothing’s perfect.
We designers do our best to maintain accuracy, but the real world produces imperfections:
- Our CAD systems assume that a drill is perfectly centered in a round pad. It never is.
- We declare specific trace widths, but when we measure them on an actual board they are always slightly thinner or thicker.
- Multiple layers are perfectly aligned on our computer screens, but the fabricators can never quite manage to duplicate it. There will always be some misregistration.
- The board design is assumed to be flat, but boards in the final product can be bowed or warped.
- We designate some traces to be impedance controlled, but our measurements differ.
I could go on and on and on, but I think you see the point. As designers, we calculate exact numbers. We design with precision. Our CAD systems show us the ideal board. In the real world, however, nothing is so precise. The final product will vary from the ideal in one way or another, but hopefully in ways that are harmless.
Editor's Note: This column originally appeared in the May 2013 issue of The PCB Design Magazine.
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