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A question about impedance controlled PCB

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Jim Zhu

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Dear Friends

When I worked in PCB factory as a QA manager, sometimes I need face the UAI (use as it ) apply of impedance cotrolled PCB from Manufacture department. Beacause the impedance PCB has some difficulties point to control. If some parameter was over control, which will cause the impedance value over tolerance. Normally the impedance tolerance that customer required is ±10%.

My question is what kind of effect to the customer if the impedance value was over tolerance. And what the biggest limited of impedance that customer can accept?

Best regards
Jim Zhu

E-mail: jim@accpcb.com
Skype: z-m-zhu
 

Tolerance in impedance may lead to ringing in digital circuits, this may affect timing and reduction of interference margin.

In analog ciruits it may result in additional phase shift, change in center frequency of filters, etc.

How much is tolerable depends on the application (and the client may say 10% because he doesn't know it). You probably have seen that some additional traces are present on a large board. These traces may be used for impedance testing and to show the client that the impedance is within specs.
 
If impedance mismatches occur, the signal instead of being effectively absorbed at the load, gets reflected back. This is because the reflection co-efficient is given by (Zl - Zo)/(Zl+Zo). Zl here is the load impedance and Zo is the trace impedance. As you can see from the formula, if Zl not equal to Zo, a reflection co-effecient is present, giving to rise to signal reflections. This creates a very basic signal integrity problem. If the mismatch is out of bound, the signal transmission simply won't be reliable. As far as, how much is tolerable, it will depend on application.
 
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