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need help in deciding scan chains

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hi everyone...
can anyone tell me for a design how can we decide how many scan chains we require?...

another question is in a particular scan chain how many flops should be put? is it dependent on the technology or is it dependent on designer or on the customer requirement?

thanks in advance for your valuable suggestions.:D:D:D
 

scan chian number depend on the clock domain and max flipflops. Don't mix different clock domains and clock edges to aviod timing issue.
and longer chian will inrease long test time, < 1000k ff's per chain.
 

There are a number of factors to decide the number of scan chains and length of each scan chain.

1. Look for your ATE specification to find how many scan chains are supported. If length of scan chain is very long, then the tester time increases and hence your cost

2. If you are using scan compression, then your scan compression methodology will determine the maximum length of each scan chain
 
I just want to add something ... As silencer said in scan compression the compression factor decide the length of the chain.But as we increase the compression factor the complexity of the circuit increases which further decreases the coverage ,so we have to take the trade off between compression factor and coverage.. By default Value of compression factor in the DFT Max (Synopsys) tool is 10 and max value is 50. Generally we take value near 22-27.The device which im working ,i use this compression value as 25.
 
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