How internal scan find defects in comb parts?

[qjlsy]

I read the netlist after scan insertion. But I found in scan mode, comb logics are bypassed. All scan cells are connected with each other directly.

Is it right? If right, how can internal scan methodology find those defects in comb logic part?

Thanks a lot!

 

[dr_dft]

qjlsy,

The way scan works is that it allows you to setup pattern at the inputs of combinational logic blocks (by shifting in the flip-flops). Once we are done shifting, we turn off scan enable, pulse the clock for one cycle to capture the outputs of the combinational blocks into scan flip-flops, then turn on scan enable again to shift the response out.
Effectively, we are generating patterns for the combinational logic as if their inputs and outputs are directly controllable and observable from pins, since we can shift in and out any pattern we want.

Hope this helps.