Explain about stuck at fault models, scan design, BIST and
IDDQ testing?
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Answer Posted / navya
IDDQ testing:usually performed at the beginning of test cycle.The test checks for leakage current to know if it is in normal range or abnormal range.If abnormal die fails,it is rejected and no further tests are performed.Iddq testing can detect clusters of gate oxide shorts(GOS) where gate voltage has no control over drain current and they tend to increase leakage levels.
BIST(built in self test): used to meet requirements such as high reliability and low repair cycle times.Bist reduces need for external testing(ATE).But the disadvantage is additional silicon area needed to implement BIST circuitry.
Scan design:test methodology built into digital chips
All flipflop are provided with alternate i/p for data as well as a separate clk i/p for scan testing.F/f connected together in scan chains.Testing is done by entering a special test mode called "scan mode" where test vectors is i/p to each scan chain and the bits clkd through all f/f's in the chain with resulting o/p chkd for errors.
Stuck at fault models:
stuck-on fault:always conducts Ids with an applied Vds,gate has no control over the operation
stuck off faults:current never flows regardless of Vgs or Vds.
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