Fault Class Hierarchies in DFT

Faults list in design are categorized into sub categories. Faults class are mainly divided into

• Testable(TE)–> Faults can be tested by some patterns.
• Untestable(UT)–> Faults foe which no pattern exits to detect the faults

• Testable Faults: There are four sub category under TE.
  • DETECTED(DT)
  • POSDET(PD)
  • ATPG UNTESTABLE(AU)
  • UNDETETED(UD)

• Detected(DT): The Faults which are detected during the ATPG process are categories under DT
  • det_simulation(DS): The faults detected when the tools performs simulation
  • det_implication(DI): The faults detected when the tool performs learning analysis
• POSDET(PD): The Possible detected, faults includes all the faults that fault simulation identifies as possible detected
  • posdet_testable(PT): Potentially detectable posdet faults.With higher abort limit we can reduce the number of these faults
  • posdet_untestable(PU): These are proven ATPG untestable and hard undetectable faults.
• ATPG_UNTESTABLE(AU): This fault class includes all the faults for which test generator unable to find the pattern to create a test. Testable faults become ATPG untestable faults because of constraints or limitations, placed on the ATPG tool such as pin constraint or an insufficient sequential depth. This faults may be detectable, if we remove some constraint, or change some limitations on the test generator
• UNDETECTED (UD): This fault class includes the undetected faults that cannot be proven untestable or atpg_untestable
  • uncontrolled(UC)
  • unobserved(UO)
    All the testable faults prior to ATPG are put in the UC category. Faults that remain UC or UO after APTG aborted, which means that with higher abort limit may reduce the UC and UO fault class