Guest
 Posted: Tue Dec 09, 2025 10:38 am Post subject: Can a Bad Board Cause Rejects to Pass? |
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| Hello to you guys,
I'm from QA and got some OQA rejects recently. After investigating the eqpt guys are telling me the failures were caused by a bad board during prod testing. Shouldn't a bad board always cause a failure and not the other way around? I mean, can a bad board cause a bad unit to pass? Just want to know if I can now close this issue. Thanks to you guys! QA Cutie |
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Joined: 20 Jun 2026
Posts: 17
| Posted: Thu Dec 11, 2025 12:13 pm Post subject: |
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| Hi QA Cutie,
Yes, a reject can pass because of a defective dut or family board. Based from my experience, this can happen if the dut board and test program are poorly designed. I've even seen tests passing even if there's no unit in the test site! Since overacceptance is dangerous, pipol must always be aware of this danger. Test Engg 99 |
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Guest
| Posted: Thu Dec 11, 2025 2:18 pm Post subject: |
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| Hello Test Eng 99,
Thanks a million for your quick reply! But can you give me specific examples? To be honest, I still can't see how a bad board can make a reject pass test? It's like fail + fail = pass. Thanks again! QA Cutie |
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Joined: 20 Jun 2026
Posts: 17
| Posted: Thu Dec 11, 2025 3:38 pm Post subject: |
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| Hi QA Cutie,
One example: you're testing leakage at a particular pin and the test program only has an upper limit and no lower limit for this test, because you don't care if the leakage is zero. If the board is defective such that no contact is made on this pin, then the leakage would be zero because it's like testing an open pin. If the dut actually has large leakage at this pin, then it will pass because of the bad board and the one-sided limit. If the board is good then the dut will fail because the large leakage will be measured. Another example: you're measuring input impedance and you only have a lower limit because you're only concerned about low input impedances. If the board is bad and the right relay did not switch properly, then no contact will be made and the tester would think that the input impedance is very high. So the dut will pass even if in reality its input impedance is very low. Test Engg 99 |
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| Posted: Tue Aug 31, 2026 4:10 pm Post subject: |
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| Hi QA Cutie,
In additioin to Test Eng 99 examples, here's another one that hopefully would suffice in addressing your queries. Consider a voltage output parameter that is spec'd at a certain resistance load(ie. 1Kohm). This test is carried out by connecting a 1kohm resistance to the output pin(usually through a relay, the other end of the load is grounded)and then we measure the output voltage(in this case +voltage). Now imagine if the relay is defective, wherein it failed to connect the 1kohm load to the output pin when we measure the output voltage. It is likely that the measured voltage would be larger than that of the actual voltage(w/ load) since the load was not connected. This would result to over acceptance since the measured voltage would likely meet/exceed the lower limit, resulting to a reject part passing the test. Hence, an experience test programmer would add a relay checker routine that would check the integrity of these relays(and sometimes load) during initialization. Asmoth |
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