A chip passes wafer sort, moves through assembly, and passes final test. Nothing in its results gives anyone a reason to hold it. Yet another chip with nearly identical results could fail in the field because of a weakness neither test exposed, whether that’s a thin timing margin, a slight resistance shift, or a combination of measurements that looks harmless when each is viewed alone.
The challenge for manufacturers is figuring out which of those passing chips deserve another look without retesting everything. Test engineers have always known that a passing result doesn’t guarantee lifetime reliability. What’s changed is how much they can learn about each device. Internal parametric measurements, manufacturing history, and predictive models can now reveal unusual behavior inside a population of passing parts, but spotting a suspicious device is just the beginning.
“The cost of test and the amount of test is exploding, and the ability to test before system-level test is a challenge,” said Kimon Michaels, executive vice president of products and solutions, director, and co-founder of PDF Solutions. “But of course, one of the big challenges is how much of your supply chain is at risk at that point, or the amount of product that you’re throwing away if it fails at that point.”
The longer a marginal device goes undetected, the more value gets invested in it. But testing every device more thoroughly isn’t free. Extra patterns eat tester time, burn-in ties up equipment, a packaged device may need another mechanical insertion, and rejecting every statistical outlier throws away some good silicon.
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