industry reliability
How many more problems will I see from this batch? Should I stop production and/or shipments of this material? And should I recall previous material that has already shipped?
Customers are also really anxious to know that the root cause has been diagnosed with complete certainty, and there is absolutely no chance that it will happen again. And afterwards, when the dust has settled and they have had a chance to ponder some more, they will eventually ask a broader question that’s along these lines: What is the risk that I’ll get more problems?
At TriQuint Semiconductor, like every big chipmaker, we have had to deal with these types of questions from customers. When it happens, we carry out our investigation in a thoughtful manner, using in-depth knowledge of our processes, combined with structured problem solving, to determine the issue. Once we’ve done that, we contain the situation, hunt down the root cause, verify the solution, implement corrective action, and prevent it from happening ever again. But the question of assessing reliability risk of material that’s already in the customer’s hands isn’t a natural outcome of the corrective action process.
Our quest to measure this risk came to fruition at the end of 2010, when a single part was returned from a customer. We submitted this part for failure analysis and uncovered a rare defect. It was a déjà-vu moment, because we had first discovered this type of defect some 12 years earlier (we’ll get back to that later). This time, however, we were able to finish the story, thanks to some good fortune on our side: The customer’s fallout had happened to coincide with a manufacturing lot that had been split during module assembly and packaging, prior to final test. This split meant that the customer got half of the material, but there were still over 30,000 virgin parts on our site, which enabled us to provide a definitive answer to the customer’s toughest question of all: What is my reliability risk?
The situation that we were in is very rare. Normally, by the time an anomalous device is returned to the supplier and analyzed down to the true root cause, the customer has consumed all of the
Figure
2.Measured fallout risk for special lot,0.5 percent total affected parts
batches in question. That makes it incredibly challenging for the supplier to assess reliability risks, because they don’t have examples on hand that feature the anomaly. So when a customer asks: “What is my reliability risk?” the supplier’s best answer is “What fallout have you seen so far?”
Digging deeper
If you dig into this awkward situation, you’ll realise that we have uncovered a true maverick lot. In other words, this is a batch of material that had no anomalies detected by the supplier, yet the lot eventually produced some fallout in the customer’s hands. At this point, all the information on the severity of the problem is coming from the customer. This means that unless the supplier can somehow recover a statistically significant sample of the batch that’s in question, there isn’t any more that can be learned outside of the customer’s experience. When this happens the customer is far from happy: When they ask questions to the chipmaker, the response they get is not an answer, but even more questions.
Let’s return to our particularly fortunate situation where samples were on hand to assess risk. In this scenario, the task is different from typical reliability success testing, which is used to qualify devices. And the goal is very different too. The focus is no longer capability testing, which is designed to wear out the entire population and measure degradation along the way – instead the hunt is on for defects, which are ideally found in a way that emulates the stress in the lifecycle of a typical consumer device. In our case we decided to age the parts and measure the resulting fallout, because this allowed us to determine the risk in timeframes that are useful for the customer.
Figure 1.Expected reliability fallout for normal lots 66
www.compoundsemiconductor.net January / February 2013
Our second slice of luck was that we could detect the particular defect and isolate it from all other forms of degradation with an external electrical measurement, which was performed with production test equipment. We aimed to complete final component testing, closely approximate customer-applied stress,
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