The devil’s in the details

Brian Fernandes, Senior Design Engineer -September 09, 2010


About 10 years ago, while working as a design engineer for a capacitor-manufacturing company, I was responsible for designing new products that were not in the company’s product line. One day, my boss passed me the specs for a new MPP (metallized-polypropylene) shunt-capacitor form factor that we needed to develop. The specs had all the relevant details on size, capacitance, and dissipation and form factor. On the first pass, however, I noticed that the details on the application for this device were missing. That is, we knew what the customer wanted but not the application—beyond circuit filtering—for the parts.

Nevertheless, we established our priorities and set out to design, fabricate, test, and qualify a sample set before the end of the week. The customer urgently required delivery because evaluation was part of the company’s business process. We also knew that the design team at the customer’s end would surely know what the application required, and this part was only a small component in a big system. So we proceeded to fulfill the customer requirements. We chose the standard design parameters and materials. The manufacturing team had to manufacture the sample set under close supervision; we then tested and qualified the samples and agreed that they were ready for shipment.

Nearly a month passed, and, as often happened, our marketing team could elicit no feedback on the samples from the customer. They were probably collecting dust on someone’s desk until they were actually needed. Surprisingly, however, nearly two months after we sent them out, we received eight of the 10 samples for field-failure analysis. The task of opening the burned and badly scarred parts and doing a critical failure analysis on them was daunting.

Nonetheless, we proceeded and determined that the failure had occurred in a polypropylene puncture that had not self-healed and had then gone on to cause the capacitor to burst. We could find no signs of any material or manufacturing defect. Besides, we had thoroughly conditioned the samples during burn-in and had shipped only the good units. We had also run samples at our in-house lab for 1000-hour testing, and we had not seen any failures.

The failures received a lot of attention from our company’s top management because the customer was strategic to our business. It was now inevitable that our marketing team would have to bite the bullet and push the customer to share the application’s details with us.

A week later, we received an e-mail from our company’s field-applications team. The customer was using the capacitors to filter motor-supply noise for dc-motor applications. It immediately hit us: The motor spikes were causing heavy overvoltage impulses across the capacitor. These spikes would breach the breakdown-threshold voltage of polypropylene capacitors, and the capacitors’ self-healing would go into overdrive, leading to the capacitors’ bursting. Two simple solutions were available. We could either adequately derate the capacitors’ voltage or overdesign the parts to meet the customer’s application requirements. Either of these options would incur additional cost. Once we explained the failure and design criteria, the customer opted for us to implement both approaches.

The lessons we learned were, first, consider the application and environmental details of a device early in the design process and, second, customers themselves may fail to impart to you the small but essential requirements of their applications. Finally, in the corporate world, speed means money—usually at the cost of careful completion.

Brian Fernandes is a senior design engineer in Singapore.

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