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Design007-Oct2018

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42 DESIGN007 MAGAZINE I OCTOBER 2018 New Landing Design to Reduce Thermal Pad Failures Connect the Dots New Column by Bob Tise and Dave Baker, SUNSTONE CIRCUITS You've finally finished your design. All the traces are correct, and the integrated circuit (IC) landings are to the manufacturer's speci- fications. A short run of test boards performs perfectly. For best results, you select a repu- table domestic board house for production and a quality assembly shop to do the soldering. When the finished boards arrive, everything looks great. You're in high spirits and congrat- ulate yourself on a job well done. Then the reports start coming in: • "My board worked for a week but won't boot up now. I need a replacement." • "We're seeing a high percentage of failures during quality assurance." • "My order is dead on arrival. Did you test it before you sent it?" You've done everything by the book and passed every preliminary test, but things are still going wrong. What's happening here? We found ourselves in a similar situation while working on a client project. The failure rates were only a small percentage, but in vol- ume, those costs quickly run into thousands of dollars. We poured over every possibility and traced the problem down to a single high- power quad flat package IC where the center thermal pad was not being soldered consis- tently. Nothing was wrong with the board or the soldering process; the default method for soldering heat slugs to boards was just inher- ently flawed. Looking at the costs and reliabil- ity problems, we knew we had to do better. After testing a variety of different designs, we found one that was vastly more reliable and

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