Fail Of The Week: When the Epoxy-Coated Chip Is Conductive

Every once in a while, you’ll find some weirdness that will send your head spinning. Most of the time you’ll chalk it up to a bad solder joint, some bad code, or just your own failings. This time it’s different. This is a story of weirdness that’s due entirely to a pin that shouldn’t be there. This is a package for an integrated circuit that has a pin zero.

The story begins with [Erich] building a few development boards for the Freescale Kinetis K20 FPGA. This is a USB-enabled microcontroller, and by all accounts, a worthwhile effort. So far, so good. The problem with the prototype boards was soon apparent. On some of the boards, the external 32 kHz oscillator was not starting. Resoldering the oscillator or microcontroller sometimes solved the problem, but not always. This is troubling, because that means the issue isn’t code, and it’s not the PCB. This is going to take a deep dive and a good inspection microscope.

One of [Erich]’s friends, [Christian B] somehow found the problem. When the Freescale K40 is manufactured, the die is carefully laid in a chip carrier and coated with epoxy, putting it in a small QFN package. The problem is, there’s an extra connection sticking out of one corner of this chip. This is just an artifact of the chip carrier, but if you leave exposed metal connected to ground, something is eventually going to go wrong.

The best guess [Erich] has is that this additional connection is from the manufacturing and packaging process, with the exposed metal pad in this application being bridged to an adjacent pad. Now, if there’s one failure to [Erich]’s design, it’s that the trace comes out of the pin on the adjacent pad at 90 degrees; this isn’t a best practice, but most of the time you can get away with it. This time, though, somebody got burned.

We don’t know how [Christian] ever found this issue. When you look at a tiny QFN package, you don’t expect there to be an extra pin attached to ground that can be easily bridged with a bit of solder paste. It’s either a lot of luck or skill to find this problem, but it’s a great example of the weird things you have to look out for.

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