Early on in this week, we received our first batch of Brainwave Pros from our contract manufacturer in China. This was a very highly anticipated event, as these boards are one of the last long(er) lead time items we were waiting on to ship printer kits.
When we receive a new batch of boards in, Mike, our electrical engineer, goes through an elaborate procedure to "bring up" a new board. It basically involves monitoring key circuit parameters while slowly energizing a circuit with a current-limited power supply. The idea is that if something goes wrong, the current limiting portion of the power supply will hopefully prevent a catastrophic failure that involves magic smoke leaving our electronics.
When this batch of Brainwave Pros were brought up, we noticed that the boards drew more power than normal. This is a very bad situation as it typically indicates that there is a short on the board.
Long story short, after some serious troubleshooting kung fu, our rockstar electrical engineer located the problem. The Brainwave Pro board features protection components called transient voltage surpression (TVS) diodes to clamp the back EMF force from the motor during a sudden disconnection. In other words, although not recommended, the board can survive an accidental disconnection of the motors while energized. Unfortunately, after the technical hand-over to our contract manufacturer, somewhere along the line someone added an extra decimal point to a part number by accident while compiling the work instructions and bill of materials. As a result of this transcription error, the wrong, under-spec TVS diodes were installed onto the boards, and these diodes were blowing and shorting out during normal operation of the board.
We have now tested a board by removing the offending TVS diodes and the board functions just fine. We have ordered replacement TVS diodes of the correct specification from Digikey and installed them to verify that the board does work with the correct TVS diodes installed (and that protection of the driver chips does work). At this point, we have instructed our contract manufacturer to purge all TVS diodes in their stock and refresh their stock with proper TVS diodes, sourced from Digikey (China).
Unfortunately, the goal of the manufacturing pilot was to verify that boards from our new CM will energize and program correctly without intervention on our part. We failed that test, and for peace of mind, we will have to do another round of manufacturing pilot. (For all we know, they may also be putting on these diodes backward - we don't know what we don't know). The delay is expected to be an approximately two week time frame. Even though the risk is very small, we just cannot afford to have non functional boards land. The cost of reworking a board here in the US to replace the offending diodes by a professional board house can be almost half the cost of the boards, brand new (material and labor) from our CM.
In the meantime we have 24 boards state side that will require rework to function. But that's a lot better than having over 400 boards that require reworking.
-=- Terence & Mike.