By Zhihong Yu
The accidents caused by a major automaker’s vehicles untended acceleration have haunted many customers and still the manufacturer’s answer is pretty vague. This reminds me of something similar that happened a year ago. I used to work for a design engineering company where we produced a line of motor controller products for small electric vehicles. Those included golf carts, scooters, motorized shopping carts, etc. One day there was a weird customer return, complaining that the cart he bought actually accelerates on its own, which, of course, sounds very terrible. We lifted the cart up, let the wheels hang in the air and tried to troubleshoot. At first the firing key seemed to perform normal, then as we went back to discuss the situation, all of a sudden the cart started to accelerate on its own.
Since we were able to reproduce the problem, we opened the PCB, and one of our technicians found the cause: the key-on control input line was shorting. But why? I did some troubleshooting and located the failure. A capacitor array we use to filter the noise on that control line got shorted. Under a microscope, I found there was a tiny black line/crack crawling across the capacitor, which got it shorted - means key-on to the microcontroller. Those caps had a good rating, but as we found that the PCB was mistakenly not conformal coated, it was very likely that some moisture combined with voltage bias caused a mechanism called Conductive anodic filamentation, or electromigration. As I understand, due to pollution, some copper gets iodized and grows across two polarities on board.
To solve the problem, we enforced a better manufacturing and testing process, and most importantly, we replaced the 4 element 0805 capacitor array with four individual ones of same size, that array has too little creepage distance and can easily get short. Although creepage distance is not needed for 5V, excessive moisture hurts more than electric potential. We never had that problem again.
Zhihong Yu is an experienced power electronics/embedded design engineer and has served multiple companies. He is now an electrical engineer in Enova System on high power electronics design for electric vehicle applications. He got his BS in China and a MS in the U.S.