Circuit Diagnosis, Third Quarter, 2012,

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Swapped Parts

One of my old clients was still producing a product I had designed for them decades before. Over the years, they had to make a few changes here and there. Most of the changes were in response to some older parts which had become obsolete and had to be replaced with more readily available parts. The company had just received a batch of new boards soldered by one of their suppliers. None of the boards worked properly. Over the years, they had never had any problems with the design or the boards so they were puzzled. I told them to send me a few boards to look at.

At first, the boards looked perfectly fine. But, after a closer inspection I spotted the problem. The circuit used one surface mounted power diode and a number of tantalum electrolytic capacitors.

As shown below the power diode and one of the tantalum capacitors looked very much alike. Both had about the same size and both had a painted strip on the body of the part. The assembly company had someone who managed to get the two mixed up. The diode was soldered in the capacitor’s spot and the capacitor was mounted where the diode was supposed to go. Fortunately, the error did no harm to any of the other components on the board. The boards were shipped back to the assembly house for retrofitting.

Most tantalum parts have values and voltage rating markings. But, this particular part from China did not have such markings. I strongly suggested that the company not buy such unknown parts and stay with parts from known manufacturers with clear markings.


Power Diodes	Tantalum Capacitors


Power Diodes	Tantalum Capacitors

Third Quarter, 2012

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