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PCB Fabrication Problems

I got a frantic call from one of my old clients.  About three years earlier I had designed a detergent soap mixing control circuit for them.  The circuit monitored the alkalinity of a soap and water mixture and by controlling the water flowing into the mixing tank; the circuit could maintain a fixed soap concentration.  They sold a few thousand mixing systems per year.  They typically bought about 500 printed circuit boards at a time and soldered the surface mounted components onto
the boards in their own shop.  For about three years they didn’t have any problems.  But, on the day I got the call, my client said they had a rash of strange problems. About 75% of the boards would not pass a functional test.  The symptoms were all over the map and were never consistent.  To keep things, going, they were forced to “cherry pick” from the 50 or so completed units and installed only those units that passed a functional test into their machines.  They put the rest of the nonworking units aside.  I requested that they send me a few of the defective boards as well as a few blank unsoldered boards.
My client assured me that they had not made any component changes.  The trouble all started when they began using printed circuit boards from the batch of 500 they purchased.  But, since the boards came from the same manufacture, that cause was in doubt.  They were perplexed.

On the following day, I received three non-working circuit boards and three blank boards.  I fired up the boards.  The symptoms were indeed strange.  I picked one defective board and started the process of tracking down the source of the fault.  It was time consuming but I finally found the problem.  I found a spot of extra copper, which was shorting out two traces on the board.  The spot was a star shaped flake of copper, under the green solder mask.  When I removed the short, the board worked perfectly.  Under magnification, I looked at every

Soap Mixing Control Board
trace and every plated through hole on the other boards.  Sure enough, in random locations, on both sides of the circuit board, I found the same star shaped solder flake. On one of the blank boards, the copper flake fell in a blank area of the board, where no traces or components resided.  Clearly, that board would have work fine, since the solder flake would not have caused any problems.  This flake was some dust or dirt that somehow got into the circuit board fabrication process and resulted in a bit of extra copper deposited on the boards in a completely random location.
So, in conclusion, my client was able to cherry pick about 50, from the stack of 500 blank boards, where the solder flake fell in a safe area.  Knowing what to look for, they also were able to fix about 50 of the soldered boards, removing the copper flake.  The rest of the blank boards were shipped back to the manufacture.  Although only about 400 boards were returned to the PC board fabricator, they quickly provided 500 new boards without charge.  In about a week, my client’s production line was up and running again.

April 2010     Issue 8

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Envelope
Circuit
Diagnosis
Experimenter's
Corner
Good Idea
gone Badly
New Products Rants &
Raves
What the World
needs Now
Wily Widget


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