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PCB Fabrication Problems |
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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. |
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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
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| Soap Mixing Control
Board |
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| 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. |
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