High failure rates after pcb coating using DC 3140 RTV

Question

We have been protecting PCBs using Dow Corning 3140 for many years now without any problems. Last couple of months however, one board after the other is failing after coating. We can't think of any other possible solutions. Hopefully one of you can point us in the right direction.

What do we do? The boards I am referring to are control boards for a specific type of machinery. They come in different types for different machine capacities. The boards are all very much alike. The problems only occure with the "professional" range of machinery.

To make sure the boards are in good condition when delivered to us, the machine with the board is tested completely. No problems so far. In the next step, the board is removed from the machine taking all possible ESD measures. The board is never touched without the necessary connections. Transport is only carried out using the pink bags. Consequently, the DC 3140 is applied using a 30 ml seringe. The boards are cured at room temperature. We do not use any cleaners, primer or whatsoever, the silicone is applied directly to the board. After coating, the boards are mounted to the same machine again and we end up with a failure rate of more than 80% ...

We have discussed this issue with the OEM but without any probable cause. To make sure that the problems are not ESD induced, we have invited an ESD expert to our workshop to check and measure anything that could be measured. Conclusion: all measures are in place, co-workers are using the equipment properly, nothing to improve.

The last possibility we could think of is the DC 3140 itself. Could this be the cause of all this trouble. As stated before, we have been using this silicone for many years now without any problem whatsoever.

Do you have any suggestions on a probable cause for our mysterious failures?

Answer 1

I am going to take an educated guess and suggest that @brahns is probably right and it's related to the process used at your contract manufacturer. Some parts of microcontroller boards (the crystal in particular and sometimes the reset circuitry) are sensitive to leakage on the board and the use of "no clean" flux can totally screw things up.

There seems to be contamination left on the board that can be affected by moisture humidity and cause excessive leakage. In the case of sensitive instrument boards, the slightest bit can cause problems. Traditional RA fluxes require solvent cleaning (which is a pain for the contract manufacturer, lots of environmental issues) but they're the best. I think water soluble is somewhere between the two.

The contamination is extremely hard to remove- solvent and mechanical scrubbing does it, but it's not easy nor is it cheap.

Anyway, that would be the first thing I would investigate, and see if they've made any changes to their process recently (of course they may be cagey about it, for fear of lawsuits or other unpleasant repercussions, so you may want to go at this laterally- compliment them on how nice and shiny the boards are or something- did you change anything?, then whack them with the 'we have a problem').