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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?

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  • \$\begingroup\$ No sort of baking process before coating? \$\endgroup\$ – Matt Young Nov 17 '14 at 19:19
  • \$\begingroup\$ No. The boards are mounted to the machine shortly after production. The boards are clean at arrival our our facilities. No treatment whatsoever by us before we apply the coating. \$\endgroup\$ – Marc Moolenaar Nov 17 '14 at 19:22
  • \$\begingroup\$ Do the boards always fail in the same way? \$\endgroup\$ – Majenko Nov 17 '14 at 19:25
  • \$\begingroup\$ I worked for a company a few years ago where DC3140 was also used as a 1st stage conformal coating (before potting the boards in resin). They had a batch of boards failing in the field which were initially working fine. The problem was eventually (and expensively) traced to ionic contamination on the PCB due to a change in the assembly contractor's flux and cleaning procedures (even though 'no-clean' flux was used). \$\endgroup\$ – brhans Nov 17 '14 at 19:31
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    \$\begingroup\$ You talk of failures but do not exapnd on that. This means nothing tangible to go on whatsoever. \$\endgroup\$ – Andy aka Nov 17 '14 at 19:37
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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').

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    \$\begingroup\$ You could be quite right ... If I am not mistaken, our supplier has moved production from Japan to Thailand. Worthwhile investigating! Thanks guys! \$\endgroup\$ – Marc Moolenaar Nov 17 '14 at 20:22
  • \$\begingroup\$ It's also possible, although somewhat less likely, that one of the contract manufacturer's suppliers has changed their process, or that the change was an uncontrolled and inadvertent process change -- everybody's favorite kind. \$\endgroup\$ – Edward Nov 17 '14 at 20:22

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