Cleanliness Testing/Conformal Coating

If I clean my assembly and find no visable contamination with the unaided eye, do I need to do cleanliness testing prior to conformal coating?

reply »

| If I clean my assembly and find no visable contamination with the unaided eye, do I need to do cleanliness testing prior to conformal coating? |

More than likely, you're okay if you can't see any contamination with the unaided eye. However, the type of flux you are using may be a big factor in whether you'll get conformal coating adhesion. In my previous life, I was a conformal coating guru (they used to call me a SILICONE-HEAD). Anyway, I found a study which showed that most NO-CLEAN fluxes WILL NOT interfere with conformal coating adhesion, for ALL TYPES of conformal coating. The study can be found at http://www.empf.org.

If you're using a water soluble flux, and then aqueously washing your boards with D.I. water and seeing no contamination with the unaided eye, then you should be okay.

I would suggest that you call a chemist at your conformal coating supplier and tell them what kind of flux you're using. They'll advise you on whether you'll have adhesion or incompatibility problems. Also, you may want to get your boards tested for SIR/ionic contamination testing after they've been through your cleaning process (just to be on the safe side). If the results are good, then long term, you won't have to do any type of cleanliness testing.

reply »

| If I clean my assembly and find no visable contamination with the unaided eye, do I need to do cleanliness testing prior to conformal coating? | Douglas,

It is highly advisable to test for ionic contaminants because a coating seals in as well as out.

As a simple test suggestion, take a container big enough to hold your circuit assembly and fill it with a mix of 75% IPA analar grade and 25% de-ionised water. Measure this liquids resistivity than put the assembly in for around 15 minutes. If you can shake the container this will help to get the liquid under all components. Now remove the assembly and re-measure the resistivity.

If you see a big difference, then it is good indication that you should clean before you coat.

I will not attempt here, to make the calculation of resistivity to a level of sodium chloride equivalence - I just want to give you a simple way of testing.

A coating can accelerate assembly failure just like putting paint on to a dirty, unprimed surface. You may not see the contaminant but then you probably cant see under the devices.

Hope this helps, Graham Naisbitt

reply »