# Conformal coating compatibility - parylene- or polyurethane-based vs aqueous NaSiO4

How compatible are parylene-based or polyurethane-based conformal coatings for tin-whisker mitigation with aqueous $NaSiO_4$ (sodium silicate) conductive coatings used for EMI\RFI\ЕSD protection in electronic circuit boards?

Which would I apply first?

Can I then pot the assembly?

The subject of tin whiskers is complex, not least because no-one knows the root cause. Because of this, mitigation is at best a guess with perhaps some empirical evidence (but no two studies appear to come up with the same answers).

This backgrounder might help a bit.

If you parylene a PCB, then rework is nigh on impossible; repair is by replacement of the board (I remember updating a quote for a major aircraft manufacturer when they asked for new units to be parylened - they were quite taken aback at the cost of repair).

There is no conclusive evidence that any particular type of conformal coat inhibits tin whisker growth, although a double coat of standard conformal coating seems to help in reducing the rate of growth.

Beware of any company that claims to have solved this; NASA has been looking at this for decades (with many commercial companies) and has yet to find a definitive answer.

Whatever coating you use, the potting options will be limited (due to the nature of the coatings, some solutions will not necessarily be available depending on the coating you use); this is dependent on the specifics of the conformal coat and you should consult the conformal coat vendor as to potting solutions available.

I know this is a (very) incomplete answer, but that is due to the fact that tin whiskers are not understood (although they have been around for a long time).

• Parylene is not a good coating for whiskers, too thin. Hard coatings, 3-4 mils minimum, do work however. – Robert Endl Jan 24 '17 at 2:22