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I've just replaced my smoke detector. It was manufactured in 2006 by BRK. This is an ionization-style detector. I took it apart and the track side of the circuit board is covered with a thick waxy substance. It's clearly part of the manufacturing process, and not a by product of nesting spiders. It's 1 - 2 mm thick and fairly uneven, translucent and soft. You can scrape it away with finger nail pressure. Applying a match flame to it causes the substance to melt and drip off just like candle wax. It's not flammable in it's liquid phase.

For all intents and purposes, it looks and behaves just like candle wax. Can we still really be using candle wax to protect pcbs this century? And if so, why?

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That substance is what is known as 'conformal coat'. It is used to protect the traces and connections from moisture.

If this is an ionization-style detector, there are nodes within the circuit that operate at extremely-high impedance. The conformal coat protects those nodes from contamination.

There are many different conformal coat products available. I've used a product similar to what you describe in the past.

GE and Dow Corning make the conformal coat products that we currently use.

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  • \$\begingroup\$ It's cheap conformal coat, too. The good stuff doesn't behave like that. \$\endgroup\$ – WhatRoughBeast Sep 18 '15 at 1:14

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