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Street light PCB was "Open to the air"..would need conformal coating?

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treez

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Hello,
We took apart a mains connected streetlight “head” from China.
The head had a hole which was for sliding onto the vertical pole (lamp post).

This hole was quite a tight fit to the pole, but obviously not an absolutely perfect seal…so in other words, air and moisture and small dust particles would be able to get inside the head through this tiny gap between pole and “head-hole”.

Now, inside the streetlight head was the LED driver inside a plastic enclosure. We noticed that the plastic enclosure was not totally enclosed, but had air gaps around the connectors at each end of the plastic box….so in other words, the LED driver PCB and electronic components were in fact “open to the air”.

Given this “Open-air” nature, do you think that the LED driver PCB would need to be conformal coated?
I mean, for example, we noticed that there was a TO220 switching FET in the LED driver enclosure which had straight legs into its PCB footprint (and so not much clearance between drain and source pins)

Or do you think that it would even need potting to prevent dust and moisture from ingressing into and around the PCB and electronic components? (obviously some way would need to be found to prevent the potting compound from escaping while it cures during the potting procedure)
 

When you buy from China at a modest price - you essentially always get what you pay for ...
 

Thanks, and it seems i detect that you are suggesting that this "open air" nature, is not such a good thing?
I mean, from the point of view of cooling alone, its obviously a good thing....but seems like cheating.
 

I would say it depends on the degree of moisture ingress and to some degree the 'flow' of atmosphere around the components. You have to ask the question: 'how does conformal coating actually help?' and the only answer is it reduces oxidization and helps to maintain an insulating barrier where moisture might cause surface arcing. Taking an extreme case, I wouldn't trust a 100% coated board if it was submerged in water, even if the effects of capacitance through the coating were considered. A board that maintained a reasonable temperature above ambient wouldn't allow condensation to form anyway.

I agree that lower price may sway a purchaser to buy a less protected product but conformal coating isn't a magic cure for all atmospheric nasties.

Brian.
 
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Thanks, so may i then put the question the other way and ask....is having the PCB and electronic components "open to the air" (as described in the top post) an excellent technique for keeping the electronics cooler?

Maybe this Chinese company are stealing a march on the Philps's, samsungs, Osram's etc etc because philips etc all have their led drivers totally enclosed inside a plastic box.
 

The answer is definitely maybe.
'Open air' means free ventilation so as long as heat can be convected inside the enclosure, it allows the whole enclosure area to dissipate it. Encasing the board, even in a thin acrylic coating such as conformal coating is like putting it under a thin blanket, most of the heat can escape but not as much as if it were uncovered. Coatings also tend to block air passage around components, for example, blocking the gap under ICs and components by filling it in with an impenetrable barrier.

So there is no right or wrong, it depends on circumstances. Personally, if a device is always run above surrounding temperature, I would leave it uncovered unless there was an obvious risk of environmental attack. Components running at below ambient temperature I would consider according to risk of damage, taking into consideration things like high voltages or the need for very low leakage currents.

There is of course the need to ensure adequate air flow but that is a different topic altogether.

Brian.
 
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