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The problem of moisture ingress in outdoor electronics equipment?

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treez

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Is it true that outdoor electronics, such as streetlighting, has wide pitch IC’s to avoid the problem of shorting due to moisture eventually working its way into the enclosure? (ie thay use SOIC8’s instead of fine pitch TSSOP’s, MSSOP’s etc). Even without moisture having to seep through the gasket, (due to ‘breathing’ due to expansion/contraction) the electronics components themselves actually contain moisture.
Also, is it true that they hang PCBs of outdoor electronics upside down, so that the moisture doesn’t settle on the PCB, but kind of drips off onto the case?

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Also, is one of the reasons that they often put squidgy silicon inside electronics enclosures for outdoor equipment because it absorbs water?
 

I think the answer is no to every point.

PCBs should always be protected if in a 'hostile' environment but coating with an insulating compound or potting in resin is the normal method.
Its true that components contain some residual moisture but in most cases the manufacturer will recommend you bake them at some temperature and for some time before assembly if they have been stored in unsealed packages. This is partly to avoid gassing during the soldering and partly to prevent long term corroding.
Hanging a PCB upside down has no advantages, and of course it assumes the board is single sided! If it is single sided, drips hanging from components would be no better than puddles around their tops.

Brian.
 
Outdoors equipment uses conformal coating. Did some preliminary work on a project that required the board be conformal coated as it would have been outdoors inside a box on a pole outside an apartment building. Pretty awful conditions for the electronics with extreme temperature variations and no internal cooling/fans all heat had to be conductive to the box.
 

The electronics within my washing machine (hot, humid and vibration) is protected (what a life) with a thin coating that means nothing can be repaired. The outdoor 12V supply for the LED street lamps also have the same (I guess that is also conformal coating) type of protection- but I really do not know.

But what is "squidgy silicon"? If you are talking about the silica gel powers often found within the boxes, they are simply useless because they have a very limited capacity to absorb moisture. Although most of them can be regenerated by heating they are useless for the purpose you are talking about.
 

Tonight my new very cheap Chinese solar garden light stopped working. Looking inside I see that most parts are rusted away. The battery connectors are rusted the most. I think they used cow dung to make it instead of plated copper.
The solar panel is on top and it has very poor sealing around it so rain on it seeped inside the product.
 

But what is "squidgy silicon"? If you are talking about the silica gel powers often found within the boxes, they are simply useless because they have a very limited capacity to absorb moisture. Although most of them can be regenerated by heating they are useless for the purpose you are talking about.
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I guess the vague description is referring to silicone gel molding material. It's a kind of soft silicone rubber, of course not intended to absorb moisture but to keep moisture out. See eg. Wacker SEMICOSIL 912, 92x
 

A very common problem Audioguru. Perhaps it never rains in China. I live in one of the wettest parts of Europe and even after the special 'Brianization' process of silicone sealant in all the gaps and silicone grease on the battery contacts, they still rot away within a few months.

Back to the original question, conformal coating works well but can be difficult to apply effectively. It will give excellent protection where it provides good covering but unless special techniques are used, it may not penetrate area underneath components and it can make matters worse if it traps moisture instead of excluding it. There is also the temperature issue, prolonged high temperatures can make it decompose as can long exposure to UV light which might be an important issue in Treez applications.

Having said that, I use HPA acrylic spray on most PCBs I make and rarely have problems.

Brian.
 

Thanks, i hope daylight doesnt gradually decompose conformal coating, or solder resist, because our products are outdoors 24/7 and have perspex enclosures..PCB and all components always exposed to daylight...accepting of course night times.
 

treez said:
Thanks, i hope daylight doesnt gradually decompose conformal coating, or solder resist, because our products are outdoors 24/7 and have perspex enclosures..PCB and all components always exposed to daylight...accepting of course night times.
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UV exposure is a whole different issue.
 

That's a very informative article.

I would add another 'gotcha' to the equation, if LEDs are used and they have soft lenses, adding a coating over them may prevent them expanding as the LED warms up. The soft lens (a silicone material) not only does the optical work, it allows expansion of the die and bond wires with minimal stress on them, if encapsulated it may impact on mechanical durability.

Brian.
 

treez said:
or solder resist, because our products are outdoors 24/7 and have perspex enclosures....
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Solder resist is epoxy material; i.e., they are 3-D cross linked plastics. Most often they colored, hence they will not be affected by UV. Good PCB (not the cheap ones) is fiber glass stuff bonded with epoxy. The solder resist will not come off the board even under harsh conditions.

The spray on conformal stuff is most likely thermoplastic; they will become soft even around 100C. The perspex is the same stuff as acrylic; use a color box (you can charge the customer extra for the color!!) that will remove all the UV for all practical purposes.
 
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Surely from a moisture point of view it would be good not to use the PIC16F18856 in 28UQFN package?..because that has just 0.2mm spacing inbetween the pads, and it wont be possible to get a sliver of solder resist in that gap, so surely it would be best to use a wider pitch verson of this micro for use in outdoor electronics?
I appreciate they can be coated, but its an extra step at manufacture time and costs money.

Pg 652 shows land pattern of 28UQFN package
https://ww1.microchip.com/downloads/en/DeviceDoc/40001824B.pdf
 

treez said:
Surely from a moisture point of view it would be good not to use the PIC16F18856 in 28UQFN package?..because that has just 0.2mm spacing inbetween the pads, and it wont be possible to get a sliver of solder resist in that gap, so surely it would be best to use a wider pitch verson of this micro for use in outdoor electronics?
I appreciate they can be coated, but its an extra step at manufacture time and costs money.

Pg 652 shows land pattern of 28UQFN package
https://ww1.microchip.com/downloads/en/DeviceDoc/40001824B.pdf
Click to expand...
So you're expecting solder mask to be adequate for your product to meet environmental concerns? No solder mask:bad. Solder mask: good enough. Remind me not to by any of your products.
 

Purely from the moisture point of view, I do not see any serious problem. When they run they run hot and that is not favourable for moisture condensation. Recall that if there is no condensation, there will be no damage. On the other hand, if it gets flooded in water, then it does not matter whether the gap is 200um or 1000um.

Can moisture condense when the device is not running? Perhaps yes and small crevices are the ideal point for the condensation to start. What I mean is that condensation can start in fine gaps and cracks and crevices (very small radius of curvature) even at 98% RH. Immediately after condensation, electrolytic corrosion begins. Once the condensate has got enough ions, the problems begin. Till that time you will be fine.

A good box is the solution; make sure the wire outlets are sealed nicely.
 

Not sure if this helps but have you researched or ever used microporous venting in your designs.
 

Good idea Kajunbee, I hadn't thought of that solution but thinking about it, hard disks suffer the same problem of needing a clean enclosure while maintaining fairly constant pressure and they use micropore vents.

More info here:

Check the link at the bottom of the article.

Basically a pressure vent to allow gas expansion but with pores small enough that water molecules can't get through.

Brian.
 

In fact, this latter approach seems to solve several problems with a single solution. The moisture in the PCB, for example, when in the presence of dirt, depending on its composition which usually has ions, forms a conductive mud impacting deeply on the circuit reliability. The microporous vents seem acting as a barrier for both dirt and humidity, but it is not clear if it should be regularly cleaned to keep its original ability to balance the internal with the external pressure.
 

If it is a viable solution it probably wouldn't be something that could be put into use without a lot of testing first.
Also since the fixture is vented the stress on seals and gaskets from pressure and vacuum changes would be considerably less. This could possibly translate into savings in construction materials cost. Money that could be spent on the electronic components. Thats if the powers that be don't put it in there pocket.
 
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All of these "outside" issues are significantly amplified on an automotive environment.
It is not only water and dirt, it is gasoline, diesel, lubricating oil, brake fluid, battery acids. SAE has a long list of potential contaminants. Which includes coffee and carbonated beverages.

The automotive industry has solved these issues via a two-part urethane potting for in-cabin applications, and hard-epoxy for engine bay or other exposed applications.

Two-part urethane can be repaired. But not hard-epoxy.
 
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