Electronics > Manufacturing & Assembly

Encapsulation for ATEX

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48X24X48X:
Hi guys,

I'm new when it comes to conformal coating encapsulation on components and PCB. The ATEX certification lab ain't recommending a specific brand and model of conformal coating encapsulation to be used as they do make money when you fail the testing. Does anyone here is familiar with this and has a regular brand or model to go to? I wanted to apply conformal coating encapsulation a LTE module which is the only component on the board that couldn't conform to the thermal ignition requirements under temperature class T4.

I have heard of HumiSeal 1A33 but not sure if there are better choices. Any suggestion would be great!

mikeselectricstuff:
Does the LTE module have any exposed components in the RF section? If so then conformal coating may change the dielectric characteristics and affect its operation.
It's a long time since I did any ATEX stuff but I don't recall conformal coating being a valid way to deal with surface temperature issues, only to reduce creepage distance requirements.   

48X24X48X:
Hi Mike,

My mistake, it should be encapsulation as I have mistakenly use that term. That is one part that I have concern on the RF although the ATEX certification engineers said "many of our clients" did this with the wireless devices. I guess that would need some testing to see how bad it affect.

floobydust:
I have some experience with this as far as Zone 0 and 60079-11.
The potted modules were so much hassle. The LTE modules required a silicone caulk to first seal them up around the can, before the final potting. Encapsulant does change the RF characteristics a lot, shifts tuning to the point they don't work if it gets inside. I think it's the antenna output stripline that gets cratered. If you have a bias-T then that will get detuned as well. Sometimes the high-band just doesn't work.

I can dig for what encapsulant we were using. It was a two-part oven cure at 60°C. Had to pass the poke/hammer test and high temperature requirement. Problems with the thermal expansion coefficient because of the product's wide ambient -40°C to +60°C. It would shear and crack SMT parts on the boards. So had to first pre-coat the board with a soft silicone encapsulant, and then pot it with the hard encapsulant.

Then we had Manufacturing bungle the antenna connector, or mix it up with GPS antenna connector. This resulted in your basic unusable potted blob. RSSI would be very low but it worked on a test fixture. Only found out after a batch of 200 modules were not working and then tossed in the garbage after X-rays revealed that mistake.
You couldn't do a F/W upgrade because the connector for that was potted, and USB is not I.S. unless you workaround. Even then F/W upgrades sometimes needed access to special pins on the cell module, they could get bricked.
It was so costly and gross, I came up with a way to design out the use of potting compound and sucessfully put the product through HazLoc approvals.

The cert lab will not reveal what encapsulants meet the standards because some other customer paid to find that out, and why should you benefit from their expense. It's just awful to be paying many dollars repeating tests others have done. If you choose the wrong compound, thousand $ wasted.

48X24X48X:

--- Quote from: floobydust on October 08, 2021, 02:33:14 am ---I have some experience with this as far as Zone 0 and 60079-11.
The potted modules were so much hassle. The LTE modules required a silicone caulk to first seal them up around the can, before the final potting. Encapsulant does change the RF characteristics a lot, shifts tuning to the point they don't work if it gets inside. I think it's the antenna output stripline that gets cratered. If you have a bias-T then that will get detuned as well. Sometimes the high-band just doesn't work.

I can dig for what encapsulant we were using. It was a two-part oven cure at 60°C. Had to pass the poke/hammer test and high temperature requirement. Problems with the thermal expansion coefficient because of the product's wide ambient -40°C to +60°C. It would shear and crack SMT parts on the boards. So had to first pre-coat the board with a soft silicone encapsulant, and then pot it with the hard encapsulant.

Then we had Manufacturing bungle the antenna connector, or mix it up with GPS antenna connector. This resulted in your basic unusable potted blob. RSSI would be very low but it worked on a test fixture. Only found out after a batch of 200 modules were not working and then tossed in the garbage after X-rays revealed that mistake.
You couldn't do a F/W upgrade because the connector for that was potted, and USB is not I.S. unless you workaround. Even then F/W upgrades sometimes needed access to special pins on the cell module, they could get bricked.
It was so costly and gross, I came up with a way to design out the use of potting compound and sucessfully put the product through HazLoc approvals.

The cert lab will not reveal what encapsulants meet the standards because some other customer paid to find that out, and why should you benefit from their expense. It's just awful to be paying many dollars repeating tests others have done. If you choose the wrong compound, thousand $ wasted.

--- End quote ---

Thank you so much for sharing such a detailed experience.

I read yesterday an article on Electrolube that mentioned it is not the resin that attenuate the RF signal but the filler material that used together with the resin. If the filler portion is either very little or not present at all, it *shouldn't* affects the RF signal. I'm not sure how true is that but I guess it will be a countless trial after this. They did mentioned some part number from the catalogue suitable for RF application. During the trial, the lab said I can use a power resistor (s) to represent the LTE module with the same exact power wattage expected. Of course if it pass the thermal ignition test, then I need to check whether the module still works and behaves as it supposed to be under the encapsulation.

Antenna and it's connector can get away from encapsulation if you can put a DC blocking capacitor (with > 1.5mm pad separation, yes it look silly when your other parts are 0402 and suddenly a giant 1206 appears) on the signal path.

The module is the only one I couldn't get it to conform to the surface thermal ignition but it seems more needs to be done from here onwards.

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