Conformal coating and PCB potting, only some modules, art projects

I found PCBs with only some modules coated like this from Festo 404 (and 101) PLC series

I’m not sure about the purpose, and why there are only some coated. It hides the labellings and also gives protection (moisture, gases, heat, etc). These here are near a connector. I also don’t know what these modules are (reminds me of RAM modules but are soldered onto the PCB). Similar modules on the left aren’t coated.
I think it would be a good art project, but needs an open PCB or acrylic case.
I never heard of that. Does anybody has experiences with that?

Is the coating there to keep that section of the PCB secret from normal people?

This looks like a controller for automation, like controlling motors or manufacturing robots.
So, the vertical, covered components, which are behind the connector are likely for dust/residue protection coming through the small gap.
This would also protect the onboard components behind, like a wall.

Yes this PLC card is for automation.
I found another card Festo E.CDC with a coated module (same style and color) horizontally in the rear and not near a connector (neither front no rear).

Also strange the left part, here a close-up from another card

The back looking the same.
Maybe just for an optical design?
Although usually inserted in a rack.
I haven’t found coated modules on PLC cards from other manufacturers.

What’s strange here? Those pads and tracks are very wide, but then each chip is a 0.5A switch, so we are looking at high currents, so low resistance (and low inductance) might well be important.

It’s hard to tell here, but those black modules just look like SIP arrays of some kind, probably some kind of passive.

The SIP boards in the first image are what are called “hybrid modules” or “hybrid circuits”, they’re very common in equipment from the mid-70s to mid-90s or so. They contain both integrated circuits (and/or discrete silicon devices) and passives that are inappropriate for inclusion in an IC package. You find them a lot in radios and similar devices, due to their need for relatively large values of inductance and capacitance for filters and resonant circuits.

Wikipedia has a page on hybrid ICs here.

I don’t know why some are potted or coated and some are not, but it is very common for them to be so; the brown coating in the first image on this post, a yellowish coating much like that frequently found on tantalum and dipped ceramic capacitors, and a flat black coating are common, in my experience. It may be related to specific military or industrial (e.g., SAE or DIN) requirements for foreign material intrusion or vibration or similar.

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Thanks very much! Another mystery solved.
I think the PLC 404 is from the (mid) 90s. So quite late for that.
Maybe it’s for compatibility with the previous product line or even same parts.

PLC technology tends to lag behind by a fair margin (at least a decade), in my experience; PLC users are usually not super keen on being early adopters. :wink: It’s possible that this style of hybrid module is still in use in military and industrial settings; I haven’t seen one in a consumer device in years.

It’s possible that some of the modules were bought from a supplier that routinely coated their modules.

I now found a card from Siemens (SIPART line) with coated short modules. I think from 1994.

3 horizontal ones on top (coated dark red)
3 non coated ones in the middle
and 2 vertical coated ones below.

I found similar non-coated thin plastic modules on Siemens SIMATIC probably S5 PCBs (1987?) white/red alternating.