Help identify and source a material

[Alex Eisenhut]

You know sometimes there's a thin sheet of rigid fiberglass-ish material that is found between the PCB of off-line switchers and the metal case?

(At least quality industrial ones). (I guess. Not my usual area of investigation!)

Um, what is that stuff? I found this

http://accurateplastics.acculam.com/advanced3/g5-g9-melamine-fiberglass/melaglas-trade-g5-g9-laminate-sheet-36-x-48-

Would that work? I don't even understand why that sheet exists on power supplies, if the board is high up enough, it won't short to the case? What is its purpose? To prevent flash-over from the sharp edges of sloppily-cut through-hole leads?

[tpowell1830]

I only glanced at the page, but I did not see anything regarding the dielectric strength or thermal properties of that material. In many power supplies, you have inductors that could potentially, under certain failure circumstances, cause a very high potential to exist. The thin sheet initially is to prevent an arc over to other components during such a failure. The other possibility could be the thermal conduction of hot components and this is simply a heat shield made with light weight non-conductive materials (as opposed to sheet metal). Sorry I don't have a name for that material, but the material on the page seems close.

Hope this helps...

[intabits]

Is it like thin but stiff cardboard?
I've seen stuff like that, also sometimes between bakelite terminal barrier strips and the case they're bolted to.
That stuff seems the same as or similar to "fish" paper, or "transformer" paper, used to separate the windings within a transformer.
It has a relatively high dielectric strength.

I bought some from Element14 a while ago, but it seems to be no longer stocked. they described it as "FIBROID FISH PAPER ELECTRICAL INSULATION" Manufacturer: GC ELECTRONICS (was Stock no 1999620)

[PA0PBZ]

If you google that fibroid etc... you get a lot of hits, like

https://www.amazon.com/Fibroid-Fish-Paper-Electrical-Insulation/dp/B018T7H792
http://www.newark.com/gc-electronics/560/fibroid-fish-paper-electrical/dp/94F4073
https://www.digikey.com/catalog/en/partgroup/prototyping-products-accessories/26149
https://www.ebay.com/itm/GC-ELECTRONICS-560-ELECTRICAL-INSULATION-FIBROID-FISH-PAPER-240-SQ-IN-/151103948385

https://en.wikipedia.org/wiki/Fishpaper

Why fish paper? I have no idea...

[T3sl4co1l]

Hard paper, fishpaper, vulcanized paper, just regular old (thin) FR-4 / G-10, or polyester sheet, etc.

Lots of options will do. 

Incidentally, standard "fishpaper" is UL-94 rated, but not V-0 (self extinguishing).  It's more subtle than that: it must burn at a particular rate!  I tested a strip of my own and it does indeed burn in a well controlled (but definitely not self-extinguishing) way.

A reminder that, just because something has UL ratings, doesn't mean it's magically safe, regardless of its composition!  UL is about consistency, about putting together elements such that the whole will be safe to operate, even if some of the elements (for whatever reasons -- good old cost being a big one) aren't the safest possible option.

And yes, insulation and flash arresting are the likely goals for those insulating sheets around PCBs.  In a pinch, you could just as well apply layer(s) of tape -- assuming good enough stuff, like polyester or polyimide, preferably 2+ layers (giving a reinforced layer even if the air gap falls to zero).  I suppose that would be fine in production as well, it's just that you can get whole, self-adhesive sheets punched to size and ready to apply.

Tim

[amyk]

Mica is another good dielectric.

[Giaime]

I don't even understand why that sheet exists on power supplies, if the board is high up enough, it won't short to the case? What is its purpose? To prevent flash-over from the sharp edges of sloppily-cut through-hole leads?

You usually are given creepage and clearance distances on the safety standard that you're following for your product. For example, IEC60950 for IT stuff such as SMPS for PCs or similar.
Since it's nice to have a smaller form factor, sometimes you would reduce those distances. For example, you have bottom-mounted components on your SMPS PCB, thus reducing the distance between active parts of your SMPS (maybe at mains potential) and the metal case. This usually ends up at more than 6-8mm of required distance between PCB and metal case: for some form factors this is too much.

Here is where the plastic sheet enters. If you place an additional insulation sheet, you can reduce those distances. Of course this is regulated in the safety standard and there they say what requirements this insulation sheet must have, and how much you can reduce the safety distances.

TL;DR: check the appropriate safety standard for your product to learn what type of material you want.

[intabits]

Ahh... Now I know the stuff you mean.
I just opened a battery/power unit from a medical cart and there was some in there. I've seen it before, and always assumed it was phenolic resin impregnated paper, as used in PCBs for consumer grade electronics. But looking at it now, it does seem more cloth/fiberglass based.
I think this is the stuff:-
https://www.alibaba.com/showroom/electrical-insulation-board.html

But I'd just throw some blank PCB into the etchant to get bare FR4...

[Alex Eisenhut]

Thanks all. Not sure where we're headed yet but I got enough clues from everyone!