# Thinest insulator between metal case and PCB?

For a miniature product, I want the smallest possible product enclosure around the PCB. I figure I can get away with a 1mm thick metal sheet enclosure.

But I (probably) also need an insulator between the circuit board and the case, so nothing shorts out. What's the thinnest way I can make the inside of the metal case insulative? Paint? Powder coat? Paper?

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What peak voltage (and max freq)? Any regulatory requirements? In what environment will it operate? – tyblu Sep 22 '12 at 4:58
5v USB used only occasionally. 3.3v logic. +/- 9v peak-to-peak low-current AC. --- FCC. --- Consumer electronics in everyday conditions; continental US. – EvolvedAI Sep 22 '12 at 5:24

(1 mm steel is thick!)

The isolation may not be required, since you're probably (S)ELV. Anyway, it's not going to cost you much space-wise. I wouldn't mess with paint sprays and such. Agreed, it's the thinnest, but I assume you can afford the thickness of a 0.1 mm PP (PolyPropylene, PP has very low water absorption) sheet?

Try to use only SMT parts, and mount them single-sided. PTH components will add at least 2 mm because of the pins sticking out at the other side. A single-sided PCB may be glued directly onto the PP sheet, which in turn you glue to the bottom of the enclosure. If you manage to do the wiring of the PCB single-sided as well you don't even need the PP insulation. It may be worth using a couple of 0 Ω jumpers to ease the layout.

You can save an extra couple tenths of mm by using a 0.8 mm PCB instead of the standard 1.6 mm. The thinner PCB is less stiff, but at the small size it's not a problem, and when glued against the enclosure it won't get any mechanical strain anyway.

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Thanks! So even if a +5v and a GND are touching the polypropylene 1 mm or 2 apart from each other, it always presents high resistance? – EvolvedAI Sep 22 '12 at 8:29
Is this the stuff? goodfellow.com/catalogue/… – EvolvedAI Sep 22 '12 at 8:30
@Evolved - Yep, that's the one. It mentions a roll width of 650 mm, though, and a roll may be 600 m worth (mine is), so that may be, er, somewhat much. You may ask them for a sample. The PP has a volumetric resistivity of > 10$^{15}$, which is, er, pretty high. At these low voltages you're absolutely safe. – stevenvh Sep 22 '12 at 8:47
For playing (and better) "Mylar" [tm][literally] will do what you want as long as you do not mechanically puncture it. OHP (remember them) projection film is/was usually Mylar. That's a version of Polyester but is going to behave well enough for what you want. – Russell McMahon Sep 22 '12 at 11:06
@Russell - I guess a lot of plastics will be suitable, but I'm not a mechanical engineer, nor a chemical one, so I don't know them all. I do know that PP has about the lowest water absorption, and a high resistivity (I forgot the dimension ohm.cm in my previous comment). – stevenvh Sep 22 '12 at 11:35