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I have a 4-layer board with no parts on the bottom side, the bottom-side solder mask fully covers all vias.

I want to stick the board with a double-side adhesive foil on the metal bottom side of a display. As I do not want to use the isolating double-side adhesive foil to cover the complete display area (you'll never would be able to separate the display from the PCB anymore in this case), I only use some stripes for sticking the PCB onto the display.

As the solder mask is not very thick I'm afraid of short circuits between the traces / vias of the PCB with the metal frame of the display.

So I would like to have something like an isolation foil (no high voltage isolation, only max. 30V) on the PCB's backside without having high costs and work. As far as know on flex-prints an isolation called coverlay can be used - could this be used for a rigid PCB as well? If yes: is this expensive? Or do you have any idea how to solve this isolation issue?

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  • \$\begingroup\$ user3531827 - Welcome :-) You use the word "foil" several times in your question. In some countries, that can be interpreted as something which is metal or with a metallic coating. I don't believe you mean that here. I am guessing that in some places you are referring to double-sided adhesive tape, and elsewhere to an isolating sheet. Can you check using those are translations for the terms you used, to see if that is what you mean? As I said, I just want to be clear whether you mean metallic foil or not (and I think you don't). Please edit the question to clarify. Thanks! \$\endgroup\$
    – SamGibson ♦
    Commented Mar 12, 2021 at 11:42
  • \$\begingroup\$ Might be a german thing like Kunststofffolie which is essentially a plastic sheet e.g. Mylar or PP \$\endgroup\$
    – Lars Hankeln
    Commented Mar 12, 2021 at 11:58
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    \$\begingroup\$ The Polish also use the word "foil" to mean plastic sheet. \$\endgroup\$
    – tim
    Commented Mar 12, 2021 at 12:31
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    \$\begingroup\$ Ooops, obviously I got a "false friend" - you are right in German "Folie" is a thin sheet (can be plastic but also metallic). Anyway, I'm talking about e.g. a Mylar sheet. My original question was if somebody has more knowledge about coverlay used for flex PCBs. \$\endgroup\$
    – user3531827
    Commented Mar 12, 2021 at 12:54
  • \$\begingroup\$ Coverlay is polyimide tape. Just thicker sometimes. \$\endgroup\$
    – ScienceGeyser
    Commented Mar 12, 2021 at 13:11

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You can get thin mylar/polyester sheets that could be glued on. Depending on amount you could get these premade to size.

(well, you can get them premade to size at any amount, but that is not going to be cost effective)

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  • \$\begingroup\$ If there are no exposed pads and your solder mask is consistent (i.e. it doesn't flake!) I'd say that for your 30V the mask itself should be sufficient. You could evaluate foam adhesive which is thick enough to avoid contact even when there's no adesive mounted \$\endgroup\$
    – Lorenzo Marcantonio
    Commented Mar 12, 2021 at 11:54
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    \$\begingroup\$ No, absolutely not. Vias poke out trough the soldermask all the time. A solder mask cannot be considered as an insulator, as described in every standard that deals with separation distances. As cost and labor is a part of the question I am assuming this is for professional use and relying on the soldermask for isolation is not only a bad idea, it is also illegal. A product with such a solution cannot be CE marked. \$\endgroup\$
    – Arcatus
    Commented Mar 12, 2021 at 12:05
  • \$\begingroup\$ It's not an insulator for creepage and clearance purposes but for very low voltages good solder mask has a specified withstand voltage. Of course if you don't have the specs an insulator or even an air gap are better. \$\endgroup\$
    – Lorenzo Marcantonio
    Commented Mar 12, 2021 at 12:13
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For medium or small runs, you could buy mylar sheets cheaply and hand-cut them with scissors to fit your size (make little cutouts at the corners and stick the double-sided adhesive on the corners, for example).

You can also get them cut to shape by a lot of different companies, for low quantities you might want to search for laser cutting services, for larger batches several companies will create a custom die for stamp-cutting this.

One note: I have in the past gotten manufacturers of cheap plastic SMT stencils to make me custom mylar sheets that i needed in weird forms for one-offs.

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Polyimide (Kapton) tape is a common solution for such things. You can find heat conducting, heat insulating, super thin, double sided, and so on to suit your specific needs.

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You could use plastic spacers to create an air gap between the metal frame and the PCB.

Some of them have a base with a sticky pad that attaches to the metal frame. The PCB is attached to a clip or stud at the top of the spacer.

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Another alternative for small runs could be spray-painting, conformal-coating, or manually adding another fiberglass layer to the back side. These will all provide a layer of continuous insulation, with increasing resilience.

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  • \$\begingroup\$ Production should become some 1000 pcs per year ... \$\endgroup\$
    – user3531827
    Commented Mar 12, 2021 at 12:57
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Instead of a corner-case solution you could turn your 4-layer pcb into a 5-layer pcb. Your current bottom layer will be an intermediate layer and the new bottom layer will remain empty, therefore providing the pcb with a FR4 insulation. Multilayer pcbs are not that expensive nowadays and the manifacturing process is very well established.

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  • \$\begingroup\$ You will need to specify buried vias at extra cost and cu layers comes in pairs in standard productions. But of course you can create a blank cu layer. \$\endgroup\$
    – Henning Larsen
    Commented Mar 12, 2021 at 23:43

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