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I have soldered several small PCBs, and I want to stack them together (using hex spacers).

However, since lack of (vertical) space it could be that some components (on the top) might touch the wires/solder junctions (on the bottom) of the PCB above.

What is a good way to prevent this?

I was thinking about thin wooden or plastic plates between the layers, however, this will cost some (vertical) space, especially where the wires/solder junctions from the above PCBs are lower than the top components of the PCB below.

So another way I thought was to use two 'layers' of electrical tape, stick together (with the 'glue' sides towards each other). So it is like a flexible 'plate'.

Would such a thin (< 1 mm electrical tape 'layer') be enough to isolate signals around 5 V, and very low current (DMX512 signals)?

Michel Keijzers
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    Why not just buy bigger spacers? – MCG Nov 06 '18 at 11:55
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    Is this a through hole PCB or surface mount? i.e. do you have pointy clipped-off legs sticking out of the board? Are the PCBs populated both sides or just one? – Jack B Nov 06 '18 at 11:59
  • @MCG Because I have lack of space, I need the PCBs as close together as possible. – Michel Keijzers Nov 06 '18 at 12:08
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    In that case, can you not change the PCB layouts so that the components do not clash? A bit of care taken in the layout stage could easily fix this – MCG Nov 06 '18 at 12:09
  • @JackB it's all through hole, I cut off most unnecessary legs, but there are many wires and solder junctions which use some height. On the upper side are all components (ICs + sockets, capacitors, resistors), on the bottom side are wires and solder junctions. – Michel Keijzers Nov 06 '18 at 12:09
  • @MCG Yes you are right, I made a mistake in the design phase not to take into account the bottom side (wires/solder junctions). – Michel Keijzers Nov 06 '18 at 12:10
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    If it is a one off project, then go with the Kapton take as mentioned in the answer by BenAdamson. If you are planning on doing more, redesign the PCB and make sure to consider these things in future – MCG Nov 06 '18 at 12:16
  • It's a one off project, actually I'm intending to make in the future an ordered PCB (if I can justify using at least 5 of them). – Michel Keijzers Nov 06 '18 at 12:20
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    @MichelKeijzers without wanting to push overseas production too much, you can order 5 PCBs for < 3€ (total) from China these days; I'd strongly recommend designing a PCB with SMD components if you're actually space-limited. – Marcus Müller Nov 06 '18 at 12:31
  • @MarcusMuller true, however, shipping cost are also added, still they are quite cheap. Actually, the reason I am space-limited is that I forgot the bottom side space when designing the boards and buying the enclosure, otherwise I would have bought a bigger enclosure. Also I learnt better soldering (I hope). But I also never designed a PCB (for ordering), so that also is a nice challenge for me. – Michel Keijzers Nov 06 '18 at 12:33
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    :) the good thing with cheap services like jlcpcb is that even if you have to learn through trial and error, it doesn't become an expensive hobby :) – Marcus Müller Nov 06 '18 at 12:39
  • @MarcusMüller True ... that's why I prefer cheap Chinese stuff for now. I ordered quite a lot of components which were not the 'best' ones for the job, or just plainly wrong, and now it's nice I have quite some assortment in components to play with (I'm not intending to make anything commercial anyway). – Michel Keijzers Nov 06 '18 at 12:51
  • @MarcusMüller Just checked, 10 PCBs are 2$, shipping is $7.30, so still very good. – Michel Keijzers Nov 06 '18 at 12:58
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    You shouldn't just worry about shorts. Mechanical stress can also break parts. And adding a layer of tape without separating the boards will only increase the stress. So I'd say don't do this, find a way to physically separate the boards. – The Photon Nov 06 '18 at 17:47
  • @ThePhoton ... I will try, but if it doesn't fit and some wires would touch the components below, Kapton tape seems perfect ... however, as you say, adding stress is also not good (even without Kapton tape I have to make sure it does not happen). I will try to separate them, but not sure if it is possible (without buying another enclosure, and spending much more time to make a ground plate again). – Michel Keijzers Nov 06 '18 at 18:01

2 Answers2

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Kapton tape is a widely available industry standard product for this scenario. You can view the specs here - from table 3 you can see that it has a dielectric strength of over 100kV/mm, plenty strong enough for your purposes. It comes in different thicknesses but any standard 'off the shelf' kapton tape will suffice.

I'd recommend it over electrical tape as electrical tape tends to leave a sticky residue and is thicker, which may be undesirable given your height restrictions.

BenAdamson
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  • I will check into it; the stickyness is not a problem, since I want to stick two layers of electrical tape between each layer, one up, one down, so the sticky sides are stick together. – Michel Keijzers Nov 06 '18 at 12:11
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    Cut-off component legs may puncture Kapton tape. Especially if it's pressed onto them by other components. If you're going this way, I would go over the bottom of the PCB with end-cutters and trim the ends of the legs carefully first. Use a wide Kapton tape, and overlap it a bit to make sure it sticks well. – Jack B Nov 06 '18 at 12:22
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    Use Kapton sheets, not tape - you can get a variety of sizes. – Jon Custer Nov 06 '18 at 16:43
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    @JackB I've seen some boards use a thin flexible layer of foam covered by plastic on one or both sides to avoid the puncturing issue. – user71659 Nov 06 '18 at 18:20
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    If the height is really close enough for different types of tape to matter, then you probably want something more abrasion resistant. – Chris Stratton Nov 06 '18 at 18:47
  • @ChrisStratton If space allows (but I doubt), I will try. – Michel Keijzers Nov 06 '18 at 19:42
  • The point is that if space does not allow, meaning your parts are likely to touch, then you really need something that won't wear through. Likely you need a re-design. – Chris Stratton Nov 06 '18 at 22:21
  • The other point is that the dielectric strength of insulating tape is (surprisingly) low -- typically 30 kV/mm max -- I have managed to burn through it with "reasonable" voltages (200-300 V) on large (~6 mm wide) copper traces before, particularly as the tape is flexible and can become thin if stretched. Kapton has a higher dielectric strength and doesn't stretch as much. You can also reapply it. – Landak Nov 07 '18 at 07:46
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As people have pointed out, you need mechanical strength against puncture and abrasion as well as insulation.

"Press Paper"/"Elephantide"/"Insulation Paper" is ideal for this task and is available in many thicknesses/grades.

Mylar film is also commonly used if you need something really thin. It is not quite as mechanically robust and has an increased cost though.

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