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We are working on a product where the entire device needs to be dissolved in liquid after the device has operated and the device is no longer usable or desired.

This is a down-hole application. The device body is either aluminum or magnesium. There is a small lithium-ion battery plus a circuit board with some electronics. There currently exists technology that can dissolve the aluminum body - a brine solution of about 5% Potassium Chloride (KCl) is circulated until the device is dissolved.

Our client would like to have the circuit board break down / dissolve as well. The board is currently FR4 glass epoxy with traces on both top and bottom layers. We will have a look to see if there is any chance that we can constrain the traces to the top-side layer only - this might allow us to use an aluminum circuit board. However, I'm not hopeful this will be possible.

I'm looking for suggestions for either suitable PCB material OR techniques that might allow the board to be dissolved.

For example, we are considering using a much more fragile PCB material (paper-epoxy) and using a small explosive charge to shatter the board into much smaller pieces. However, I'd like to learn about other techniques that might achieve our goal.

Note that is NOT a shopping question. If someone can suggest a PCB material that would directly be suitable - that's awesome. But I'm after other techniques that might achieve a similar outcome.

I'm aware that the individual components won't be dissolved by the brine solution. However, the goal is to make the pieces small enough that they can be pumped without clogging the system - the pieces can be filtered out and discarded.

[Edit]

From the comments below:

1) Not military

2) PCB is currently about 1.5" x 1.0". Was larger but we've been shrinking it.

3) Operate time from deployment to end of life is measured in hours. I'm not the lead engineer on the project but I think there is sufficient battery capacity for about 24 hours of operation.

4) PCB is sealed inside a heavy-wall aluminum canister. Circuit board is not exposed to any liquid during operational life.

5) Max temperature that we have been testing to is 100C. Surprisingly, the particular Lipo battery that we are using is quite happy at that temperature.

6) The unit dissolving or breaking into smaller pieces is simply so that it doesn't cause obstruction when it has finished its job. Nothing nefarious - just sort of "cleaning up after itself".

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. Any conclusions reached should be edited back into the question and/or any answer(s). \$\endgroup\$ – Dave Tweed Sep 25 at 11:57
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Researchers from the National Physical Laboratory (NPL), in London, in cooperation with partners In2Teck Ltd and Gwent Electronic Materials Ltd, have developed a 3D printable circuit board that separates into individual components when immersed in hot water. The goal of the ReUSE project was to increase the recyclability of electronic assemblies in order to reduce the ever-increasing amount of electronic waste.

enter image description here
Source: http://environmentaltestanddesign.com/dissolvable-printed-circuit-board-recycled-with-hot-water/

If that doesn't work, nitric acid will work on just about everything.

Oh, if you wanted to 'roll your own' manufacturing process, you could find a dissolveable material (maybe a some kind of cellulose?) and print on it with on of these PCB conductive ink printers: https://www.voltera.io/

As per Edgar Browns suggestion, also this idea for dissolving polyimide for flat flex:

Try a mixture of Methanol:THF=1:1 , but it will take 1-2 days; The easiest way to dissolve Kapton - is to use 0.1-0.3M NaOH in water. By using alkaline solutions you can completely decompose the Kapton - down to initial monomers.

https://www.researchgate.net/post/can_polyimide_filmskapton_dissolved

NaOH is lye, I don't know in what concentration you would have to have to get kapton to dissolve but that seems like it would be easy to experiment with.

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    \$\begingroup\$ NaOH is sodium hydroxide (lye). "Bleach" typically refers to sodium hypochlorite (NaOCl), which is a very different chemical. \$\endgroup\$ – duskwuff Sep 23 at 22:34
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    \$\begingroup\$ Your right, for some reason I always think bleach is watered down lye, it is not. Thanks for the correction \$\endgroup\$ – Voltage Spike Sep 23 at 22:35
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    \$\begingroup\$ Oh, and "0.1-0.3M" is a description of the concentration required. It's pretty weak. \$\endgroup\$ – duskwuff Sep 23 at 22:40
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    \$\begingroup\$ Seems like something I should try in the lab... \$\endgroup\$ – Voltage Spike Sep 23 at 22:43
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    \$\begingroup\$ If this is down hole application it is already really hot so heat only solution is bad idea, typically polyimide PCB is already used for high temp down hole application so you may be able to adapt the flex solution for polyimide pcb. You still have to disolve the components \$\endgroup\$ – crasic Sep 23 at 23:44
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You should reconsider metal core PCBs Example. I've used them for high power LEDs, and we etched in house using basically standard processes. This is the one we bought.

Of course they do place limits on your design (and they're annoying to hand-solder), but they can be double sided (example from the same supplier as above, not someone I've ever used). They'll give you a solution that will dissolve in anything your Al case will dissolve in.

The insulating layer is typically 100 µm thick, and it appears to be epoxy-based prepreg. I'd assume that if surface-mount components can be dealt with, so can small pieces of polymer insulation, which are likely to break up. It could be scored by routing, slotting the board, or even by hand with a scriber so that it breaks into smaller pieces (I don't know whether this is a research 1-off or a production run, so I don't know what processes are plausible).

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like aluminium, Alumina is soluble in potassium hydroxide, and is available as a substrate from many manufacturers, also, some manufacturers will do double sided aluminium.

Probably the most soulble solution would be aluminium metalisation on alumina substrate, special solders and fluxes will probably be needed to attach the parts, but all the interconnect should dissolve in your alkaline salt solution. I'm not aware of any place that can provide that as a standard option.

wood pulp bonded with a soluble salt would be another interesting experiment, but would require the use of only water-free processes during manufacture

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For FR4 you only need to dissolve or decompose the epoxy in between the fibers. The usual process is to pyrolyse it.

Next to FR4 there are other materials to make a PCB from. Polyimide film is often used in flexible boards, and this can be dissolved.

https://electronics.stackexchange.com/a/221926/148363

Unaware of application you might need to glue this flexible pcb to another more easily dissolved substrate for rigidity or thermal purposes.

Flexible PCB will also be easier to burn away. Some defective products already have damaged flexible PCB due to water from drinks.

Tight collaboration with your PCB house is required. Since this is a rather unusual product requirement.

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Consider using a flexible PCB and using a "can-crusher" design to compress it in one axis, then compress again with another in a 2nd axis. You will be left with a pellet which can be released from the enclosure easily.

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The easiest way to dispose of the PCB is during the design phase of your project, not after it's deployed. That is, don't use a PCB at all.

Your circuit can use a stiff piece of non-coated cardboard as a substrate. Long-leaded components (resistors, diodes, etc) can stick right through the cardboard. Shorter-leaded components (ICs) might need a socket. Instead of traces, make your connections using good old-fashioned wire wrap techniques. I've used this as a poor-man's prototyping technique for ages.

When it comes time to dispose of the device, cardboard is fairly easy to destroy (source: the packages on my porch any time it even slightly rains). What you'll be left with is the components themselves and a rat's nest of Kynar-coated wire. Kynar is resistant to acids, but there are solvents that will destroy it (some of your electronic components likely have Kynar in/on them so you'll need this chemical anyway). If possible, choose a solder that breaks down in the same acid that you're using to dissolve the casing.

The main downsides to this approach are that the devices are harder to manufacture (more labor, less automation) and that they're much less sturdy (less of an issue since yours will be in an enclosure). If your circuit is extremely complex, you might have to go with a larger board size, or make a multi-layered circuit by stacking several boards on top of each other.

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  • \$\begingroup\$ I'm not sure if you read the edit that I made to my question. The circuit board is double-sided about 1.0" by 1.5" with components on one side only. Most of the components are 0603 size along with a QFN package and other packages. But trying to solder 30 AWG Kynar wire-wrap wire to that QFN package is pretty much a non-starter. \$\endgroup\$ – Dwayne Reid Sep 27 at 1:23
  • \$\begingroup\$ I haven't yet mentioned that the expected production quantities are expected to be many thousands per year. Building these by hand is out of the question. \$\endgroup\$ – Dwayne Reid Sep 27 at 1:24
  • \$\begingroup\$ @DwayneReid You're correct that you need through-hole parts. If a part doesn't have a through-hole version available, you might be able to find a socket or carrier board for it. Machines that automate assembly of a wire-wrap board indeed exist, but they're harder to find nowadays than they were in the 60's. Wire wrap still sees use in some niche applications, so you never know. \$\endgroup\$ – bta Sep 27 at 2:16

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