We're encapsulating an underwater sensor instrument inside rigid epoxy. We need to accurately position the sensing elements just under the surface of the epoxy to within say 0.2mm.
Coming up with an arrangement to easily, quickly and repeatedly position the PCB within the mould is giving me grief.
Anyone with experience doing this sort of thing care to offer suggestions or point me toward resource?
Thank you.
Mold the unit and apply the sensor to the surface, then cover the entire thing with .2mm more epoxy?
the external surface.
Its not really practical to assemble as a two step process as the sensors are soldered to the pcb assembly.
Can you turn it "upside down" so the board can be positioned on spacers that position your sensor 3mm above the bottom of the mold? Any irregularity in the top surface then doesn't affect your sensor location.
Maybe you can use locating pins (I'm thinking if something like a 3mm polished steel ejector pin) to hold the PCB in position. That will leave you with small holes that have to be filled.
Or consider extending the PCB out through the epoxy (route away material so there are just narrow legs sticking out). The legs then have to be trimmed off. If the geometry works out you may be able to use V-grooves leaving minimal stubs at little to no cost. I would arrange it carefully to maximize the length along the PCB surface (no straight path in) so that risk of ingress of moisture is minimized.
Or press (perhaps brass) custom made (if necessary) pins into the PCB and use those to press against the inside of the mold and locate the PCB, depending on the brass-epoxy seal, and again maximizing the distance along the seals.
--sp
Yes, that's our current approach. It works fairly well.
There is a little variation unit to unit (maybe 0.25mm) so I'm looking for a practical way to adjust this out.
heavier boards (so they don't float) precision spacers. use a torque wrench on the spacers if applicable. don't pull on the wires.
mechanical regidity and a consistent and repeatable process.
Attach a 'window' or 'spacer' to the sensor which is exactly the desired thickness. Cover this spacer with Kapton tape. Pot the assembly. Peel off the tape.
Modify your mold so you have a 3mm step directly in front of the sensor. You could drill your mold and put in a pin/plug that gives you your 3mm sensor to inside edge of the mold. Position the unit so the sensor is securely resting on the pin and fill. Release from the mold and you have a small 3mm deep "hole" directly in front of the sensor. Clean the hole and fill with expoy.
I have almost no experience, but have read a bunch. In casting metals they use chaplets. Pieces of metal to hold cores in place. The chaplets become part of the casting.
You could do something similar, You could mold epoxy bits to hold the sensor in place during encapsulation. The epoxy bits would become part of the assembly.
Dan
For one thing, you will get failures with PCBs in rigid epoxy if you do not first conformally coat them to keep the epoxy from getting under parts and breaking them during thermal cycles.
That said, you can make corner stands that nest the PCB corners and space off the board the exact appropriate amount before the potting commences. Though 0.2 mm is a very small tolerance, so your cases need to be very precise and the aforementioned stanchions need to be as well.
If you need to conduct heat off the board use a fiberglass or silica powder filler in the epoxy mix to increase its theta numbers. Aluminum finned heats sinks still work to carry heat into the epoxy better as well, so do not merely count on a single surface interface between a heat source component and the epoxy.
What a stupid suggestion! If 0.2mm is his max, then you have just made it a zero tolerance placement by incorporating his gap into your method.
You are a true idiot.
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A picture of what you've got would help me a lot. Can you make the epoxy a bit thicker and then machine it down to size? (or will that be too expensive?)
George H.
We have a company policy about potting: never do it.
But if you must, use a potting shell. Put some Spehro-style spacers or bumpers on the board, invert it and drop into the shell, add goo, degass, cure.
I wouldn't trust a 0.2mm thickness of epoxy.
I's use a 0.2mm sheet of a suitable insulator, bent to anchor within the potting, and make that the wall surface.
If 0.2mm is only the tolerance of a thicker barrier, then I don't see an issue. Pot and mill.
RL
Can you 3d-print a core to represent your board, and cast an epoxy shell with all the critical dimensions against it? After it cures, place the sensors and shoot with encapsulant. The dimensions of the shell can be adjusted by machining, if necessary.
Post encapsulation machining would cost too much, we are going the encapsulation route to try to reduce costs.
Unfortunately I'm not able to put to many details up here for commercial reasons.
I like that idea but others dont like the aesthetics of filling holes post encapsulation.
The PCB extension is a good idea & one that we have toyed with as the epoxy bonds well to the PCB material. It's also cheap.
We're a bit worried about using metals as spacers or standoffs as the completed units will live in a underwater marine environment.
Thanks for the ideas.
Yep, we've done a few tests using PVC.
I hope not. The epoxy is supposedly ok for direct encapsulation.
This is our current approach.
Luckily the unit is low power so head removal is not an issue.
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