Casting a circuit in resin

[PeterDove]

Hi All,

I was wondering if it would be feasable ( and the circuit would still work ) if I were to cast a PCB which has a matrix of LEDs in resin so that the resin covers the back of the pcb and a little up on bases of the LEDs essentially rendering the circuit weather resistant?

Has anyone else done such a thing?

Peter

 

[Nigel Goodwin]

It's quite common, and you can even buy resin specifically designed for the purpose - obviously it renders repairs pretty well impossible, so you need to make sure it's 100% OK before you do it.

 

[PeterDove]

It's quite common, and you can even buy resin specifically designed for the purpose - obviously it renders repairs pretty well impossible, so you need to make sure it's 100% OK before you do it.

What sort of container can I use to put the PCB into and pour the resin into? I am worried about the resin binding to the container I am pouring it into.

Peter

 

[ericgibbs]

hi Peter,
If you use a 2 part epoxy mix you can pour into a cheap, open topped pvc box.
They are in fact called 'potting cases', its usual, as they are so cheap,to leave the box as part of the potted assembly.

You could pour upto the level you want, let it harden, then trim off the excess pvc box with a utility knife.
It a 'once and for good' method.

There are silicon based potting compounds, which are semi-permanent, it can be cut away, the module repaired then resealed.

The best way I have found is to get a small IP65 box [water resistant seal].

Cut out a rectangular hole in the top cover, [ the size of your LED outline] cover this with 1-2mm thick perspex or polycarbonate, on the inside of the lid, seal and secure the perspex.

If you use a 'red' filter instead of the perspex its even better.

Regards
EricG

edit: to clarify, when using the water resistantIP65 box, no need to pot the pcb/assembly.

 

[Hero999]

It's called potting and you can buy resin specifically for the purpose. I've heard about people using the transparent resin used to make ponds and lakes in model railways but make sure the resin doesn't heat up too much when it cures.

 

[HarveyH42]

What sort of container can I use to put the PCB into and pour the resin into? I am worried about the resin binding to the container I am pouring it into.

Peter

You need a mold release agent the doesn't react with the type of resin you use.
I work with pewter (not so much these days), and coated my model and anything I didn't want stuck to the mold material with petroleum jelly. Not sure if this will work for you, I made molds out of concrete or plaster of paris. I'd search some sites on resin casting, I'm sure you will find some kind of spray on mold release, maybe something common around the house. I'm guessing that you want just the resin piece when you are done.

 

[Sceadwian]

A very clean debris free board is really important before you do the resin pouring.
Another thing to keep in mind is it's going to effect the heat dissipation characteristics of the board. I'm sure various resins have different heat transfer ratios.

 

[Oznog]

It's important to note that this significantly reduces the dissipation of the LEDs, which is already a problem being in close proximity. You may need to run the LEDs at a lower current.

How much dissipation is lost depends part on how deep the potting resin is poured, but actually IIRC LEDs in those 5MM pkgs get most of their dissipation through the leads and the PCB traces. If so whether the PCB is potted or not may be more important than the potting around the LEDs.

 

[hjames]

I think polyethylene is the material of choice if you need some sort of temporary non-stick cup/tray to do the pouring. Pretty much nothing sticks to it.

**broken link removed**
These guys have a ~.25" lens on top of a .25" solid molded/poured base. I have one with some pretty ugly cracks in the resin, probably because the manufacturer tried to pour the entire thing in one layer.

Also, I think some materials (like RTV silicone) will react with copper and corrode it a bit, so keep an eye out.

 

[ClydeCrashKop]

Poly Vinyl Alcohol, PVA is mold release for fiberglass type resins. Brush it, spray it, nothing sticks to it. Water soluble.

 

[Oznog]

PVA is difficult to put onto plastics like a project box because it will bead up and fail to cover. Really, a good mold release is just petroleum jelly. Very easy to get a reliable coating all over.

The reason people like to use PVA is it's water-soluble, thus very easy to clean off. Petrolum jelly will just take a few paper towels.