Electronics > Manufacturing & Assembly

isolating heatsink from traces underneath?

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coppercone2:
I have a linear extrusion heat sink that has traces running under neath. For my prototype its fine I can just put some washers to space it a bit.Like brass or copper washers, that way they solder to the solder pads and raise the sink.

But it leaves something to be desired, for instance bowing PCB shorting out. Silkscreen works at this range but IDK if you could call that a real solution.

I have also cut strips of teflon before and mounted a heatsink over it, but unless its a screw down type, it seems a bit bootleg.

is there any techniques or hardware to do this better? The best I could think of is to make another thin PCB with two big Vias, so it does the same thing as the brass washers do, but lays against the entire heatsink bottom. Maybe glue it down too. Not sure how any of this would effect the integrity of the solder joints.

what do the pros do?

Jeroen3:
Run traces on bottom or internal layer.

jpanhalt:
Is the heatsink anodized?  If so, that's a pretty good insulator.  But of course, it doesn't take much to scratch it.  Be careful and live dangerously.  In the alternative, I used a 3M, non-conductive heat sink adhesive.  Unfortunately, McMaster no longer shows it.  This may work: https://www.3m.com/3M/en_US/p/d/b00017502/  but it doesn't specify electrical conductivity.  Acrylics are naturally insulators.

Ian.M:
An insulator between the PCB and the heatsink is not 'bootleg' provided it is of an appropriate material, thick enough to provide the required isolation, positively retained in place and extends to or beyond the heatsink footprint perimeter so it is inspectable.  Your Teflon strip should have been notched or punched to fit round/over the heatsink mountings.   For functional insulation of low voltage traces, consider using Kapton tape, as an adhesive bond to the PCB prevents buildup of dirt under it, which could promote surface tracking from the traces to the heatsink mounting points.

coppercone2:
What I did not like about the teflon strip is that its possible to move it around under the heat sink. It just seems bizzare that there is a loose element. If its bolted down, then its solid, but if you put it with a solder joint its never going to be tight. Not even sure if its good for the PCB to do something like press it down and then solder it. I guess if you chamfuer some screws to tighten down the teflon bit and put the heat sink over it and then solder it in normally, that would get rid of that problem.

actually damn that might creep and lead to loose nuts in chassis. acrylic probobly is more stable, but I am thinking if you get boards just make a snap off bit in the cutoff that snaps off to mount under heat sinks. that is manufacturable and you can get it with vias, for whatever they are worth. seems OK to just add it on the board perimiter with some milled slots to nibble through.

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