Author Topic: isolating heatsink from traces underneath?  (Read 1872 times)

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Online coppercone2Topic starter

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isolating heatsink from traces underneath?
« on: November 27, 2023, 11:42:02 am »
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?
 

Offline Jeroen3

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Re: isolating heatsink from traces underneath?
« Reply #1 on: November 27, 2023, 12:02:04 pm »
Run traces on bottom or internal layer.
 
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Online jpanhalt

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Re: isolating heatsink from traces underneath?
« Reply #2 on: November 27, 2023, 12:12:34 pm »
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.
 

Offline Ian.M

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Re: isolating heatsink from traces underneath?
« Reply #3 on: November 27, 2023, 12:29:48 pm »
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.
 

Online coppercone2Topic starter

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Re: isolating heatsink from traces underneath?
« Reply #4 on: November 27, 2023, 01:16:38 pm »
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.
« Last Edit: November 27, 2023, 01:20:40 pm by coppercone2 »
 

Offline Ian.M

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Re: isolating heatsink from traces underneath?
« Reply #5 on: November 27, 2023, 01:23:23 pm »
So what if the Teflon can move around a bit?  As long as its trapped by notches or holes for the heatsink mountings it cant fall out.  Clamping thin teflon sheet would be a bad idea as its soft and tends to cold flow, so over time the heatsink edges could cut through it.

You can buy insulating sheets specifically for this and the similar situation of providing insulation between a PCB and a metal chassis.
« Last Edit: November 27, 2023, 01:26:33 pm by Ian.M »
 

Online jpanhalt

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Re: isolating heatsink from traces underneath?
« Reply #6 on: November 27, 2023, 01:39:45 pm »
What I did not like about the teflon strip is that its possible to move it around under the heat sink.

1) Don't use teflon.  There are almost countless bondable plastic films.
2) Teflon can be easily bonded.  I have been doing that since the 1960's.  The trick is to properly etch it.  Use sodium dissolved in a non-reactive (non-protic) solvent, such as naphthalene (recent times) or liquid ammonia (1960's).  There are other options.  It turns a very light brown.  We called it chemical charring, but the more accurate name is probably etching.  Epoxy and even super glues will bond to it.  I preferred epoxy for my applications.

In your case, I would use something with an adhesive such as in the link I gave.  Kapton tape or even 3M packing tape would also work.  Presumably you don't need the temperature tolerance of Kapton or Teflon.
 

Offline CatalinaWOW

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Re: isolating heatsink from traces underneath?
« Reply #7 on: November 27, 2023, 04:34:16 pm »
You have an implied requirement for electrical connection to the heat sink.  Why not just specify thicker washers?  As thick as required to make you comfortable with the spacing.
 

Offline ajb

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Re: isolating heatsink from traces underneath?
« Reply #8 on: November 27, 2023, 08:24:31 pm »
How big is this heatsink?  The only kind you generally see soldered into place are the little individual TO-whatever kind, which wouldn't be big enough for the board to bend much between the mounting points.

Why not just use a bit of tape, either on the PCB or on the heatsink?  No additional fixing required.  Or see if your PCB assembler has a label feeder and can place one under where the heatsink goes during pick-and-place. 
 

Online coppercone2Topic starter

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Re: isolating heatsink from traces underneath?
« Reply #9 on: November 27, 2023, 08:56:05 pm »
You have an implied requirement for electrical connection to the heat sink.  Why not just specify thicker washers?  As thick as required to make you comfortable with the spacing.

top of chassis!
and its a linear extrusion the long king 50+mm
 

Offline Jeroen3

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Re: isolating heatsink from traces underneath?
« Reply #10 on: November 28, 2023, 07:15:00 am »
You could also place a few unused sacrificial surface mount resistors to keep the heatsink away from the board surface.
 

Online coppercone2Topic starter

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Re: isolating heatsink from traces underneath?
« Reply #11 on: November 28, 2023, 07:40:46 am »
hmm that is interesting but beware of ceramic dust possibly getting in your chassis. needs to be solid metal
 
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Offline rhb

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Re: isolating heatsink from traces underneath?
« Reply #12 on: November 28, 2023, 01:25:18 pm »
I use adhesive thermal transfer pads (3M) and have not encountered any problems.  I have a 5kV megger and the resistance is in the GigOhm range.

Have Fun!
Reg
 

Offline T3sl4co1l

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Re: isolating heatsink from traces underneath?
« Reply #13 on: November 28, 2023, 02:04:50 pm »
Tape (polyester, polyimide, take your pick)
Polypropylene fabric tape, or paper
Fish paper
Polyester sheeting (esp. die cut, tacked down with dabs of glue, spray adhesive, or constrained by heatsink hardware)
Any other plastic or composite sheeting; Delrin, PTFE, phenolic paper, G10/fiberglass, etc.
Conformal coating

Some of these might only afford spacing (fabric or felt tape, margin tape, etc.), some, full insulation (fish paper, depends on the grade; a rubber-vulcanized version is available that's been used for decades).  Some more rigid, some more flexible; how much depends on mechanical considerations as well (e.g. something stiffer plus dabs of adhesive might be preferable for high-vibration environment).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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Online coppercone2Topic starter

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Re: isolating heatsink from traces underneath?
« Reply #14 on: November 28, 2023, 05:22:36 pm »
Fish paper is a good one the same as transformers and water proof
 

Offline TimFox

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Re: isolating heatsink from traces underneath?
« Reply #15 on: November 28, 2023, 06:37:55 pm »
For similar purposes, I have had good luck with silicone-adhesive "skived PTFE tape".
https://www.mcmaster.com/products/skived-tape/
 

Online Gyro

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Re: isolating heatsink from traces underneath?
« Reply #16 on: November 28, 2023, 06:51:44 pm »
Nobody seems to have asked what voltages are involved here. 5V? 50V? Mains?
Best Regards, Chris
 

Online coppercone2Topic starter

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Re: isolating heatsink from traces underneath?
« Reply #17 on: November 28, 2023, 09:37:11 pm »
50 v seems reasonable
 

Online jpanhalt

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Re: isolating heatsink from traces underneath?
« Reply #18 on: November 28, 2023, 09:50:46 pm »
Fish paper is a good one the same as transformers and water proof

I have never seen such a logical choice made based on the names sounds similar.  Go for it!
 


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