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Old 15th September 2006, 07:26 PM   #1
hotscot is offline hotscot  Scotland
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Default Mounting chip to Heatrsink

Thanks for all the help so far.
I do search and read all the posts but I always look for clarification to be safe.

Regarding mounting the chip to the heatsink.

Is this acceptable...

Chip attached to sink with small nut and bolt.

Heatsink attached to chassis wall with small bolts.


Is electrical isolation an issue between the chip and sink and if so how do I tackle it? I often see (electrically?) insulating paste or grease mentioned so do I use that? What about the mounting nut and bolt?

Regards
Alan
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Old 15th September 2006, 08:19 PM   #2
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Check with the datasheet for the chip you are using. Most will mention if the chip needs to be isolated or not. Even when the tab of the chip is at ground attaching it directly to the heatsink can introduce noise to the circut. Also there are several versions of some chips. For instance the LM3875 and LM3886 come in isolated and non-isolated packages. Typically if you can see metal on the back of the chip you will need to insulate it. You'll need to get yourself a heatsink pad, or thermal tape of some kind, thermal grease alone will not insulate your chip from the heatsink. For testing purposes I sometimes use a piece of the antistatic bag that IC's sometimes come in, also laser printer transparencies work well. Remember these are NOT permanant solutions.

As for insulating the mounting hardware, there are washers with a flange you can use, these are usualy made of nylon. Also nylon screws will work fine if you don't overtighten them.

When in doubt test for continuity with a multimeter between the chip flange and the heatsink.

Good luck with the project

G.
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Old 16th September 2006, 10:40 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
some of the clear self adhesive tapes (3M and "sellotape") are surprising resistant to higher temperatures. A few are even made of kapton (I think a slightly lower temp version cf the Kapton heatsink tape).
What happens to it when a cool (150degC) soldering iron is placed on it?

This is a good insulator and makes a good short term thermal conductor. You may find, after thorough testing, that some brands could be used on a long term/permanent basis if your temperatures are not too high.

If you have an enclosed heatsink then you can isolate the sink from all else. This may require some capacitive coupling to either ground or supply rail. BUT maintenance must be carried out carefully.
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Old 16th September 2006, 06:24 PM   #4
hotscot is offline hotscot  Scotland
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To clarify....if I have an insulated chip, which I think I have, does that make it simpler?

Thanks all
Alan
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Old 16th September 2006, 10:26 PM   #5
jaycee is offline jaycee  United Kingdom
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If the chip is the TF version (the tab will be black), then all you need is a nut and bolt. This chip is already isolated so you just need to bolt it to the heatsink. Some thermal paste between the chip and heatsink will improve thermal transfer quite a bit.

If it is the T version (the tab will be silver) then you need insulating hardware, for both the chip, and the bolt. In my own Gainclone, I used a TO3P silpad and insulating bushing that I "rescued" from a dead PC power supply.

Basically, what you need is Rapid code 38-0432 and 38-0230. For the silpads, no thermal grease is needed. Some people don't like silpads but for a Gainclone they are fine.

Maplin also have a suitable insulating washer RN97F, but this is Mica, which means you will need thermal paste applied to BOTH sides. I couldn't find the insulating bushings on Maplin's site, though...

Hope that helps.
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Old 17th September 2006, 03:12 AM   #6
Leolabs is offline Leolabs  Malaysia
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Quote:
To clarify....if I have an insulated chip, which I think I have, does that make it simpler?
Yes.Why not?
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Old 17th September 2006, 02:20 PM   #7
jarre is offline jarre  Norway
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Hei

Look at this link:

http://no.farnell.com/jsp/search2/br...questid=112250

This are thermal pads for chips that are/are not insulated. It has the same effect that thermal grees.


Jarre
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Old 17th September 2006, 06:10 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
you only need a thermal insulator when the back of the chip is not isolated.

Grease (thermal compound) is far better than any insulator.

A good thermal insulator can be upto 3times better than a bad thermal insulator

Choose carefully.
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