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Old 5th May 2008, 04:49 PM   #1
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Default Lapping heatsinks?

Having searched the forum I found references to lapping heatsinks but no mention of how effective it is or how flat a surface should be.

I'm servicing an old 500W/chan MOSFET amp. I know that heat was a concern with that model.

The fets are bolted to an "L" section which is bolted to "U" section, which in turn. has 2 progressivly smaller "U" sections nested inside it. The previous application of thermal grease was unevenly "lumpped" in.

Am I insane to even bother?

edit:-
P.S. I'm talking about the junctions between the lumps of metal, rather than the TO3s themselves.
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Old 5th May 2008, 05:05 PM   #2
AndrewT is offline AndrewT  Scotland
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Can you omit any of the intervening couplings?
The closer your FETs to the heatsink the better.
A flat and polished surface is desired. But most will settle for a flattish surface finished with 600grit.
All air spaces at the interfaces must be filled with thermal compound and the maximum areas in direct contact the better.
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Old 5th May 2008, 07:07 PM   #3
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Thanks for the 600 grit number.
Unfortunatly, I can't build it any differently to its design. I might be able to do something about air flow through the sections.

The early model had a more basic heatsink. When I got one the lid had been thrown away to ventilate it. The Mk2 had a more elegant heatsink and more air holes - all signs that there may have been a cooling concern.

That said, I'm sure my Mk2 led a full and happy life before I got it. I'm just trying to enhance. When I get it back together I'll do some Left/right comparrasons. If the rebuilt right side heatsink gets warmer than the left, it must be removing more heat.
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Old 5th May 2008, 08:04 PM   #4
taj is offline taj
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Just out of curiosity, can you post a photo of this arrangement?

..Todd
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Old 6th May 2008, 09:01 AM   #5
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Surface finish must be 0.002" to use white thermal compound, 0.001" to use clear.

Surfaces mated with clear must be re-surfaced when dis-assembled and re-assembled, or use the white.
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Old 7th May 2008, 07:10 AM   #6
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I'll try and get a photo tonight.
Meanwhile, someone told me that thermal transfer pads have improved a lot in the last 20 years so I might get more practical results replacing them. I'll still lap between plates though.
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Old 7th May 2008, 07:21 AM   #7
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If it helps, I use 600 grit taped to a sheet of glass, resting on the bed of my table saw.
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Old 7th May 2008, 08:05 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Pbassred
... someone told me that thermal transfer pads have improved a lot in the last 20 years so I might get more practical results replacing them. I'll still lap between plates though.
Hi,
the thermal pads have a variation from best to worst of about 10:1 in conductivity.
In the middle comes a typical 0.002inch mica washer with thermal compound to both faces.
A direct metal to metal interface with thermal compound (to exclude the air) will far outperform any thermal pad or other type of conductor.
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Old 7th May 2008, 08:44 AM   #9
Nordic is offline Nordic  South Africa
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I have a bit of experience with lapping heatsinks (and CPUs) for overclocking in PC's.

sheet of water paper glued to a mirror...


Overall the increase in thermal performance is small enough to be almost negligible... maybe gain 3C of heat headroom... I would rather look at getting the sinks anodised black (if they are not allready) this will produce a much higher margin in my opinion....
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Old 7th May 2008, 08:54 AM   #10
weissi is offline weissi  Europe
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Another idea is to use Kapton (polyimid) Isolation Tape, which will also provide excellent heat transfer due to it's minimal thickness. I've bought those 2 rolls for less than $20 including shipping. Just search the well known internet shopping platform for Polyimid... (from China)
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