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Enough heatcompound on output transistors?
Enough heatcompound on output transistors?
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Old 30th September 2015, 01:39 AM   #1
MaccAu is offline MaccAu  Australia
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Question Enough heatcompound on output transistors?

Enough heatcompound on output transistors?
Hi All

My first time at doing output transistors, a friend helped me with a video of how to do the bias etc on my Technics SU-V8

I fixed and smoothed the scratches and appreciate the help from that thread I made

I am a bit concerned , i may have used too little heat compound on the ICs. I got myself some Wakefield 120, and a source for Mica washers for MT200.

I know too much is not good either for heat transfer but too little could be a problem also

Enough heatcompound on output transistors?


Enough heatcompound on output transistors?


Enough heatcompound on output transistors?
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Last edited by MaccAu; 30th September 2015 at 01:43 AM. Reason: adding 1 more pic
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Old 30th September 2015, 01:56 AM   #2
ostripper is offline ostripper  United States
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MT-200 devices have much better thermal resistance than most other
packages.
With 2 screws torqued I'm sure your good. I just use a line of compound
between holes , torque till I see a small bead all around the device.
Wipe away the excess , you are just concerned about the few uM on both
sides of the mica.

OS
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Old 30th September 2015, 02:02 AM   #3
Richidoo is offline Richidoo  United States
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That's plenty. Heatsink grease should be thinnest layer possible, only thick enough to fill microscopic voids that would otherwise have only air to conduct the heat. Any thicker will reduce heat transfer unnecessarily. Use very thin layer, then squeeze out as much as you can. The applied layer is so thin it looks more like clear than white.
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Old 30th September 2015, 02:20 AM   #4
Ian Finch is offline Ian Finch  Australia
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You seem to have applied puh-lenty of compound there - witness the bead of compound exuded from the washer/heatsink and washer/transistor case gaps as you clamped them up. As you can't see any residual air gaps under the transistors, that's all you can hope for and that is why it's important to have the faces as flat as possible so that the film of compound is as thin and uniform as possible and the amplifier can meet the design specifications safely. Though it's necessary as a kind of gap filler with some benefit to conduction, thicker layers of it just reduce heat transfer. Compared to metal, it doesn't really conduct heat well at all.

You can measure the temperature in operation by using a thermocouple probe with your multimeter etc. applied to the centre top of each device and so check that all devices are shedding heat evenly as they should if they're correctly fitted. There will be some variation due to each transistor's location relative to the others but 10% higher than others could be a warning that more attention is needed.

BTW, what was the source of the transistors.....Chinese Ebay store?
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Old 30th September 2015, 02:49 AM   #5
MaccAu is offline MaccAu  Australia
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Thanks OS, I torqued the screws nice and tightly, and the concern was just 2 transistors, and the mica, not looking like they had enough compound

Thanks Richard, its good to know,that thin is fine and I have enough on mine. I screwed them in tightly

Thanks Ian, Good to know, i have enough compound there. They were screwed in tightly to spread the compound. I spread it thinly across the heatsink, then a thin layer using a spare mica insulator on the back of the sankens to cover up any metal (just a thin coat). Variation also in the heatsink compound, but all is fine

No, I dont like using Chinese ebay stores. Mine came from Wagner online here in Australia. The code is YC90, they came in a pack of 10
http://www.wagneronline.com.au

Sam
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Last edited by MaccAu; 30th September 2015 at 02:50 AM. Reason: link to store
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Old 30th September 2015, 01:14 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Richidoo View Post
.......Heatsink grease should be thinnest layer possible, only thick enough to fill microscopic voids that would otherwise have only air to conduct the heat................
Yes.
exclude all the air in the interface and only just enough to minimise the thickness of the remaining goop.

If the mounting allows, I bolt up gently and rotate the device back and forth to spread and squeeze the goop out as I continue tightening. This seems to bring centrally located goop out to the edges.
Capillary action should hold the goop liquid in the ultra thin gaps.

When I come to remove the devices I find what I would describe as "too little" and this must indicate that the goop really was that thin.
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Old 2nd October 2015, 08:41 PM   #7
MaccAu is offline MaccAu  Australia
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Thanks Andrew for the reply. What worried me a bit, i may have tightened the screws too much, but i am sure there is a tiny bit of goop

I lined up one heatsink, and I am a bit concerned, maybe it was already like that with the Technics SU-V8, the heatsink shield doesnt cover the black transitor or whatever the tiny things are. Again a thin layer was used, however the screw holes lined up on the heatsink

Enough heatcompound on output transistors?

Enough heatcompound on output transistors?


Thanks Sam
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Old 3rd October 2015, 05:09 AM   #8
mt490 is offline mt490
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The best general purpose technique is a small splotch or line in the centre of the surface(s) and then assemble down without re-attachement and minimum side to side motion. You may want to initially perform this on one transistor/surface and detach your work to check you have the right metering of paste to provide adequate coverage - clean the surfaces off and reattach with the correct metering, don't reuse the paste!. This will also tell you if the surfaces are adequately flat and free of protruding defects.

You want only a minimum to squish out as this also means minimum chance of entrained air in the paste, and the dot technique generally results in the thinnest layer possible out of simple application techniques.

The above is just a good technique to use in DIY, but is it actually that critical? Not really, you could easily just dump paste on like a nutter and squish it in and you won't tell the difference without looking for it the majority of the time.

Getting a consistent, pocket free layer is a big thing for power electronics fabs where they're dealing with much larger surfaces - factories use a pasting machine to ensure a good initial metering and distribution to produce a quality product.
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Old 8th October 2015, 03:23 AM   #9
roger2 is offline roger2  United States
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MaccAu, what is your source for MT-200 mica insulators?
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Old 1st November 2015, 07:09 PM   #10
MaccAu is offline MaccAu  Australia
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Quote:
Originally Posted by roger2 View Post
MaccAu, what is your source for MT-200 mica insulators?
Hi Roger,

I get them in Australia from wagner net, they might post internationally

Wagner Online Store | Wide range of products and accessories

code to search is YC90

Cheers
Sam
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