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Options for power transistor mounting to the heat sinks ?

yogibear

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#1
What do you guys use to electrically insulate BUT thermally couple the power transistors to the heat sink ?

Thanks.
 
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#2
There are multiple types of thermal compounds/pads,etc..
Thermally conductive/electrically insulating
Thermally conductive/electrically conductive
Thermally conductive/adhesive
,etc..

Adhesive thermal pads are like a "sticker" that you simply apply and can rely on the stated thermal resistance more than you can with a hand applied grease application from assembly to assembly.

This is an interesting product :-
High thermally conductive and electrically insulating 2D boron nitride nano-sheet for efficient heat dissipation of high-power transistors
http://iopscience.iop.org/article/10.1088/2053-1583/3/4/041009
 
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yogibear

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#3
Thermal grease or compound would be used any way. I am looking for available viable / commercial options to do the job of electrically insulating and thermally coupling power transistor to the heat sinks.

Silicon pads are not reliable....
 

yogibear

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#6
Why do you say that?
The ones I have, cannot trust them for mounting the transistors for my Class A amp......

Looking for options. Anything with good thermal conductivity and excellent electrical insulation even at high temperatures.
 

all4music

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#7
i've used Kapton tape for insulation along with some good thermal paste in my Class A amp. they have been performing just fine wrt to heat dissipation.
 

yogibear

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#8
Anybody used mica sheets ? And why ? Some figures here:

Aluminum 205 W/mK, Alumina Ceramic (Al2O3) 50, Mica 0.46, Teflon 0.25
 

yogibear

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#9
i've used Kapton tape for insulation along with some good thermal paste in my Class A amp. they have been performing just fine wrt to heat dissipation.
Source of kapton tape ?

Kapton tape is said to be heat resistant too..... How would it aid in effective thermal transfer ?
 

surfatwork

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#10
Been using mica for years and never had a problem with heat dissipation or shorting.
While the other options do provide better heat transfer characteristics, achieving those in practice is highly dependent on several factors like perfect polished heat sink surface, clamping force etc. In my opinion, for consumer applications such as home hifi, mica should work just fine.
 
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yogibear

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#11
I have the option of mica only but its use despite low thermal conductivity surprises me.

It seems the options to electrically insulate at high temperatures seem to be limited and hence a compromise....
 

spirovious

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#12
Fitting mica with screw on transistor need special care as screw is directly connected on transistor and other to heatsink which can short thw things.Need to isolate properly.
 

manniraj

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#13
I have used Mica sheets along with the Arctic silver thermal compound (a bit expensive for its small tube size) but very efficient for my Sony VFET amp build.
 

avesbilal

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#14
I have the option of mica only but its use despite low thermal conductivity surprises me.

It seems the options to electrically insulate at high temperatures seem to be limited and hence a compromise....
I have used both mica and silicon pads whichever was available at the time.

Whenever I use mica I strip down it to the minimum thickness, haven't seen any negative effects of reduced thickness of mica till now.

My personal opinion is that silicon pads are better in heat transfer job but maybe silicon pad should of good quality for endurance with a Class A amplifier, according to me even the thermal compound will act as a electrical insulator even if the silicon pad fails.

Regards
Bilal
 

avesbilal

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#17
Bilal,

Could you point to a source of good silicon pads ?

I fished out this from On Semiconductors regarding ways of mounting power devices on heatsinks:

https://www.onsemi.com/pub/Collateral/AN1040-D.PDF
Here is a good one though expensive

http://shopping.kitsnspares.com/Element14/led_product_description.asp?id=347164

or

http://in.element14.com/bergquist/sp900s-0-009-00-104/sil-pad-900s-009-to-3p-pk10/dp/8783578?ost=SP900S-0.009-00-104&ddkey=http:en-IN/Element14_India/search

Edit: It is recommended not to use thermal grease with these pads.
 
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all4music

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#18
Source of kapton tape ?

Kapton tape is said to be heat resistant too..... How would it aid in effective thermal transfer ?
i sourced it here from SP Road [Bangalore].

i've used it along TX-4 thermal paste - temperature near the transistor hovers around 60 degree C and stays the same irrespective of the usage hours.
you can also see the mounting method in this pic [you can refer to Rod Eliiott's site or my thread in DIY section for more detalis]:
 
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#19
Hello Yogi Bear,
Mica works quite well when used with thermal grease. However, commercially available mica sheets tend to be far too thick. Try splitting them to half their width with a sharp razor-blade. I've also used Bergquist sil pads successfully. If you do use them, purchase from a reliable vendor like Mouser or EFY/Element 14. However, good sil pads tend to be expensive and, in my opinion, not worth the extra cost over a thin slice of mica. As an aside -- the most common reason for thermal failure is not the TIM. In my experience they are: (1) excessive torque on the mounting screws (2) incorrectly drilled and tapped holes (3) poor surface finish on the heat sink. Transistors with single point mounts such as TO-220, T247 and so on are particularly prone to over-tightening, which causes the contact surface to bend. This is not a problem with TO3 types. Again, incorrectly drilled holes, especially when used with self-tapping screws intended for fastening sheet-metal or plastics can cause flexion at the point of contact. The photograph posted by All For Music shows an excellent technique to side-step this problem -- use a top plate firmly to push the transistor on the heatsink. If you have access to a junked computer motherboard, you might be able to repurpose the heatsinks that cover the processor's power supply MOSFETs with the added advantage to be able to re-use heatsinks for different projects without the need ever to drill more than two holes. For more information on correct transistor mounting technique, do look at the photographs of amplifiers posted by other members, such as Shaan and Aniket. I have not used kapton tape for this purpose, but it looks quite alright to me if the adhesive doesn't dry over time. If it does, then the voids left by the dried adhesive would hamper thermal transfer.
Cheers,
~hp
 

yogibear

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#20
Thanks, gives me an idea to use additional individual heatsinks available for TO3 for the top side too.
 
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