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Transistor heat transfer capability
Transistor heat transfer capability
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Old 5th December 2020, 01:54 AM   #21
Duke58 is online now Duke58  United States
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5 mils thick.

How do you measure 5 mils thick?
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Old 5th December 2020, 01:56 AM   #22
jneutron is offline jneutron  United States
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With a caliper or micrometer, or just trust the vendor.

You can buy silpad in various thicknesses in sheet form, cut with scissors
Jn
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Old 5th December 2020, 05:03 AM   #23
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Quote:
Originally Posted by amplidude View Post
I am working on a hiraga 30w amp, it has an iddle at 1.65 amps and produces alot of heat, I was not happy about die temperature versus heatsink temperature.

SKIPPED

Anybody else tried this approach?
If I were you I would of checked the flatness of the device before you do the polishing. Second of all proper lapping of a soft metal like copper is extremely difficult and require counter intuitive tricks. By the pictures you have had ruined rather flat surface. In some instances to-247 flatness is given. It's around 20 micron per 100 mm / less than 1 mil over 4".
Please check cpu lapping videos to get the idea.
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Old 5th December 2020, 08:27 AM   #24
amplidude is offline amplidude  Denmark
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Very good explanation jneutron, and I will try on my next project which is a pass amp, your method with copper and graphite.
An idea, the device shown is clean smooth copper, could one not simply solder a 2 by 2 inch copper plate with smd solder paste and then reflow the whole thing?
The reason I want a datalogger is to be able to verify the improvements from this method, it produces nice graphs or spreadsheets
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Old 5th December 2020, 08:33 AM   #25
amplidude is offline amplidude  Denmark
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A good method for checking flatness? Yes picture shows schratches, I could quickly see the device needed alot of work , so to speed up process I applied more pressure, then later eased of, and in the end almost no pressure.
But I will check the computer forums for further ideas
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Old 5th December 2020, 11:06 AM   #26
altec9440 is offline altec9440  France
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I used to spray paste, but I follow silpad evolution... and experience of others.

a link for design guide:
http://www.farnell.com/datasheets/99609.pdf
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Old 5th December 2020, 11:38 AM   #27
jneutron is offline jneutron  United States
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Quote:
Originally Posted by amplidude View Post
Very good explanation jneutron, and I will try on my next project which is a pass amp, your method with copper and graphite.
An idea, the device shown is clean smooth copper, could one not simply solder a 2 by 2 inch copper plate with smd solder paste and then reflow the whole thing?
The reason I want a datalogger is to be able to verify the improvements from this method, it produces nice graphs or spreadsheets
The overmolded plastic can be damaged by soldering heat. One of the last steps in the stamping process is to form mechanical features which will hold the plastic in place. For example, the plastic edge on the bottom of the device is actually a mechanical lock, the edge of the copper has a taper and is wider at the top so acts like a key. Heating the entire bottom to 250 plus C can compromise that function.
Should the mechanical bond between the plastic and the copper be compromised, flux can wick into the device. The semi and bond wires in the past were covered with "glob top" to protect the semi and wires from the injection molding plastic process and the.different thermal expansion coefficient of the overmolded plastic. But I've been out of that industry for 30 years now, so am not sure if they just use a better overmold formulation.
Alexberg... while the spec may be very good, the actual product out there is not that good. Next time I go into work, I will look for one of the failed units, take it home and trammel it. I have dro's on my mill and a dial indicator good to go, I'll just have to 3D print a holder to lock the indicator in the R8 collet. Since I can't perfectly align the backside, I will take data every mm or so, then do a trendline fit on excel to see how it curves.
My check at work was to use a straight edge and backlight, but I will agree that is a great test of flatness with absolutely no calibration of actual curve.

Data works for me, I will try. May take a week or so.

Jn

Ps.. amplidude, unbelievably, graphene is a hugely thermally conductive material, far far better than solder. Also, using SMT paste and flux guarantees bubbles under the device due to outgassing while reflowing. In the device, they will usually stamp the copper bonding surface with a pattern of grooves who's sole purpose is to allow the outgassing a path to exit under the die.

PPS. When I wrote the thermal article for Linear Audio, I researched the junction to case values (more specifically, case to heatsink)for a large part of the IR mosfet line. They tended to use a single value for a wide variety of die size. That indicates to me that they do not use the 45 degree spread model, but are assuming the backside is being used isothermally and that it is the area of the backside used in the calculation. The reason to do that is to safely accommodate lack of flatness as well as inconsistency in mounting force.
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Last edited by jneutron; 5th December 2020 at 11:53 AM.
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Old 5th December 2020, 01:45 PM   #28
amplidude is offline amplidude  Denmark
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I will be looking forward to your findings, and study heat transfer more indeepth.
So I got both channels mechanically assembled, a process that could have taken few hours, have now stretched over several days due to lapping and polishing all heat transfer surfaces, but I'm confident it was worthwhile, and its lifespan is prolonged.
Thanks guys for all the technical insights
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Old 5th December 2020, 02:35 PM   #29
jneutron is offline jneutron  United States
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Just finished the design in openscad, going to print it now, s/b about 2.5 hours.
Going to try using an embedment nut, so have to make an adapter for my soldering iron in brass on the lathe. Hopefully, my first try at embedment goes perfectly..

I do have some 254's in parts, so may have to trammel some.

Jn
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Old 5th December 2020, 06:22 PM   #30
jneutron is offline jneutron  United States
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my 3D printed trammel fixture worked well. I did have to chase two holes a bit, the software recently updated and I lost the compensations for internal hole size, about 400 microns too small, no problem.


I had a batch of to-254's, IRFP-250 mosfets, from the time dinosaurs roamed the planet.

Flat. totally flat to within .001 inch throughout the copper surface. The package encapsulation epoxy was almost .002 thicker on one edge than the other, but absolutely of no concern.

I will look for those non flat parts Monday at work.

jn
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