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Re: [Amps] 'heat spreader' sizing ??

Subject: Re: [Amps] 'heat spreader' sizing ??
From: Steve Thompson <>
Date: Sat, 05 Feb 2011 16:36:44 +0000
List-post: <">>
A potential problem with soldering a transistor down is ensuring 
there aren't any voids. Where transistors have a carrier and 
flange soldered together it's normal to do it in a reflow oven 
with an inert atmosphere, using preformed inserts.

Both Freescale and NXP discuss mounting the latest high power 
devices in application notes. For example, see NXP #10800 para. 2.5.

It's interesting to note the difference between the regular white 
heatsink goo we're all used to and some of the new stuff prompted 
by CPU cooling demands. At 500W dissipation in one of the kW 
devices there could be several 10s degrees (C) improvement.

I'm hoping to find time to experiment with these transistors - my 
plan is to lap the transistor onto the spreader with ultra fine 
grinding paste or jewellers rouge and then use Arctic Silver 5 or 
Shim Etsu 7762 thermal grease. Both are readily available for 
about 2% of the transistor cost.

> Your thought of soldering also needs to take the differential thermal 
> expansion to be taken into account - solders vary widely in their ability to 
> take repetitive thermal cycles .
> I would think the device manufacturer would supply you with the materials 
> used in the package and their thermal expansion properties as well as 
> recommended mounting techniques. It is certainly to their benefit to do so.
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