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Author Topic: Best practice in mounting a "chip" attenuator to a heat sink  (Read 1677 times)
N1JR
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Posts: 15




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« on: December 14, 2012, 07:12:49 AM »

I have a "chip" 30 db attenuator (about 1" square metal bottom) which I am mounting to a substantial aluminum heat sink.  For best heat removal purposes, am I better off to mount (bolt down with thermal paste) the chip directly to the aluminum heat sink or, instead, to bolt the chip first to a 1/4" thick copper plate (say 2" x 2" or so) and then mount that copper plate to the aluminum heat sink?  I'm wondering if the heat transfer improvement I may achieve with the larger copper plate to the aluminum is offset (neutralized?) by the existence of the second thermal boundary (i.e. between the chip bottom and the copper).  I would appreciate any actual experience and/or any actual engineering knowledge here.  Thanks.  Jim Robbins, N1JR
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KA4POL
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Posts: 2028




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« Reply #1 on: December 14, 2012, 08:17:40 AM »

For an exact answer you one would need to know the thermal resistance introduced by the two contacts versus the one contact. Just from experience I would use only one heat sink as you said bolted and using thermal paste. I guess you don't need to keep it electrically isolated.
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G3TXQ
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Posts: 1524




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« Reply #2 on: December 14, 2012, 02:53:24 PM »

You may want to take a look at some of the heat sink arrangements used on high power semiconductor amplifiers; for example Motorola's Engineering Bulletin EB104 "Get 600W RF from four power FETs".

To quote:

"It must be noted, that special attention must be given to the heat sink design for this unit. With the 200 – 300 watts of heat generated by the transistors in a small physical area, it must be conducted into a heat sink efficiently. This can only be done with high conductance material, such as copper. If aluminum heat sink is used, a copper heat spreader is recommended between the transistor flanges and the heat sink surface."

Communication-Concepts supply an 8"x6"x3/8" copper spreader to go between the FETs and the aluminium heat-sink on an EB104 amplifier.

73,
Steve G3TXQ
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VK2TIL
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Posts: 331




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« Reply #3 on: December 14, 2012, 03:18:27 PM »

Jim; PM sent.
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