removing heat with thermal tape and small heatsinks

By a factor of at least 5.

Would the heat sink go on the bottom of the board? That would benefit from strategic use of copper pours and vias to transport heat from the parts.

Thin tape may not conform to the surfaces, and leave air gaps. Compressible gap-pad material would be better.

I'll probably also add a

At 5.5 watts, you could just blast air down onto the board, sort of like this:

Again, copper pours make a huge difference in getting heat out of small parts. 0.75 watts isn't bad for a 1206 with decent end-cap cooling. I'm using some AlN 1206 resistors rated for 11 watts!

The SOT23s, with the usual tiny pads, might be 300 K/w. That sounds grim at 1.1 watts.

Internal copper layers, a ground plane and some copper pours, can make a small board almost isothermal, so it wouldn't matter too much where you blow the air.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

If you have room for a fan, then why not just spread the components out?

Cheers

Klaus

Hey, Win, please post some pics of the top and bottom of the board.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

--
 Thanks, 
    - Win

Things will move around a bit to improve the power situation.

--
 Thanks, 
    - Win

Right. Bergquist makes stuff called "Gap Pad" in several thicknesses. The stuff is a bit sticky, but not good enough to hold a heat sink onto the board. So, you'd need some sort of screws and spacers to anchor the heat sink in place with the right crush and pressure to maintain thermal conductivity. And, the Gap Pad stuff is not real high thermal conductivity, either. We've used it to keep some medium-power chips cool even when run in vacuum. But, it might not be good enough for higher power densities.

Jon

Interesting trace pattern on the bottom side, near R30.

If you're going to spin the layout, you might add more copper and thermal vias to move heat to bottom side pours, where there's more room. Especially if capacitances don't matter much.

When I do a guard trace that dead-ends, like your bottom leftside, I usually get a call from the fab house telling me that I have a trace that dead-ends.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

What John said, more copper. (Some power R's spec thermal resistance for different pad sizes.) Can you use transistors in a package with a thermal pad?

George H.

I use some stuff from 3G, 2mm thick, that is rated 6 w/m-K. It's soft and fairly compressible and a little tacky on both sides. A square inch, compressed down to 1 mm thickness (with modest force) is under

0.3 k/w. It's a good dielectric, too.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I like SOT89s for a couple of watts, but there aren't a lot of parts in that package.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Here is (as is usual with EETimes) a very bad article about gap pads.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Right, those.

GH

It doesn't mention graphite cloth, which is electrically as well as thermally conductive, but worked fine for me (as is mentioned in my 1996 millidegree thermostat paper).

--
Bill Sloman, Sydney

Winfield Hill wrote in news: snipped-for-privacy@drn.newsguy.com:

Skip the tape and use thermal epoxy. Need to retain serviceability?

Place those elements on a daughterboard with the heat sink, and stand it off the main board on pins. Make more than one and trade out the entire unit upon failure. Service the failed units.

Looks like you are pushing them pretty hard.

I was amazed that the Euro radial leaded resistor form factor has the ratings they do.

Our US (read Vishay, most of the industry) 1/8 W units get pretty hot at their rated value. The Euro (read British) resistor in the same sized package is rated at 1W. Not sure I believe them.

Anyway, the daughterboard route allows you to modify those elements as well and create the improved set up all properly integrated onto the sinking element.

Jon Elson wrote in news:N9ydnaLhN snipped-for-privacy@giganews.com:

The amount of heat he is talking about coming off of these is way more than the gap pads are designed to dissipate heat from. That rate is so slow as to be counterproductive in this case.

He needs a hard, thermal connection to a real, finned heat sink and his fan. And THAT will do it. Otherwise, the numbers he states are really an indicator that the entire circuit segment is a flawed or under examined design.

So if this is a patch job for a single board... sure. If this is a production solution for a problemeatic design, it will likely see a lot of failures in production and in the field once deployed.

Yes, that first up on the next pass.

Like John said, not much choice about it. My HV amplifier design specializes in wider bandwidths and faster slew rates, so I'm forced to stick with low-capacitance parts. But my design allows me to shift much of the heat to a series string of resistors, hence the 150C temps they were experiencing. I note that according to the datasheet, 150C is the expected temperature at the rated power with a room-temp ambient, but I'm going to see how far down I can push that with heatsinks.

--
 Thanks, 
    - Win

Gap Pad(R) is probably on the older side of things; new materials with quite high K are available, like t-Global's lines,

which are available in loose gooey form, perfect for filling gaps (even moreso than Gap Pad).

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/

Good suggestions. My basic high-production PCB will be slower, and will not have severe heat dissipation issues. I can add a few pads and have plenty of height to allow for an appropriate high-power daughterboard for the fastest versions, good idea!

--
 Thanks, 
    - Win

Winfield Hill wrote in news: snipped-for-privacy@drn.newsguy.com:

serviceability?

trade

has

pretty

the

And the elevated board can also benefit from the airflow you ride onto the sink tines. You could even sink the bottom side too.