0805 size high power resistors, 250mW ok on FR-4?

Sounds OK to me. We push regular 0805's harder than that. Zero fails so far.

We experimented with various resistors, 0603 through 1206, with a thermal imager that has plenty of resolution to find the hot-spot temp on all the parts.

Turns out that the actual part thetas are astronomical, and what cools resistors is the copper pads and traces, more than the bodies themselves. An 0603 soldered to big copper pours can dissipate as much as a 1206 ditto; the hot spot temps are the same. That makes sense physically, as theta is determined by the L/W ratio and the substrate thickness, and all the resistors we checked were all 20 mil thick alumina.

A better substrate, BeO or AlN, would help a bit, at great expense.

If there's a ground plane close, that helps a lot.

The easiest thing to do is fire up the board and scope it with a Flir, and see how hot stuff gets. You can borrow mine, or come here and use it.

ftp://66.117.156.8/IR_0026.jpg

ftp://66.117.156.8/IR_0032.jpg

John

Thanks, John. I think we are ok then up to 250mW. At least the resistors are rated for that. I've got to visit you guys some day and look through the FLIR. And maybe empty a pitcher at Zeitgeist.

Another peculiarity that surprised me was that the TO18 was stated at

Thermal vias are your friend. Vias in pads are now standard procedure for most PCB houses. Put a thermal via in each pad and one under the center of the part. Tie TVs to heat spreader on another plane. JL, tell us more of your FLIR, type, cost and performance. Cheers, Harry

It's a model E45 with the wide-angle (germanium!) lens. You can basically touch a part and still focus, so you can easily resolve the hot spot temp on an 0603 or a SOT-23. It's amazing... image a board and learn all sorts of stuff.

The PC interface, sadly, is insane. It's USB, but it's a network device, so you have to use their very klunky PC software. They should have made it work like all the other cameras in the world.

Cost about $10K, and worth it. I think some new technologies are coming on board, so prices will drop.

In that second pic I posted, we thought that the FPGA (under the blue heat sink) was getting too hot as clock frequency increased, so we were looking into clock gating and tedious stuff like that. A few minutes with the Flir showed that the dacs (the white-hot blobs) are in fact heating the FPGA.

John

On a sunny day (Wed, 23 Jul 2008 11:52:06 -0700) it happened John Larkin wrote in :

Xcuse me, but would not a finger test show that too? That teh DACS were much hotter then the FPGA? I am not against modern tech, but 10k is a lot of money for something you can also do with a temp probe or your finger. ;-)

Consider a series of images, of the whole board and of regions, taken at different clock rates. That tells us, very quickly, where the power is going, and sometimes it's unexpected. It's very hard to quantify, or even feel, the temperature of an 0603 resistor or a SOT-23 opamp, and one board can have hundreds. If you stick a thermocouple to a small part, you suck all the heat away. And the simple IR thermometers have rotten spatial resolution.

One of our VME boards can sell for more than the Flir cost. It's been a great tool, sort of an instant thermal design review.

It's also cool for troubleshooting. Suppose a power rail is collapsed. You can often image the board and find the part that's hogging the power. Or add another power supply, force a few amps, and image the traces.

And with the tax considerations, the net price is about half.

John

Some time ago I posted a little auto-zeroing microvolt meter that I used at GenRad to trace drops on PCB traces, to locate shorts.

...Jim Thompson

I just donated it to a client but in the days when that still happened a lot I used a >10A power supply for that. Crank it up ... *WHADDABAM* ... aha, t'was that high-faluting PWM driver again. Only with eye protection, of course. Sometimes all it takes is a D-cell.

I used to "cure" pin-hole shorts in alumina thick-film hybrids with a

...Jim Thompson

But that was in the days before LDO chips that short out for no particular reason and "SMPS in a can" that lose clock and where the only current limit is some Rdson.

Yep. It was a _real_ supply, took two people to carry it ;-)

...Jim Thompson

Nowadays, most bench DVMs have sub-microvolt offset and resolution, so you can probe trace drops directly. The Flir lets you *see* the current paths.

John

So, it was portable?

Sure.

It was like those instruments designated by Motorola, "Try to move it by yourself, and you're fired" ;-)

...Jim Thompson

I was thinking about the early Ampex 1" R-R VTR with a pair of slide out coffin handles. Two people could lift it a few inches, to put it on a very strong cart. If you weren't VERY careful, you would end up in a coffin. :(

I remember those ;-)

...Jim Thompson

You remember when Bill whatshisface had a job, too! ;-)

only

You misspelled "whatshitface" ;-)

I remember "I remember Mama" (trivia test of the day ;-)

...Jim Thompson

That is older than me.

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