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

83.3C/W to case by ST and the TO92 at the same by ON. Junction to ambient was stated lower (!) for the TO92 even though it's plastic. Strange. I haven't used TO packages in roughly two decades but now I have to, for the proto run. I thought TO18 would have to be better but it carries the collector and then some day it touches something ... bzzzt ... POOF.

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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

Can I interest you in 0402 at 10 watts? Or 0505 at 50 watts?

John

Ouch! That ought to do it for an impressive meltdown procedure on FR4. Reminds me of a tube in a ceramic socket on phenolic. I was still a kid, blissfully unaware of the limitations of some materials. Transistors were very expensive in Germany so I built most stuff from salvaged tubes, plucked out of radios and TV sets. Well, this thing worked for months. Then one fine day there was fireworks, hissing and smoke. The ceramic socket including tube had fallen through the charred hole that had developed. Now I knew where that faint "amperage smell" was coming from. Luckily the plate connector was up top and the tube fell in, but didn't tip over. Cuz I had "economized" on the power supply and ran straight from 230V mains into a voltage doubler. The fuse was, ahem, home made.

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My first ever job interview, I made the comment that I generally preferred tubes (which I got free) to transistors, because transistors were too easy to blow up. The prig said "that won't do" and didn't hire me. Next interview, I said the same thing and that guy laughed and did hire me. I designed about $100M of stuff for the second guy.

John

Similar things happen to consultants. Sometimes people present a task, then ask whether I am versed in VHDL, Cadence and all that. I often tell them "No, but a chip design wouldn't make sense here, we can do that at lower cost with off-the-shelf parts". Sometimes they walk away. The companies of those who do are usually not heard of much anymore. Some of them keep burning an enormous amount of cash until their VCs turn off the spigot. The sad cases are where I could have really helped them with $50-100K in cost, they decline, keep trying on their own, blow through another few million bucks and then kablouie.

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I had that same experience with Metricon. I had a bog simple, cheap, proven way to do what they needed. They had a complex mess, but said it didn't matter because "silicon is free." Remember Metricon?

John

Self-desoldering?

Best regards, Spehro Pefhany

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I'm beginning to suspect that, in many cases, that (the last sentence) is in fact the whole point of the operation. The high-level technical people may or may not believe in the product/technology being developed, but the executives are just in it for the boondoggle, living high on the hog for as long as the investor money lasts, then moving on to a different project.

Even the investors are not entirely unhappy with this situation -- they're often looking for ways to take a loss without being too obvious about it.

It's just frustrating for the lower-level technical people who probably don't realize what's going on and put a lot of good work into something that just ends up in the dumpster. I once worked for a company where I now firmly believe that this is what was (and may still be) going on.

-- Dave Tweed

Believe it or not, even successful companies can have to much money. I worked at a place where their main product line was quite successful. When it came to new development they never had to make hard decisions, they could always throw money at a problem. As a result a vast software and hardware infrastructure grew to resolve various perceived (often self-inflicted) problems. Combine that with ISO 2000 and such and you came to realize how Kafka came to his story; and how bureaucracies form and survive. The particular causes were many and in a lot of cases not due to overt empire building; that made little difference when people who didn't understand what they were doing organized a project. Of course the opposite also leads to insanity, penny pinching at a microscopic level.

To paraphrase some Greeks, Yogi's, and Buddhists Balance is all, and to be cherished.

RayR

These guys?

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Luckily none of my clients, many of whom are VC-financed, is like that. They all want to make it happen and I am sure most of them will. There is always going to be risk, as in "venture" :-)

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No. They were in Silicon Valley. They started as an RF electric-meter reading company, staffed by a bunch of ham-radio types. The meter they designed was too expensive for any utilities to buy (geez, I only wanted a buck a meter in royalties) so they hopped on the Internet bandwagon and migrated their mesh technology into Ricocet.

The guru was

and the prez, when I was involved, was Bob Dilworth, formerly the head of Zenith Data Systems, the early laptop pioneer.

Burning hulks everywhere.

John

Quote "Ricochet sent its Denver based customers an email at night on March 28th 2008, that service will cease as of March 29th 2008."

Yikes! Talking about customer valuation. Drop the ball and run.

Quote "Overall, Ricochet is the only consumer priced, mass market, high speed, wireless data network that enables you to send unlimited data."

ROFL! I can just picture what'll happen when everyone is online and you want to download that 4MB PDF datasheet.

Pretty sad. This repeater technology has a hard time. Companies try to push it via protocols such as Zigbee but in order to make that work customers usually can't start out small, you have to have critical mass. Data volumes are low and when a node at a critical juncture goes down a whole chunk of the network becomes orphaned.

In ham radio packet transmission is rather successful and very useful in emergencies. But that's because ham groups can mobilize a large pool of motivated volunteers very fast and data volumes are low.

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Oops, sorry, I spelled the name wrong, as usual. Metricom, not Metricon.

Another example of mass insanity.

John

None of my current clients is like that, either. It's just that I've seen a few organizations that just couldn't possibly be that badly managed by accident alone.

Now that I'm a full-time consultant, I have plenty of work from a fairly diverse set of clients. I get paid straight time and materials, while they take the risks and reap the rewards (or not). This suits me just fine.

-- Dave Tweed