DUTs

Nice and thanks. I really like when my calculations, all based on somewhat erroneous assumptions, land fairly close to the test result. It appears that 62 msec is the minimum the 50 ohm load will survive at

30 volts (18 watts) overload. Cracking the resistor in half seems rather odd. If the chip was unable to propagate the heat to the copper PCB heat sink in the short length of time allowed, I would have expected both this thermally isolated test and the heat sink version to have nearly identical times. Instead, adding the heat sink resulted in 5x longer time to failure. Therefore, it would seem that there is sufficient time for much of the heat to propagate and that a heat sink is a useful improvement to survivability. Also, since the substrate is conducting some of the heat to the copper heat sink, a silicon nitride substrate will yield an additional improvement in survivability time.

These might be of interest:

"Power Dissipation in High Precision Vishay Sfernice Chip Resistors and Arrays (P Thin Film, PRA Arrays, CHP Thick Film)"

"SMT Resistor Thermal Design and Layout"

"The heat is on: high-power surface-mount resistors" There are new technologies using aluminum nitride (AlN) ceramics in place of aluminum oxide that boast extreme power ratings; for example, >100 W for a 1206 case size.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Skype: JeffLiebermann     AE6KS    831-336-2558

Filet de solder isn't normally on the menu but does taste better when served on a copper plate.

Sorry, but I couldn't resist(or).

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Jeff Liebermann wrote in news: snipped-for-privacy@4ax.com:

The term we usually used was UUT. For "Unit Under Test".

Am 28.04.2018 um 04:57 schrieb Jeff Liebermann:

That's what the 1206 says: SCNR!

(we had that recently in d.a.netdigest :-)

Gerhard DK4XP

What kind of 'Al2O3' are you thinking of? My tube furnace was an Al2O3 item that took lots of stress, at cherry-red temperatures, without any problems. Metallization would fail first, or adhesion, I suspect.

Could there be cumulative damage even if you detect the condition in

1ms?

It looks like 62 milliseconds when it blows up, but about 25ms when it starts to go.

When I pulsed the 47 ohm resistor with short pulses, to check the setup, current increased a bit with each pulse, 18 watts and 1 ms. Might have been thermal, more likely something going on. It did seem to settle after 10 or 20 pulses, maybe something like annealing.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

How thick is the ceramic tube wall in your furnace? Fairly thick would be my guess(tm): Using a 2" tube as a random example, the wall thickness is: (50.80 - 44.45) / 2 = 3.18 mm

Meanwhile, the garden variety 1206 thick film SMD resistor is only

0.55 mm thick, which is far more fragile than a tube 6 times thicker. A ceramic tube oven is designed to heat the tube very evenly, thus preventing stress cracks that might appear from uneven heating and expansion. Meanwhile, the 1206 resistor starts out by heating at one spot. Without a heat sink, the resistor will heat quite unevenly.

From the above data sheet, thermal expansion appears to be fairly typical for alumina (8 * 10^-6/C) which would be the same for the resistor.

Bottom line. The resistor cracked because the substrate was thin. If the resistor had a 3 mm thick substrate like the tube, it would not crack.

Drivel: In the distant past, I worked for a company that made radio communications accessories. Part of the product line were CTCSS (continuous tone controlled sub-audible squelch) boards, which used a twin-T filter to decode and encode the tones. Frequency accuracy and stability over a wide temperature range were critical, so we settled on making the twin-T filters using hybrids consisting of carefully selected chip caps and silk screened resistors on a ceramic substrate (fired in an electric tube furnace). The resistors were trimmed to frequency originally using an air abrasive sand blaster (bad idea) and eventually a do-it-thyself CO2 laser.

When the laser was aimed at the ceramic substrate to see how well it would scribe the substrate, we had a small explosion which launched pieces of ceramic everywhere. So, we turned down the power and played with the duty cycle. A point where the substrate wouldn't fly apart was eventually reached, but it was too low to be useful for scribing. Lesson learned: Keep the laser dot moving.

The laser problem was similar to the 1206 resistor cracking in half. Apply too much heat at one point, and the ceramic under that point will expand, causing the substrate to crack.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Awesome dremeling (I'm assuming). How'd you get such straight lines with it?

Ed

Lot of practise, mostly; I do this a lot. I whiteboard the schematic layout first, then draw lines on the FR4 with a sharpie, usually freehand or maybe with a straightedge, and then route them under my Mantis magnifier. I use a Dremel with a carbide dental burr. For important stuff, I use gold-plated FR4. This can usually be done in minutes.

Each prototype and resulting experiments are assigned a part number and archived in a dedicated folder on a server, available to other employees theoretically forever. So there is peer pressure to make things pretty.

I did my kilovolt pulser prototype this way and took it to a customer meeting. They were reportedly very impressed, and it might result in some decent business. I also use a lot of hand-drawn sketches and whiteboard photos, which are usually well received. I think a lot of people are tired of silly PowerPoints and such.

This is going to be in a proposal going out to a giant aerospace company on Monday.

I thought about spending an hour doing this in Visio or something, but bailed on that.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

"Device Under Test" surely? If not, then kindly enlighten me, John. Forgive my shameless and wanton ignorance on this issue.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Using a straightedge is an excellent pro-tip! What sort of straightedge? It's become apparent to me that my primary metal cutting tools are a 4" circular saw and a Dremel. They're good enough for my prototypes one offs. OK, sometimes a few dozen pieces are actually rolled out. Although it takes time to Dremel metal destined for a client's site, it takes even more time to master a metal working beast, such as the one you showed off in sed a few weeks ago. Life's too short for me to tackle such things and there's no employees here to do it for me. Your hand drawn presentation for your pitch looks great. When it comes to fashions, sometimes old becomes new again. Your hand drawn sketch stands out simply because it's /not/ yet another Powerpoint presentation.

73,

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Don Kuenz, KB7RPU

Using a straightedge is an excellent pro-tip! What sort of straightedge? Oh! Nevermind. It just occurred to me that you use the straightedge with the Sharpie, not the Dreme. It's become apparent to me that my primary metal cutting tools are a 4" circular saw and a Dremel. They're good enough for my prototypes one offs. OK, sometimes a few dozen pieces are actually rolled out. Although it takes time to Dremel metal destined for a client's site, it takes even more time to master a metal working beast, such as the one you showed off in sed a few weeks ago. Life's too short for me to tackle such things and there's no employees here to do it for me. Your hand drawn presentation for your pitch looks great. When it comes to fashions, sometimes old becomes new again. Your hand drawn sketch stands out simply because it's /not/ yet another Powerpoint presentation.

73,

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Don Kuenz, KB7RPU

Anything. A steel ruler, or the edge of a plastic template (I still have those!)

To make accurately spaced lines parallel to an edge, I lock my dial caliper at some setting and slide one flat jaw along the edge of the FR4 and make a scratch on the copper with the pointy part of the other. For controlled-impedance lines and such.

We have an old cheap rusty shear/roll/brake thing that I use to chop up the pcb outline. I'm not allowed to use the good foot shear on FR4.

A sabre saw will cut FR4 pretty well. Rub it on a piece of sandpaper, flat on a table, to finish the edges.

We have a Tormach n/c mill, and one of my guys is really good at programming and using it. I've never learned to use a mechanical CAD program.

We have one stuffy european customer that I wouldn't try the hand-drawn stuff on.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

It occurred to me after my first followup that your straightedge is for the Sharpie, not the Dremel. But, you followed up before my Supersedes reached you. A 1/2" X 1/2" aluminum angle secured with c-clamps is used as a straightedge for my 4" circular metal hand saw. The saw's plate squares up with the aluminum angle to make perfect cuts in sheet metal. My Dremel 4000 actually hangs from the wall with a 225-01 flexible shaft used as a hand-piece. The hand-piece has a small hilt 1.5" up from the tip of the bit that's about 0.25" wider than the bit. This thread makes me realize that a short length of 2" X 2" aluminum angle is tall enough to use as a straightedge. For the Dremel the hilt contacts the aluminum angle straightedge and the hilt functions as a saw plate.

73,

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Don Kuenz, KB7RPU

LOL! An accurate description indeed. :-D

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Cursitor Doom wrote in news:pc50ck$5oo$26@dont- email.me:

Perhaps you should, for once in your life, read an entire thread before posting.