We were thinking about adding a feature to one of our boards, sort of a programmable circuit breaker thing. It would just be firmware, but it could add some thermal stress to some 1206 resistors. So I did a quick measurement:
A 1206 resistor, mounted normally on layer 6 of a 6-layer board, with a solid ground plane on layer 5, about 8 mils away. Hot-spot theta, measured with an IR viewer, was about 80 K/W. So this would be comfortable at, say, half a watt dissipation.
John
What's your maximum allowable ambient temperature near that resistor?
Those thermal imagers are the bees knees. They're still a bit expensive, but very worth it (ours was about $2.5k).
Bob
60C maybe. It's a VME module, so we can expect pretty good air flow across the board. I did the resistor measurement in still air.
What kind is that? In knew prices were coming down, but that's impressive.
Our FLIR is a couple of years old now. It cost $12K. But it's pretty good. It can measure the hot-spot temperature of an 0603 resistor.
Here's a tiny NEC dual transistor with power being dissipated in one side.
ftp://jjlarkin.lmi.net/UPA800_80mW_one-side.jpg
So much for any hopes of thermal matching.
John
Ours is made by FLIR. I didn't buy it so I'm just going by what I was told. I was showing it to our receptionist and took a moment to image her. It's amazing what it reveals.
So, your 1206 will be running at 100C? Is it rated for that?
Bob
These resistors, common Panasonic thickfilms, are rated for 1/4 watt or 1/8 watt dissipation, depending on which specs you prefer. Whatever that means.
I don't trust any of these specs. The hot-spot temperature is wildly dependent on how well you get the heat out of the end caps. An 0603 can dissipate a watt if you solder it to really big copper pours. This
1206 has the standard pcb pads, but the nearby ground plane sucks a lot of heat out. I figure a hot-spot of 100C is conservative.I tried pulse testing these resisors, too. They die at about 60 watts for 1 millisecond.
John
Looks like 2 dies in one package.
Yes. This, like many dual transistors, is an isolated pair that shares only epoxy. Thermally, it's hardly better than two SOT23 transistors side-by-side.
ADI did recently revive their monolithic duals.
John
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