0805 size high power resistors, 250mW ok on FR-4? - sci.electronics.design

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

John Larkin

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More options Jul 24, 4:55 am

Newsgroups: sci.electronics.design

From: John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com>

Date: Wed, 23 Jul 2008 13:55:18 -0700

Local: Thurs, Jul 24 2008 4:55 am

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

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On Wed, 23 Jul 2008 19:06:23 GMT, Jan Panteltje

<pNaonStpealm...@yahoo.com> wrote:
>On a sunny day (Wed, 23 Jul 2008 11:52:06 -0700) it happened John Larkin
><jjlar...@highNOTlandTHIStechnologyPART.com> wrote in
><urue84dk5os9eiihvr5d990o7lvhl7q...@4ax.com>:

>>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
>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

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