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, 2:52 am

Newsgroups: sci.electronics.design, sci.electronics.components

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

Date: Wed, 23 Jul 2008 11:52:06 -0700

Local: Thurs, Jul 24 2008 2:52 am

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

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On Wed, 23 Jul 2008 18:40:04 GMT, "HarryD" <har...@tdsystems.org>
wrote:

>"John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in message
>news:n1qe84pvvg5bfomsm6uvu2e8a42mf35ouk@4ax.com...
>> On Wed, 23 Jul 2008 09:59:26 -0700, Joerg
>> <notthisjoerg...@removethispacbell.net> wrote:

>>>Hello All,

>>>Got a circuit where in hindsight the client decided they need >10 times
>>>the juice than initially spec'd. This was truly unforeseen for them and
>>>we have to make it work at least for a proto series. Of course, now all
>>>those 0805 resistors in the power path would exceed their usual 1/8W
>>>spec. Found these at 1/3W:

>>>http://www.susumu.co.jp/english/pdf/products-j01-07.pdf

>>>I plan to run them around 200-250mW. Was expecting those to be really
>>>tall to vent off some heat but they are surprisingly flat, only 0.4mm
>>>high. Will this become a heat problem on FR-4 for stuff that can run
>>>24/7? Any experience?

>> 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

> 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

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