>> 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