kicad-developers team mailing list archive

[Lorenzo Marcantonio <l.marcantonio@xxxxxxxxxxxx>]

On Sat, Jul 07, 2012 at 12:38:09AM +0200, Miguel Angel Ajo Pelayo wrote:
>  It's a pad that gets connected by thermal reliefs?,

It's simple: tht pads get reliefs, smd pads get full copper

> Just out of curiosity: In which case wouldn't you want a SMD pad to
> disipate heat to the PCB? :-)

It's the contrary! Full explanation:

Thermal reliefs are *only* a manufacturing/reworking convenience: they slow
down heat conduction so that you can hand solder or get properly heated during
wave soldering; otherwise you need a way bigger soldering iron: for example I
have a 70um copper board with about 10mm tracks and to replace the terminal
blocks you *need* at least 150W of iron (or a big heat gun).

At the same time reliefs get in the way *dissipating* heat (since it's what
they are designed to do!) so you need to balance spokes width and antipad (also
to mantain current carrying capability); the whole design method IIRC is
explained in IPC-2222 (a couple of formulas), maybe I'll do a patch for it.

SMT components instead have none of these manufacturing issue, unless wave solder them which is not the preferred way... unless you have a big volume and you need to lower the price of the board (then you need different pads, align stuff to avoid bridging and so on). During reflow the *whole* board is heated and wattage is not an issue (desktop ovens are about 2KW and you don't want to know the power drawn by multizone mass production models :D). So there is no need to thermal spokes: they would impede heat dissipation!. Instead you can copper fill all around; in fact the pad becomes mask defined, not copper defined (look in your power component datasheet if that's acceptable).

Practical example: look at the IRF8304M datasheet; you have a 6x5x0.7mm case which can dissipate 100W *if* you can keep the case at 25°C. There are drawing of the test layouts for thermal resistance and there is no thermal spokes (obviously).

OK, now think of a zone which has inside *both* a terminal block and the drain of an smd fet. How would you define this? 'connected' would make bad solder joints on the block (due to the absence of thermals), 'thermals' would add spokes to the drain (blocking dissipation).

The patch simply allow that situation.

-- 
Lorenzo Marcantonio
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