Thermal Relief Pad

We "flood over" pads and vias all the time. It's no big deal. If the board is surface mount, or wave soldered, it's fine. Our boards are mostly surface mount, with a few thru-hole parts hand soldered after reflow. With a good assembler and a good Metcal iron, soldering the non-thermal pads is still no big deal.

Thermal resistance and electrical resistance are inseparable. For most metals, including copper, the ratio is about 140,000 K/w thermal resistance per ohm electrical. So a typical thermal pad, with four spokes (1 net square of 1 oz copper) and maybe 0.5 milliohm of resistance, has a thermal conductivity of about 70 K/w to the plane.

John

Yes - This is the sort of job where a Metcal really shows how much better it is than most other irons.

Yup, best money I ever spent on a tool.

Wouldn't oven reflow soldering be more tolerant of thermal-less pads? I would think that they'd heat up the solid copper as much as everything else, which avoids the cold joint problem. Same with hotplate reflow like I use; the ground planes are plenty hot by the time the paste melts.

than most other

But heating those flooded pads still slows things down. If a milliohm of electrical resistance is OK, the 140 k/w thermal resistance will make soldering a lot easier. You've just got to run the numbers.

John

is than most other

A milliohm shouldn't be a problem (0.064W / 0.008V drop), my concern was the current handling capacity of the spokes. After doing the calculations for the track width the spokes didn't seem to add up.

Infact doing my own calculations on the spokes give each one to

Thanks for all your replies.

Andrew

is than most other

Re. current-carrying ability, remember that each end will be heavily heatsinked, so the temp rise of the spoke will be a lot lower than a piece of track of the same width.

it is than most other

heatsinked, so the temp rise of

Again, approximate math is simple. a 10x10 mil spoke has 500 uohms resistance. Run one amp through it and it dissipates half a milliwatt. The thermal resistance from the center to the pad/plane is about 35 k/w, and the average is about half that. So the center hot-spot temp rise is ballpark 10 millikelvins.

John