Thermal Management

[Al Keyworth]

I need to modify D-Pak land pattern to improve thermal performance. I am using a TI 3KTE PowerFlex part. The spec calls for soldermask over Cu under the part in area not contacted by thermal tab stating it "improves thermal performance". Huh?? How? Following that direction, should I soldermask areas outside of footprint that I add for increased dissipation? Can anyone suggest resources for such issues - I have three different part footprints with similar requirements.
Al

[randychase]

I am trying to grasp how adding soldermask improves thermal performance. Anyone else?

[Ross Clunie]

I've never heard of it either, I would suggest that you get in touch the parts vendor and ask them to clarify their recommendation.

[David Ricketts]

Dark radiates heat better, and soldermask is darker than copper or tin/lead. When was the last time you saw a heatsink that wasn't black?

[randychase]

Actually a lot of heatsinks are gold chem filmed. I have a collection of heatsinks here that are red, blue, or clear.

I think the small increase from black would be offset by the thermal insulating properties (though small) of the coating on pcb copper.

[smfaucher]

I'm reading the upowerflex abstrac and canot find where they say that solder mask over CU is better than CU. They say to help power thermal managment by using a large portion of sm over cu around the chip.

I guess you would be better of without soldermask, but in some application you canot because oxidation of the CU is to high.

http://focus.ti.com/lit/an/szza015/szza015.pdf

Sébastien

[Al Keyworth]

This is the manufacturer's response;
AL, what they are saying here is that it is common practice to extend the AREA of the heatsink plane onto the surface and the exposed added ground plane would then be covered in soldermask, as it normally would.

The arrow that points to the small grey area is a little misleading. The A-2 page just shows the minimal footprint required. Since the copper the IC is soldered to acts as the heatsink, you can extend it as needed to meet the thermal dissipation of the regulator.

This is referenced in FIGURE 2 in the SZZA app note and in the text that explains how the pad area effects THETA JA.

Bottom line, if your board has a ground plane then just connect the parts tab to this plane and use it for thermal relief. The tab on the bottom of the IC is the same potential as the center pin, COMMON or ground.

Ed Walker
TI POWER SUPPORT
972 644 5580

[THREAD ID:1-IVECA][SYS:Done]
-----Original Message-----
PROBLEM:
Ref document SZZA015 (April 1999)PowerFLEX SMT Alternative for Through Hole PackagesPage A-2 Note D states "Soldermask placed over Cu can be used for improved thermal performance". Please explain how this improvement is accomplished.

STEPS NEEDED to RECREATE PROBLEM:

[robert Tarzwell]

just so the infomation is correct, solder mask decreses thermal disapation, it is a insulator with onlt .3 w/mc , and sorry black does not radiate heat significantly better, the surface roughness is the most important, to increase thermal output of a printed circuit you can have the board manufacture coat it with black oxide , for about a 10-15 percent increase. its not the black but the oxide is comprised of many little fingers that increase surface area and therefore disapation.
robert Tarzwell pcb books at megadawn.com