# Electronic – Needed PCB surface to cool a SOIC-8 EP package

heatsinkpcbsurface-mount

I have got a part in a SOIC-8 EP package. The "EP" indicates it is a package with a exposed pad which can transfer heat to the PCB. I would like to have a better understanding how much pcb surface I need to cool the part at different power consumption levels. Lets say 1 Watt 1/2 Watt and 0.1 Watt.

I did read some white papers. They basically tell:

PD = (TJ − T A)/θJA

Where
θJA = theta ja (junction to ambient) in C/W
TJ = junction temperature rating in C
T A = ambient temperature in C
PD = power dissipated in watts

θJA can be split into three parts which add up:

θJA = θJC + θCS + θSA

Where:
θJC = theta JC (junction to case) °C/W
θCS = theta CS (case to heatsink) °C/W
θSA = theta SA (heatsink to ambient) °C/W

The datasheet of the part tels me:
θJC = 10 °C/W
TJ = 150 °C

I can think of a ambient temperature lets say 22°C

But then I am still missing the following: θCS and θSA. I could imagine that θCS is negligible, is this true? θSA I find difficult I plan to use via's to get the heat to the other side of the PCB but I can not find any data giving me a idea what number I can use for θSA. I also find it hard to figure out if I need 35um (1oz) or 70um (2oz) PCB.

This is actually a pretty deep question. Fortunately, there is an extensive layer of literature on this subject. You are basically on right track, but, for some reason, didn't get to the right articles.

Yes, if the pad is soldered down, you can assume the θCS as zero.

Regarding vias, typical thermal resistance of via over a 1.6mm FR4 PCB is 130 to 250 °C/W, depending on stack-up and via size. So you would need few of them to have any effect. Or you can make one 2-mm diameter via, and fill it with solder. There are plenty of calculators on this subject, Google for "Thermal resistance of via calculator".

All details are perfectly explained with formulas and practical examples in this Application Note AN-2020.

Final results will depend on details of ambient conditions, whether the board is vertically oriented or horizontally, are there obstacles to natural convective airflow, or there might be some forced ventilation around. A thermal image of PCB will help tremendously to evaluate the board thermal condition, and, if necessary, design corrections should be made.

But for a 1W dissipation and 3x3mm thermal pad soldered to 1.5 oz PCB, I wouldn't worry much, given TJ = 150 °C.