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.