Thermal dissipation for gull wing transistor

[csiess]

Hi,

I'm looking for printed circuit guidelines for thermal vias under RF LDMOS power amplifier (like MHT2000GN) land pattern. 

Thanks,

Carine

[LPP · Accepted solution]

Please read about mounting options in "Designing with Plastic RF Power Transistors":

https://www.nxp.com/docs/en/white-paper/RFPLASTICWP.pdf

"There are two options for the heat removal: one is the use of a via farm pattern, and the second is the use of a metal coin or pallet. The via farm has higher thermal resistance than the metal coin or pallet. However, the via farm has an advantage with regard to cost: the RF PCB has many ground vias, so the addition of a few more closely spaced vias in the source pad of the RF PA (power amplifier) device will have almost no cost impact, while the addition of coin or pallet will have a significantly higher cost impact."

In the case of via farm pattern, the device is soldered to PCB that is mounted to a heat sink.

Usefull links on the topic:
https://www.nxp.com/docs/en/application-note/AN4005.pdf
https://www.nxp.com/docs/en/application-note/AN1949.pdf

Thermar resistance of the via farm can be estimated using thermal design CADs. You can find multiple thermal via calculators on the WEB.

http://www.pighixxx.net/tools/pcb-via-calculator/

For example:

MHT2000GN, PCB thickness 60 mils, thermal via diameter 0.4mm, 1mil copper plating, 0.8mm x0.6mm via hex farm pitch

 thermal resistance of the via 112'C/Watt
 via count 19*11 = 209
 thermal resistance of the via farm 112/209=0.54'C/Watt

RF power 25w, efficiency 30% -> Thermal power 59 Watt -> Junction to Heat sink temperature rise 59W*(0.54+1.2)'C/W=103'C

Maximum heat sink temperature allowed 225'C-103'C = 122'C

Have a great day,
Pavel

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