Thermal Conductivity
Understanding Thermal Conductivity
Thermal conductivity is critical in RF power module design. The heat generated by a PA die must travel through the die attach material, substrate, and heatsink to be dissipated. Each material in the thermal path contributes thermal resistance inversely proportional to its thermal conductivity and directly proportional to its thickness.
Material Thermal Conductivities
| Material | k (W/m-K) | Application |
|---|---|---|
| Diamond | 2000 | GaN-on-diamond substrates |
| SiC | 490 | GaN substrate |
| Copper | 401 | Heatsink, ground plane |
| Aluminum | 237 | Heatsink, enclosure |
| AlN | 170 | Ceramic substrate |
| Silicon | 150 | CMOS substrate |
| Alumina | 35 | Thick-film substrate |
| FR-4 | 0.3 | Standard PCB |
R_th = t / (k x A) (C/W)
where t = thickness, k = conductivity, A = area
Temperature rise: delta_T = P x R_th
Example: 0.5mm AlN substrate, 5x5mm die:
R_th = 0.0005 / (170 x 25e-6) = 0.12 C/W
At 50W dissipation: delta_T = 6 C
Frequently Asked Questions
What is thermal conductivity?
Thermal conductivity measures how well a material conducts heat. Higher values mean better heat transfer. Copper (401 W/m-K) is excellent; FR-4 PCB (0.3 W/m-K) is very poor. Substrate thermal conductivity is critical for high-power RF device mounting.
Why is SiC used for GaN substrates?
SiC has high thermal conductivity (490 W/m-K) compared to sapphire (35 W/m-K) or silicon (150 W/m-K). This efficiently removes heat from the GaN transistor channel, enabling higher power density. GaN-on-SiC is the standard for high-performance GaN PAs.
How does PCB thermal conductivity affect RF?
FR-4 has very poor thermal conductivity (0.3 W/m-K). High-power RF components on FR-4 require thermal vias (copper-filled holes) to conduct heat through the board to a heatsink below. Metal-core PCBs or ceramic substrates are used for very high power.