Bolt-Down Package
Understanding Bolt-Down Packages
The flange serves as both the electrical ground (source/emitter) and the thermal path. Copper flanges (400 W/m·K) are standard; copper-tungsten (CuW) or copper-molybdenum (CuMo) are used when the CTE must match ceramic substrates to prevent die cracking under thermal cycling.
The package includes a ceramic window frame (Al2O3 or BeO) that provides electrical isolation for the input and output leads while maintaining a hermetic seal. Internal matching networks (MOS capacitors, bond wires) pre-match the transistor die impedance from sub-ohm levels to a more manageable 2-5 Ω.
Rth,jc = junction to case (0.1-0.3 °C/W)
Rth,cs = case to sink (0.05-0.2 °C/W)
Example (200 W, 50% eff):
Pdiss = 200 W, Rth = 0.4 °C/W
ΔT = 80 °C above heatsink
Package vs SMT Thermal Comparison
| Package | Rth (°C/W) | Max Pdiss | Mounting |
|---|---|---|---|
| Bolt-down flange | 0.1-0.5 | 1000+ W | Screws to heatsink |
| SMT (QFN) | 5-20 | 5-10 W | PCB solder |
| Over-mold (PQFN) | 1-5 | 10-50 W | PCB + thermal pad |
| Coin/slug (exposed pad) | 2-8 | 20-50 W | PCB + via array |
Frequently Asked Questions
Why not SMT?
SMT limited to ~10 W (FR-4 thermal bottleneck). Bolt-down: direct metal-to-metal heatsink path, 0.1-0.5 °C/W, 1000+ W capable.
Package types?
Standard flange (2 holes), earless (clamp), pill/stud (UHF). Sizes: 10×7 mm (50 W) to 30×15 mm (500+ W).
Mounting?
Flat surface <25 μm deviation. Thermal compound 0.02-0.05 mm. Torque to spec (0.5-1.0 N·m). Verify with thermal imaging.