Thermal resistance of the TIM layer is R_TIM = BLT / (k_TIM * A), where k_TIM is the thermal conductivity (0.5-10 W/m-K for greases, 20-80 W/m-K for solder, 200-400 W/m-K for direct bond). Doubling BLT doubles the thermal resistance. For a 100 W device with 10 mm x 10 mm die: 25 microns of grease (5 W/m-K) adds 0.05 C/W. 100 microns adds 0.20 C/W, a 15 C junction temperature increase.

Particle-filled greases: BLT controlled by particle size (25-75 microns). Phase-change materials: melt during reflow, flow to minimum BLT (15-25 microns). Thermal pads: fixed thickness (50-500 microns), compress under clamp pressure. Solder die attach: BLT 10-25 microns, controlled by solder preform thickness and reflow profile. Flatness of both surfaces is critical.

BLT in RF power amplifiers?

GaN HEMT dies are soldered to CuMo or CuW flanges with AuSn solder (BLT 10-20 microns, k = 57 W/m-K). The flange bolts to the heatsink with thermal grease (BLT 25-50 microns). Total BLT path: die to solder (10 um) + solder to flange (through bulk) + grease to heatsink (25-50 um). Each layer adds thermal resistance that must be minimized for reliable high-power operation.

Power & Thermal

Bond Line Thickness

Bond Line Thickness (BLT) is the gap between a heat-generating component and its heatsink or substrate, filled by a thermal interface material (TIM). BLT directly determines the thermal resistance of the interface: R_TIM = BLT/(k·A). Typical BLT values range from 10-25 μm (solder die attach) to 50-500 μm (thermal pads). Minimizing BLT is critical for high-power RF devices where every 0.1 °C/W matters.

Category: Power & Thermal

Range: 10-500 μm

Understanding Bond Line Thickness

The TIM fills microscopic air gaps between imperfectly flat surfaces. Air has thermal conductivity of only 0.025 W/m·K, so even tiny air gaps create significant thermal resistance. TIMs (greases, pads, phase-change materials, solder) replace air with a material of 0.5-80 W/m·K conductivity.

BLT is controlled by surface flatness, clamping pressure (for pads/grease), particle size (for filled greases), and reflow parameters (for solder). Non-uniform BLT creates hot spots where the gap is largest. X-ray or acoustic imaging can verify BLT uniformity in production.

TIM Thermal Resistance

R_TIM = BLT / (k_TIM × A)
BLT in meters, k in W/m·K, A in m²

Example (10×10 mm die):
Grease (k=5): BLT=25μm → R=0.05 °C/W
Grease (k=5): BLT=100μm → R=0.20 °C/W
Solder (k=57): BLT=15μm → R=0.003 °C/W

TIM Material Comparison

TIM Type	k (W/m·K)	BLT (μm)	Application
Thermal grease	0.5-10	25-75	CPU, PA module
Phase-change	3-8	15-25	Server, MMIC
Thermal pad	1-15	50-500	Gap filling
AuSn solder	57	10-25	Die attach
Indium foil	82	25-125	RF flange

Common Questions

Frequently Asked Questions

Why does BLT matter?

R = BLT/(k·A). Doubling BLT doubles thermal resistance. 100 W device: extra 0.15 °C/W = 15 °C hotter junction.

Controlling BLT?

Grease: particle size sets minimum. Pads: fixed thickness. Solder: preform thickness + reflow profile. Surface flatness critical.

RF PAs?

GaN die: AuSn solder (10-20 μm). Flange to heatsink: grease (25-50 μm) or indium foil. Minimize each layer.

Related Terms

← BOM Bond Pull →

Thermal Solutions

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