TIM selection by thermal conductivity
A thermal interface material is specified by the interface it fills. The same pad is correct at one plane, wrong at the next.
| Plane | Interface | Controlled by |
|---|---|---|
| Die to lid, inside the package | Chip packager | |
| Lid to heatsink base | Module integrator | |
| Lid or heatsink to cold plate; module to rail | Rack integrator; the plane this catalogue serves |
Single-component power passes 1,000 W.
Thickness follows the gap
| Form | Window | Minimum bond line |
|---|---|---|
| Micro-gaps | 0.05 mm | |
| 0.20–1.0 mm | 0.20 mm | |
| 0.5–6 mm common · 0.25–20 mm die-cut | 0.5 mm |
Two flat surfaces in dry contact touch on under 3 % of the apparent area; the interface sets the temperature.
Impedance at a stated pressure
| Material | Conductivity | Impedance | At |
|---|---|---|---|
| Gap filler | 7.5 W/m·K | 0.154–0.539 °C·in²/W | 10 psi |
| Gap filler | 7.5 W/m·K | 0.074–0.203 °C·in²/W | 30 psi |
| Gap filler | 7.5 W/m·K | 0.061–0.129 °C·in²/W | 50 psi |
| Grease | 3.5 W/m·K | 0.02 °C·in²/W · 0.015 min. | 30 psi |
| Phase change | — | 0.0078 °C·cm²/W | As tested |
Impedance moves three- to four-fold across a range a bracket can deliver or miss.
ASTM D5470 returns impedance
ASTM D5470 measures apparent thermal impedance at a controlled pressure and thickness, and it returns impedance. Conductivity is derived: measure at several thicknesses, regress linearly, take the intercept as contact resistance and the slope as the inverse of conductivity times area. A conductivity figure without its pressure and thickness is not a specification.
Direction is the second trap. Oriented graphene at up to 130 W/(m·K) and graphene films at 1,450 W/m·K are in-plane figures. A through-plane pad runs 0.8–15 W/m·K. The two axes are never tabulated together.
Where the interface is the insulation
| Property | Gap pad | Phase change | Method |
|---|---|---|---|
| Breakdown voltage | 5.0 kV/mm or more | 3.0 kV/mm or more | ASTM D149 |
| Dielectric constant, 1 MHz | 7.1 | — | ASTM D150 |
| Volume resistivity | 8.7 × 10¹³ Ω·cm | — | Per item |
| Mechanical ceiling | 2.4 g/cc, 40 Shore 00 | 3.5 W/m·K grease | — |
Where the cold plate is not isolated from the electrical net, the electrical rows come first; the thermal row only grades the choice.
Three mechanisms decide what is left
| Mechanism | Figure and condition | Consequence |
|---|---|---|
| Die and cold plate expand at different rates | Cycling pumps the paste out of the bond line | |
| below 0.2 % weight loss, 125 °C for 48 h | Against a 150 °C grease rating | |
| oil bleed 1 % or less, 25 % compression, 125 °C for 48 h | REACH SVHC carries D4, D5 and D6 |
Pressure selects the class
| Application | Gap, pressure | Material class |
|---|---|---|
| Lid to cold plate, thin | 0.05–0.2 mm, high | Grease 3.5 W/m·K, 0.02 °C·in²/W at 30 psi |
| Lid to cold plate, cycling | 0.20–1.0 mm, moderate | Phase change, softening 45–55 °C, ASTM D3418 |
| Rail to manifold | 0.5–6 mm, low | Gap pad 5.0 W/m·K, 40 Shore 00 |
| Warped lid, low-pressure bracket | 0.5–3 mm, very low | Ultra-soft pad 1.2 W/m·K, 10 Shore 00 |
| High-flux, direction-critical | 0.25–2 mm, moderate | Oriented graphene pad, 70 % rebound |
| Silicone-restricted build | 0.5–3 mm, low | Silicone-free pad 2.0 W/m·K, 45–55 Shore 00 |
Conductivity only grades the choice.
Sample and drawing
Grease EMS-TM-G35. Gap pads EMS-TM-P50-20 and EMS-TM-P20-10 silicone-free. Phase change EMS-TM-PC85. Graphene EMS-TM-GR120 at 1.5 mm.