Lesson 9/1369%
MODULE 09

Thermal Interface Materials (TIM)

Master the critical layer between heat source and heat sink. Learn how TIMs fill microscopic gaps, compare TIM types, calculate thermal resistance, and apply installation best practices.

Learning Objectives

Explain why TIMs are necessary at every metal interface
Compare thermal grease, pads, PCMs, and gap fillers
Calculate TIM thermal resistance from k, BLT, and area
Understand bond line thickness (BLT) trade-offs
Apply TIM installation best practices
Recognize TIM degradation and pump-out failure modes

No matter how flat two surfaces appear, at the microscopic level they are rough. When a heat sink is placed on a component, actual metal-to-metal contact occurs at only 1-3% of the apparent surface area. The remaining 97-99% is filled with trapped air (k = 0.025 W/m·K) — an extremely poor thermal conductor.

Rcontact = Rconstriction + Rair gaps

A thermal interface material fills these microscopic voids, replacing the trapped air with a medium that has 40-320× higher thermal conductivity. This can reduce interface resistance by 5-20×.

Without TIM, a typical heatsink-to-package interface has a thermal resistance of 1-3 °C/W. With proper TIM application, this drops to 0.05-0.3 °C/W — often the difference between a working design and thermal failure.

Interactive: Gap-Fill Visualization

Move the slider to see how TIM progressively fills the microscopic air gaps between two mating surfaces, replacing insulating air with a conductive medium.

TIM Fill Progress0%
Effective Interface k
0.025 W/m·K
Relative Resistance
100%
TIM Type Conductivity (W/m·K) Typical BLT (μm) Advantages Limitations
Thermal Grease 1 - 5 25 - 75 Lowest thermal resistance, no curing, reworkable Pump-out over thermal cycles, messy application
Thermal Pads 1 - 6 200 - 2000 Easy handling, gap filling, electrically insulating options Higher resistance due to thickness, limited compression
Phase-Change Materials 3 - 8 20 - 50 Thin BLT at operating temp, no pump-out, long life Requires initial heat cycle to activate, moderate cost
Gap Fillers 1 - 3 500 - 5000 Bridges large gaps, conformable, vibration dampening High thermal resistance due to thickness, low conductivity
Engineering Note — Bond Line Thickness (BLT): the final compressed thickness of the TIM layer is the single most important factor in TIM thermal performance:
RTIM = BLT / (kTIM × A)
Even a high-conductivity TIM performs poorly if the BLT is excessive. A 5 W/m·K grease at 25μm BLT outperforms a 6 W/m·K pad at 500μm BLT by 16×. Always minimize BLT through adequate mounting pressure (typically 20-60 psi for thermal grease).

Enter contact area and power, then compare thermal resistance and temperature drop for several TIM options.

TIM Comparison Entries

This plot shows how thermal resistance increases with bond line thickness for five representative TIM types, assuming a 400 mm² contact area. Note the log-log scale — resistance scales linearly with BLT but conductivity shifts the entire curve.

Log-log plot: Bond Line Thickness (10-2000 μm) vs Thermal Resistance (°C/W), 400 mm² area.

Surface preparation
Clean both surfaces with isopropyl alcohol (IPA) to remove oils, dust, and old TIM residue. Surface roughness should be <10μm Ra for grease.
Correct amount
For grease, apply a thin center dot or X-pattern. Excess grease increases BLT and acts as an insulator, not a conductor.
Uniform pressure
Use spring-loaded fasteners with specified torque. Uneven pressure degrades thermal performance by 20-50%.
Avoid air entrapment
Lower heatsinks from one edge to sweep air outward. Never drop straight down.
Thermal cycling
Grease can pump out after 500-2000 cycles. For >10 year life, prefer phase-change materials or cured adhesives.
Electrical isolation
Use ceramic-filled pads (alumina/boron nitride) rather than grease. Verify breakdown voltage exceeds system voltage by 2×.
Never reuse thermal grease after heatsink removal. Once the bond is broken, air re-enters the interface. Always clean and reapply fresh TIM during rework. Thermal pads can sometimes be reused if undamaged and still tacky.
TIM Degradation and Pump-Out: Thermal greases contain silicone oil as a carrier for thermally conductive particles (alumina, zinc oxide, boron nitride). During thermal cycling, differential expansion between heatsink and component creates a pumping action that gradually pushes grease away from the center. After thousands of cycles, effective thermal resistance can increase by 2-5×. Mitigations: phase-change materials that re-wet each cycle, grease containment barriers (dam-and-fill), higher viscosity greases, or indium foil TIMs for extreme reliability (k=80 W/m·K).