How to Select the Right Thermal Interface Material (TIM)

Even machined surfaces that appear flat have microscopic roughness. When two surfaces are pressed together, actual contact occurs at only a small fraction of the apparent area—typically 1-5%. The remaining space is filled with air, which has a thermal conductivity of just 0.026 W/mK.

The Impact of Air Gaps:

Consider a 1cm² interface with 50μm average air gap:

• Air thermal resistance: 0.05mm / (0.026 W/mK × 1cm²) = 19.2°C/W

With a 3 W/mK thermal pad filling the gap:

• Pad thermal resistance: 0.05mm / (3 W/mK × 1cm²) = 0.17°C/W

The TIM reduces interface resistance by over 100×.

Real-World Example:

A 100W IGBT module with junction-to-case thermal resistance of 0.2°C/W:

• Without TIM (air gap): Tj-Tc = 100W × (0.2 + 19.2) = 1940°C (impossible)
• With thermal grease: Tj-Tc = 100W × (0.2 + 0.1) = 30°C (manageable)

The interface can easily dominate total thermal resistance if not properly addressed.

Components of Interface Resistance:

Total interface resistance includes:

1. Bulk TIM resistance: t/(k×A)
2. Contact resistance at each surface
3. Spreading resistance if heat source is smaller than interface

Bulk conductivity matters, but contact resistance often dominates for thin bond lines. A lower-conductivity material that wets surfaces well may outperform a higher-conductivity material with poor contact.

TIMs come in various forms, each with distinct advantages and trade-offs.

Thermal Greases:

• Thermal conductivity: 0.5-8 W/mK (typical 2-5 W/mK)
• Bond line: 25-75μm (thinnest possible)
• Advantages: Lowest thermal resistance, excellent surface wetting
• Disadvantages: Messy, can pump out, requires controlled application
• Best for: High-performance applications with stable assembly

Thermal Pads (Gap Pads):

• Thermal conductivity: 1-15 W/mK
• Bond line: 0.5-10mm (conformable)
• Advantages: Easy handling, fills large gaps, electrically isolating options
• Disadvantages: Higher thermal resistance than grease
• Best for: Production assembly, varying gap sizes

Phase Change Materials (PCM):

• Thermal conductivity: 0.7-4 W/mK
• Bond line: 25-125μm at operating temperature
• Advantages: Solid at room temp, flows when hot, consistent bond line
• Disadvantages: Requires initial thermal cycle to wet
• Best for: Repeated assembly/disassembly, consistent manufacturing

Thermal Adhesives:

• Thermal conductivity: 1-4 W/mK
• Bond line: 50-250μm
• Advantages: Bonds components, no mechanical fastening needed
• Disadvantages: Permanent, higher resistance than grease
• Best for: Attaching small components without fasteners

Graphite Sheets:

• In-plane conductivity: 400-1500 W/mK
• Through-plane conductivity: 5-15 W/mK
• Advantages: Excellent spreading, very thin
• Disadvantages: Requires flat surfaces, can be electrically conductive
• Best for: Heat spreading applications

Liquid Metal:

• Thermal conductivity: 20-80 W/mK
• Advantages: Highest performance
• Disadvantages: Electrically conductive, can corrode aluminum
• Best for: CPU/GPU cooling, not typical power electronics

Choosing the right TIM requires balancing multiple factors:

1. Thermal Conductivity (k)

Higher is generally better, but context matters:

• Below 3 W/mK: Budget applications, large gaps
• 3-6 W/mK: General purpose, good balance of cost/performance
• 6-10 W/mK: High performance applications
• 10+ W/mK: Premium applications, diminishing returns

2. Bond Line Thickness (BLT)

Thinner is better for thermal resistance (R = t/kA), but must accommodate:

• Surface flatness (typ. 50-150μm for machined surfaces)
• Thermal expansion mismatch
• Assembly tolerance stack-up

3. Contact Resistance

Often more important than bulk conductivity. Affected by:

• Surface wetting ability
• Applied pressure
• Surface roughness

4. Operating Temperature Range

Must exceed your application requirements:

• Consumer: -20 to +80°C
• Industrial: -40 to +125°C
• Automotive: -40 to +150°C
• Extreme: -55 to +200°C

5. Electrical Properties

Consider whether isolation is needed:

• Thermal grease: Usually not isolating (can add particles)
• Gap pads: Available in isolating versions (4-10 kV/mm)
• For power modules: Often need 2.5+ kV isolation

6. Long-Term Stability

For 10+ year products, consider:

• Pump-out resistance (greases under thermal cycling)
• Dry-out (loss of volatile components)
• Compression set (permanent deformation of pads)
• Outgassing (contamination of nearby components)

Proper application technique is as important as TIM selection.

Thermal Grease Application:

Screen Printing (Production):

• Most consistent method for volume production
• Typical thickness: 50-100μm
• Stencil thickness = desired BLT + ~50%

Dispensing:

• Dot pattern or X pattern for good spread
• Amount: ~1mg per mm² of area
• Let mounting pressure spread material

Manual Application:

• Apply thin layer with spatula or card
• Avoid air bubbles
• Wipe excess from edges

Thermal Pad Application:

Preparation:

• Remove protective films from both sides
• Handle by edges only
• Pre-cut to size if needed

Placement:

• Center on heat source
• Avoid stretching pad
• Don't trap air bubbles

Compression:

• Apply specified mounting pressure
• Typical: 10-50 psi for solid pads
• Use spring washers for consistent force

Phase Change Material:

Pre-attach to heatsink (if possible):

• Apply at room temperature
• Initial thermal cycle activates material
• Re-flow fills gaps completely

Assembly:

• Align carefully—repositioning is difficult
• Apply mounting pressure evenly
• First power-up completes wetting

Critical Considerations:

Surface Preparation:

• Clean surfaces with IPA (isopropyl alcohol)
• Remove any residue from previous TIM
• Ensure surfaces are dry before applying

Mounting Pressure:

• Most TIMs require pressure for optimal performance
• Too little: High contact resistance
• Too much: Material extrusion, potential damage
• Use calibrated torque or force-limiting fasteners

Coverage:

• TIM should cover entire heat source
• Slight overhang is acceptable
• Large overhang wastes material

Long-term performance is critical for power electronics with 10-20 year expected lifespans.

Thermal Cycling Effects:

Pump-Out:

• Greases can migrate out of interface under thermal cycling
• Caused by differential expansion between surfaces
• Mitigation: Use gel-type greases, PCM, or pads

Compression Set:

• Soft pads permanently deform, losing compliance
• Can create gaps as surfaces move
• Mitigation: Use firm pads, adequate preload

Dry-Out:

• Volatile components evaporate over time
• Increases thermal resistance
• Mitigation: Select non-volatile formulations

Environmental Factors:

Temperature:

• Verify TIM rating exceeds max operating temp
• Consider peak transient temperatures
• Some materials soften excessively at high temp

Humidity:

• Most TIMs are moisture-resistant
• Verify for outdoor/humid environments
• Consider sealed interfaces for demanding apps

Chemicals:

• Check compatibility with cleaning agents
• Avoid silicone contamination if painting nearby
• Some TIMs incompatible with certain plastics

Testing for Reliability:

Thermal Cycling Test:

• 500-1000 cycles between extremes
• -40 to +125°C typical for automotive
• Measure thermal resistance before/after

High Temperature Aging:

• 1000-2000 hours at maximum rated temp
• Monitor for dry-out, degradation
• Accept max 20% resistance increase

Power Cycling:

• More severe than thermal cycling
• Junction temperature swing creates local stress
• 10,000+ cycles typical requirement

When to Use Which:

For high reliability (automotive, industrial):

• PCM or gel greases (resist pump-out)
• Premium pads with low compression set
• Thorough qualification testing

For production ease:

• Pre-cut gap pads
• PCM pre-attached to heatsink
• Phase change eliminates grease application variability

Based on your application requirements, here's a practical selection approach:

Step 1: Determine Gap Requirements

Measure or estimate:

• Surface flatness of both surfaces
• Assembly tolerance stack-up
• Thermal expansion mismatch

If gap < 100μm: Grease or PCM If gap 0.1-1mm: Thin gap pad or thick PCM If gap > 1mm: Soft gap pad

Step 2: Calculate Required Performance

Target interface resistance: Rth_int = (Tj_max - Ta - Rth_jc × P - Rth_sa × P) / P

If Rth_int < 0.1°C/W: High-performance grease needed If Rth_int 0.1-0.5°C/W: Standard grease or quality pad If Rth_int > 0.5°C/W: Basic pad adequate

Step 3: Consider Application Requirements

High Volume Production: → Gap pads or pre-applied PCM for consistency

Frequent Rework Expected: → PCM (reflows on re-assembly)

Vibration/Shock Environment: → Pads (won't pump out)

Extreme Temperature: → Specialty high-temp greases or pads

Step 4: Validate Selection

1. Obtain samples from 2-3 vendors
2. Build test assemblies
3. Measure thermal performance
4. Run accelerated reliability tests
5. Evaluate production compatibility

Quick Reference:

| Application | Recommended TIM | |------------|-----------------| | High-power IGBT | Premium grease + precise mounting | | TO-247 devices | 0.2-0.5mm gap pad | | PCB-mount FETs | Thermal adhesive or pad | | LED arrays | PCM or thin gap pad | | Battery cells | Soft gap pad (conforms to cylindrical) | | Variable gap | Conformable silicone pad |