Thermal Interface Materials: Pads, Grease and Gap Fillers Compared
Thermal grease wins on thermal performance, pads win on manufacturing, and gap fillers exist for the awkward gaps in between. A good grease with a thin bond line reaches 0.1-0.5 K·cm²/W of thermal impedance; a typical 1 mm pad is 5-20 times worse but needs no dispensing and no cleanup; a gap filler is the only sane choice when the gap is 1-5 mm and irregular. Choose by impedance target, production volume, and how the part gets disassembled — in that order.
No two solid surfaces touch perfectly. Under a microscope, even a machined face has peaks and valleys, and the air trapped between them is an excellent insulator. A thermal interface material (TIM) fills those voids so heat conducts across the joint instead of creeping through dead air. This guide compares the three families of TIMs with real numbers, and tells you when the cleverest material is the wrong answer compared with fixing the mechanical design.
First, the Metric That Matters: Thermal Impedance
Datasheets shout bulk thermal conductivity in W/m·K, but what your joint actually feels is thermal impedance — conductivity divided by how thick the material ends up, times the area. A 5 W/m·K pad that is 2 mm thick can be worse than a 1 W/m·K grease squeezed to 0.05 mm. The honest spec line is impedance in K·cm²/W at a stated pressure and thickness.
| TIM type | Bulk conductivity | Practical thickness | Typical impedance |
|---|---|---|---|
| Silicone grease (ceramic-filled) | 0.8-4 W/m·K | 0.025-0.1 mm | 0.1-0.5 K·cm²/W |
| Phase-change material | 3-8 W/m·K | 0.03-0.1 mm (after melt) | 0.1-0.3 K·cm²/W |
| Soft thermal pad | 1-6 W/m·K | 0.5-3 mm | 1.0-6.0 K·cm²/W |
| Gap filler | 1-5 W/m·K | fills 1-10 mm | 3-15 K·cm²/W |
| Graphite sheet | 5-20 W/m·K (in-plane) | 0.05-0.5 mm | 0.2-1.0 K·cm²/W |
Takeaway: quote TIMs by impedance at your actual clamp pressure, not by conductivity alone. The 2 mm pad with the flashy W/m·K number usually loses to a thin grease, and only the impedance spec reveals it.
Thermal Grease: Best Performance, Messiest Process
Grease is a pumpable paste of thermally conductive filler in silicone or synthetic oil. Squeezed thin between flat faces it gives the lowest impedance of the non-soldered TIMs, which is why it still dominates CPU, GPU, and power module assembly. The price is process: it must be dispensed in controlled volume, spread or clamped to the right bond line, and it can pump out or dry out after years of thermal cycling.
Grease suits: single flat interfaces, moderate-to-high power density, applications where a reworkable joint matters. It fails you when assembly is automated and mess-free, when the parts will be removed often, or when the interface is large and uneven. And one warning from the bench: more grease is not better — excess thickness adds resistance, so the thinnest uniform layer that covers both surfaces is the goal.
Thermal Pads: Manufacturing-Friendly, at an Impedance Cost
Pads are pre-formed sheets of filled silicone or acrylic that you peel, place, and clamp. No dispensing, no cure, no cleanup, and they tolerate some surface irregularity and component height differences. That convenience costs thermal performance — pads need a few times more thickness than grease, and they need real clamping pressure to deform into the surface. Typical recommendation is 10-50 psi of mounting pressure, which is why pad-mounted sinks need spring clips or screws rather than gravity.
| Pad hardness | Shore 00 / Shore A | Best for |
|---|---|---|
| Very soft (Shore 00 ~20-40) | Conforms at low pressure | Fragile components, low clamp |
| Medium (Shore 00 ~50-70) | Balance of contact and handling | General PCB-to-housing fills |
| Firm (Shore A ~30-50) | Handles well, needs more pressure | Precision flat joints |
Takeaway: choose the softest pad your handling and budget allow — a pad that cannot conform under your available clamp force leaves air pockets that no datasheet conductivity can fix. Pads shine on stamped heat sinks and clip-on assemblies where automation places sink and pad together.
Gap Fillers: For Gaps, Not for Flat Joints
Gap fillers are soft, highly conformable materials — sheets, putties, or dispensed liquids — designed to fill spaces from about 0.5 mm up to 5-10 mm between a board and a housing or heat sink. Use them where components sit at different heights and no single flat joint exists: the filler squishes around tall capacitors and short inductors alike, transferring heat from each to the metal wall. Their impedance is high compared with grease, but the alternative — no contact at all — is far worse.
The common mistake is using a gap filler where a grease or pad belongs. If your surfaces are flat and close together, a 3 mm filler sheet is pure thermal penalty; design the joint flat and thin instead. Fillers also need containment: they are soft, so design a dam or pocket so the filler stays where it is placed during assembly and rework.
Electrical Isolation and Other Selection Axes
Do you need the TIM to isolate electrically? Ceramic- or alumina-filled pads and greases isolate while conducting heat; graphite conducts electricity and must never bridge live traces; metal-backed pads are for grounding and shielding. Voltage isolation is a separate spec — if the module baseplate carries high voltage, choose a TIM rated for that voltage and verify it under the actual clamp pressure, since thinner always means less dielectric margin.
| Requirement | Grease | Pad | Gap filler |
|---|---|---|---|
| Lowest impedance | Best | Fair | Weak |
| Automated assembly | Needs dispenser | Best | Good |
| Rework / removal | Messy | Easy | Moderate |
| Electrical isolation | Filled types isolate | Most isolate | Most isolate |
| Filling irregular gaps | No | Limited | Best |
Takeaway: map your three real constraints — impedance budget, assembly method, isolation — onto this table and the candidate list collapses quickly. When two materials tie, pick the one your production line can apply repeatably; a slightly worse material applied consistently beats a better one applied badly.
The Mechanical Fix That Beats Any TIM
The best thermal interface is the one you do not need. Flatter surfaces, higher clamp pressure, and shorter heat paths beat any material choice. A milled, flat base on a CNC-machined heat sink lets a thin grease layer do its job; a rough or bowed base forces you into thick pads that no material can rescue. If your joint needs a 3 mm pad to bridge a bow, fix the flatness instead — the thermal resistance guide shows how quickly interface resistance eats a thermal budget. When you send a heat sink inquiry to a factory, include the interface plan: which TIM, what thickness, what clamp pressure — it changes the base flatness and surface finish the sink needs.
Email sc@bquq.com or WhatsApp +86 137 1315 7787 with your PDF/DXF/STEP file. An engineer reviews it and replies with price, lead time and DFM notes on working days.
Frequently Asked Questions
Q: What is the difference between thermal conductivity and thermal impedance?
A: Conductivity (W/m·K) describes the material; impedance (K·cm²/W) describes the installed joint, including thickness and contact. A thick high-conductivity pad can perform worse than a thin low-conductivity grease, so compare impedance at your actual bond line and pressure.
Q: Is thermal grease always better than a thermal pad?
A: Thermally, yes — a thin grease layer typically beats any pad. But pads win on assembly speed, cleanliness, and rework. Pick grease when performance leads and volumes are low; pick pads when automation, serviceability, or component height variation dominates.
Q: How much pressure do thermal pads need?
A: Most soft pads need roughly 10-50 psi of clamping pressure to conform fully to the surfaces. Below that they leave air gaps; that is why pad-mounted heat sinks need clips or screws with controlled force rather than loose placement.
Q: When should I use a gap filler instead of a pad?
A: When the gap is irregular or taller than about 1 mm — for example, filling the space between a populated PCB and a housing wall. On flat, close joints a gap filler is pure thermal penalty; design the joint thin and flat instead.
Q: Can thermal pads provide electrical isolation?
A: Most filled pads and greases isolate, but verify the voltage rating at your actual thickness and pressure. Graphite sheets and metal-backed pads conduct electricity and must never bridge live circuitry.
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Data Sources and Verification
Tolerances, cycle times and price ranges in this guide come from BQUQ production records at our Dongguan plant, where CNC machining (±0.005 mm), stamping, custom springs and heat sinks run under one roof. BQUQ is an ISO 9001:2015 certified factory; the certificate and batch inspection reports are available on request with every quotation.
Related Resources
- About BQUQ: an ISO9001-certified source factory in Dongguan running four production lines under one roof.
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Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs and heat sink lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
