IGBT Module Baseplates and Thermal Interfaces
Short answer: An IGBT module baseplate is the metal layer that carries heat from the ceramic substrate to the heat sink, and the thermal interface between them is usually the single largest thermal resistance in the whole stack. For a typical 62 mm or EconoDUAL-class module, a baseplate flatness of 50–100 µm over the contact area, a machined heat-sink surface of Ra 0.8–1.6 µm, and a 50–100 µm grease bond line will keep interface resistance in the 0.1–0.3 K/W range. Poor flatness or a 200 µm bond line can double that. BQUQ machines baseplates and heat sinks to ±0.005 mm and quotes in 12 working hours.
Why the baseplate matters more than the die
Engineers spend most of their thermal budget modelling junction-to-case resistance, then lose the gains at the last 100 µm. The path from an IGBT die to ambient has four stages:
1. Die to ceramic substrate (solder or sinter)
2. Substrate to baseplate (solder, often the second-largest resistance)
3. Baseplate to heat sink (thermal interface material, TIM)
4. Heat sink to coolant or air
Stage 3 is where mechanical manufacturing decisions dominate. Unlike solder joints, which are made under controlled furnace conditions, the baseplate-to-heat-sink joint is made by whoever bolts the module down — and its quality depends entirely on the flatness of two metal surfaces and the thickness of the paste between them.
A useful rule of thumb: for a 100 µm grease bond line with a typical 1–3 W/m·K silicone grease, the interface alone contributes roughly 0.1–0.3 K/W on a 50 × 50 mm contact area. Halve the bond line and you halve that. Double it and you may add more resistance than the entire die-to-case path.
The mechanical stack, layer by layer
| Layer | Typical material | Thickness | Function |
|---|---|---|---|
| Die | Silicon IGBT/diode | 100–200 µm | Active device |
| Die attach | Solder or silver sinter | 20–100 µm | Electrical + thermal |
| Substrate | Al₂O₃, AlN, Si₃N₄ | 0.25–1.0 mm | Insulation + spread |
| Substrate attach | Solder | 50–150 µm | Thermal + mechanical |
| Baseplate | AlSiC, Cu, Al | 2–5 mm | Spread + mount |
| TIM | Grease, pad, phase-change | 25–200 µm | Fill air gaps |
| Heat sink | Al or Cu | 5–50 mm | Reject to coolant |
Each layer has a different coefficient of thermal expansion (CTE). Copper baseplates sit around 17 ppm/K, AlSiC around 7–9 ppm/K, and aluminium heat sinks around 23 ppm/K. That mismatch is why baseplate flatness changes with temperature and why power-cycling tests matter.
Which baseplate material should you choose?
Three families dominate, and the choice is a trade between thermal conductivity, weight, CTE match, and cost.
| Material | Thermal conductivity (W/m·K) | CTE (ppm/K) | Density (g/cm³) | Typical use |
|---|---|---|---|---|
| Copper (C11000) | ~390 | ~17 | 8.9 | High-power, high-duty modules |
| AlSiC (typical) | 180–200 | 7–9 | 3.0 | Traction, cycling-critical |
| Aluminium (6061) | ~170 | ~23 | 2.7 | Cost-sensitive, lower power |
| Cu-Mo-Cu (typical) | 200–300 | 6–10 | 9.5 | RF and high-reliability |
Copper gives the best raw spreading but the worst CTE match to ceramic. AlSiC gives the best reliability under thermal cycling but costs several times more and machines differently — it is abrasive and typically needs diamond tooling. Aluminium is the compromise for lower-power modules where cost and weight win.
For the heat sink side, most IGBT assemblies use extruded or skived aluminium, or a copper cold plate when the heat flux exceeds what air can carry. BQUQ produces both extruded heat sinks and CNC-machined heat sinks with the flatness and surface finish that interface performance requires.
Flatness and surface finish targets
This is the specification most often missing from drawings. Two numbers matter:
- Flatness (or profile tolerance) across the contact area. For large modules, 50 µm is a good target; 100 µm is often acceptable; beyond 150 µm you are filling the gap with paste, not mating metal.
- Surface roughness Ra. Between 0.8 and 1.6 µm is a practical sweet spot. Too smooth and paste squeezes out entirely, creating dry contact spots; too rough and the asperities hold the surfaces apart.
Note that flatness and roughness are not the same thing, and a surface can be flat but rough, or smooth but warped. Both need to be specified.
How much does the thermal interface really cost you?
Bond line thickness (BLT) is the controlling variable. The thermal resistance of a TIM layer is:
R = t / (k × A)
where t is bond line thickness, k is conductivity, and A is contact area. For a 50 × 50 mm area (0.0025 m²):
| TIM type | k (W/m·K) | BLT (µm) | R (K/W) |
|---|---|---|---|
| Silicone grease | 1.0 | 50 | 0.020 |
| Silicone grease | 1.0 | 100 | 0.040 |
| Silicone grease | 3.0 | 50 | 0.007 |
| Gap pad | 3.0 | 200 | 0.027 |
| Gap pad | 6.0 | 500 | 0.033 |
| Phase-change | 3.5 | 50 | 0.006 |
| Sintered/brazed joint | 50+ | 50 | 0.0004 |
The table is indicative and assumes perfect contact area coverage. In practice, contact resistance at both interfaces adds another 0.05–0.15 K/W unless the surfaces are well prepared.
Two conclusions follow. First, a high-conductivity grease at 50 µm beats a mediocre grease at 100 µm by a wide margin — but only if flatness allows the thin bond line. Second, bolted or sintered joints are an order of magnitude better, which is why the industry keeps pushing toward direct-bonded copper and sintered interconnects.
For a deeper look at paste selection, see our guide on thermal grease selection.
What does mounting pressure do to the interface?
Pressure spreads the paste, closes air gaps, and reduces BLT — up to a point. Beyond roughly 1–2 MPa on a typical module, returns flatten and you risk cracking the ceramic substrate or deforming the baseplate.
Practical guidance:
- Follow the module datasheet's mounting torque. It is there for a reason.
- Use a torque wrench, and tighten in a cross pattern in two stages.
- Specify a heat sink material with enough stiffness that it does not bow under load. Thin aluminium plates deflect, and a deflected plate creates a wedge-shaped bond line.
- Consider a spring-loaded or Belleville-washer mounting if the assembly sees thermal cycling, so pressure stays roughly constant as materials expand.
For assemblies where clamping is impractical, a bonded interface using thermal adhesive is an option, though it is usually permanent and harder to rework.
Manufacturing the baseplate and heat sink
Both parts of the interface are machined metal, and both benefit from being made in the same shop so the tolerances are complementary rather than independently optimistic.
Baseplate machining
Copper baseplates are typically milled flat, then nickel- or gold-plated if solderability or corrosion resistance is required. Key process points:
- Clamp lightly and stress-relieve; copper moves when you cut it.
- Machine both faces if the module is double-sided cooled.
- Measure flatness after plating, not before — plating can add 5–15 µm of unevenness.
- Deburr thoroughly; a burr at the edge of the contact area creates a local high spot.
Heat sink machining
The heat sink is usually the larger part and the harder one to hold flat. Options include:
- Extruded profiles — economical, good for natural and forced convection, but the mounting face often needs a secondary face-mill.
- Skived fin — high fin density, good for tight footprints.
- CNC-machined plate with pockets — best flatness control, higher cost per part.
- Cold plate with embedded channels — for liquid cooling above roughly 1 kW per module.
BQUQ machines heat sinks to ±0.005 mm on critical features, which comfortably covers the flatness targets above. Our heat sink overview covers the full range of processes.
Inspection: how do you verify flatness?
Coordinate measuring machines (CMM) and optical profilometers are the standard tools. For production, a simpler and faster check is a surface plate with a dial indicator, or a feeler-gauge check against a known-flat reference. Whichever method you use, define it on the drawing — "flat within 50 µm measured by CMM over the full contact area" is a specification; "flat" is not.
Our quality inspection workflow describes how we verify flatness, finish, and thread quality before shipment.
Common failure modes and how to avoid them
| Symptom | Likely cause | Fix |
|---|---|---|
| Hot spot under one die | Baseplate warp or local high point | Tighten flatness spec; check after plating |
| Rising Rth over time | Pump-out of grease under cycling | Use phase-change or pad; reduce ΔT |
| Cracking after cycling | CTE mismatch, over-torque | Switch to AlSiC; follow torque spec |
| Dry contact patches | Surface too smooth, paste squeezed out | Increase Ra to 0.8–1.6 µm |
| Uneven bond line | Heat sink bowing under load | Thicker plate or stiffener ribs |
| Corrosion at interface | Galvanic couple Cu/Al with moisture | Plate the copper, or use a barrier |
Pump-out deserves a note. Silicone grease migrates out of the joint under repeated thermal cycling because the two surfaces move relative to each other. Phase-change materials and cured gap pads resist this better, at some cost in initial conductivity.
Design checklist for a new IGBT assembly
Before you release a drawing, confirm:
1. Baseplate material and CTE are matched to the cycling profile.
2. Flatness is specified numerically, with a measurement method.
3. Surface roughness Ra is specified, typically 0.8–1.6 µm.
4. Mounting torque and pattern are documented.
5. TIM type, thickness, and application method are defined.
6. Heat sink stiffness is adequate for the clamping load.
7. Plating is specified on both mating surfaces where galvanic risk exists.
8. A thermal test point or thermocouple location is defined for validation.
A broader version of this list appears in our heat sink design checklist.
Frequently Asked Questions
Q: What flatness should an IGBT baseplate have?
A: For most large power modules, target 50 µm flatness across the contact area; up to 100 µm is often acceptable. Beyond roughly 150 µm, the thermal interface material has to bridge the gap rather than the metals mating, which raises interface resistance sharply. Always specify the measurement method and the area over which flatness is evaluated, since a baseplate can be flat overall but bowed in the centre.
Q: Is copper always better than aluminium for a baseplate?
A: No. Copper conducts heat roughly twice as well as aluminium, but its coefficient of thermal expansion is a poorer match to ceramic substrates, which shortens power-cycling life. AlSiC offers the best CTE match at higher cost and weight. Choose copper for raw thermal performance, AlSiC for cycling reliability, and aluminium for cost-sensitive, lower-power designs.
Q: How thick should the thermal grease layer be?
A: Aim for 50–100 µm bond line thickness on a machined surface. Thinner is better thermally, but below about 25 µm you risk incomplete coverage and dry contact spots. The achievable minimum depends on flatness and roughness. A 100 µm layer of 1 W/m·K grease adds about 0.04 K/W over a 50 × 50 mm area, which is often comparable to the die-to-case resistance.
Q: Can I use a thermal pad instead of grease?
A: Yes, and pads are cleaner to assemble and resist pump-out better. The trade-off is thickness: pads are typically 200–500 µm thick, so their thermal resistance is usually higher than a well-applied grease layer. Use pads when assembly consistency matters more than the last few tenths of a K/W, or when the gap between surfaces is inherently large.
Q: Does BQUQ machine both baseplates and heat sinks?
A: Yes. BQUQ runs CNC machining to ±0.005 mm, metal stamping, custom springs, and heat sink production across four lines in one ISO9001 factory in Dongguan. We machine copper and aluminium baseplates and heat sinks, and quote in 12 working hours with flexible MOQ. Send drawings to sc@bquq.com or WhatsApp +86 13713157787.
Related Resources
- About BQUQ and our Dongguan factory: /about/
- Heat sink product range: /heat-sinks/
- Extruded heat sink profiles: /extruded-heat-sinks/
- CNC-machined heat sinks: /cnc-machined-heat-sinks/
- Industry trends in power electronics cooling: /industry-dynamics/
- Technical articles and engineering guides: /bquq-blog/
- Case studies: /case/
- Contact the engineering team: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
