Key takeaways
- The junction to ambient path includes the die attach, the substrate, the baseplate, the interface and the sink.
- Power cycling fatigue often starts at a solder or interface layer, not at the heat sink.
- Module mounting pressure and baseplate flatness belong in the sink drawing, because the module datasheet assumes them.
- Liquid cooling raises the achievable loss density, but it adds a pump, a fluid and a maintenance plan.
The stack from junction to air
An IGBT module is a stack of materials with different expansion rates. The silicon die is soldered or sintered to a ceramic substrate, the substrate is joined to a baseplate, and the baseplate is mounted on a heat sink through a thermal interface. Every layer has a resistance and a fatigue life.
The junction to case resistance is quoted by the module maker for a defined condition. It includes the die, the die attach, the substrate and the baseplate. The case to sink resistance belongs to the interface and the mounting. The sink to ambient resistance belongs to the fin geometry and the cooling route.
The materials matter because they set the cycling behaviour. Silicon, ceramic, copper and aluminium expand by different amounts. A soft solder can absorb some of that mismatch, but it creeps and fatigues over time. A sintered joint is more resistant but needs a different process.
Why power cycling is a thermal problem
A power module heats and cools with the load. Each cycle strains the joints, and the strain accumulates. The failure may appear as a lifted wire bond, a cracked die attach, a delaminated substrate or a void in the thermal interface. The junction temperature rise is small at first, then accelerates as the thermal path degrades.
The cycle range matters more than the average temperature for some failure modes, and the absolute temperature matters more for others. A module that runs at a high steady temperature may creep faster, while a module that swings widely may fatigue faster. The datasheet usually gives a power cycling curve for a defined case temperature swing, and the design should be checked against the actual mission profile rather than a single worst case.
The heat sink design affects the swing. A sink with a large thermal mass slows the rise and fall, which reduces the strain per cycle but does not remove it. A sink with a low resistance lowers the absolute temperature, which helps creep and often helps life.
Sizing for a duty cycle
Start with the loss at the worst operating point, not the nominal load. Conduction loss rises with current squared, and switching loss rises with voltage, current and frequency. A drive that runs at light load most of the day can still have a short overload that sets the junction temperature.
Take an illustrative module that dissipates 250 W, with a maximum junction of 150 degrees Celsius, a maximum ambient of 45 degrees Celsius and a junction to case resistance of 0.10 K/W. The interface adds 0.03 K/W and the proposed sink adds 0.20 K/W. The total is 0.33 K/W, so the junction sits at 45 plus 250 times 0.33, or about 128 degrees Celsius. That leaves 22 K of margin for a hotter day, a dirty sink or a higher switching loss.
The margin should be checked against the mission profile, not only the datasheet maximum. A design that runs at 148 degrees Celsius at nominal load has no room for overload, and the cycling curve at that temperature may be much shorter than at 125 degrees Celsius. A larger sink or a lower loss device may be cheaper than a field failure.
The worst case is a profile
A single ambient and a single power figure hide the overload, the fan failure and the hot day. List the cases and size for the one that sets the junction temperature.
Module mounting and pressure
A large module is clamped to the sink with a defined pressure, often through a spring or a calibrated torque sequence. The pressure has to be uniform across the baseplate, because a high spot at one corner leaves the rest of the interface thick. The module datasheet usually gives a mounting force and a flatness requirement, and the sink has to meet both.
The baseplate is not perfectly flat, and it bows when it is clamped. A concave baseplate contacts at the edges, and a convex one contacts at the centre. The interface material has to fill the difference, or the contact area is much smaller than the footprint.
Use the fastener sequence in the module instructions. A cross pattern tightened in stages is normal for a large module, and a single pass at full torque can trap a corner. If the module uses a pressure plate or a clip, check that the force is centred on the baseplate rather than on the plastic housing.
Isolation and creepage
A module may be isolated internally or it may need the sink to provide isolation. An isolated module has a ceramic substrate that stands off the baseplate, so the baseplate can be earthed. A non-isolated module has the baseplate electrically live, and the sink must be isolated from it and from the enclosure.
The isolation path is a series of clearances and creepage distances, not only the interface material. A thin pad provides electrical isolation and thermal conduction at the same time, and its thickness is a compromise. A pin hole, a cut edge or a screw that bridges the pad defeats the isolation.
The earth bond is the other half. An earthed sink needs a conductive path to the enclosure, and an anodised sink is insulating at the contact faces unless they are masked. A bonded earth strap can bridge the coating, but it needs a defined contact area and a corrosion protection at the joint.
Air or liquid cooling
Air cooling is simpler and easier to maintain, but its capacity is limited by the air volume and the fin area. A forced air sink can handle a few hundred watts per module in a reasonable enclosure. A natural convection sink handles much less, and it depends on the mounting orientation and the surrounding space.
Liquid cooling moves the heat to a cold plate and then to a radiator or a chiller. It allows a much higher loss density and a smaller sink, which matters in a traction inverter or a fast charger. The cost is a pump, a fluid, a hose, a leak path and a maintenance plan. A liquid system also needs a corrosion inhibitor and a filter, and the cold plate needs a pressure drop that the pump can meet.
The choice is not only thermal. A sealed liquid system suits a high density unit, while a fan cooled sink is easier to service in a panel. The enclosure and the IP rating decide as much as the loss.
| Route | Illustrative loss band | Strength | Watch |
|---|---|---|---|
| Natural convection | Up to about 100 W per module | No fan, no fluid, quiet | Orientation, dust and available surface area |
| Forced air | About 100 to 500 W per module | Simple service and a familiar supply chain | Fan life, filter blockage and airflow bypass |
| Liquid cold plate | Above about 500 W per module | High loss density and a compact footprint | Pump, hoses, leaks, fluid maintenance and pressure drop |
Loss bands are illustrative and depend on the ambient, the airflow, the interface and the allowable junction temperature.
Acceptance and test
A power module assembly should be tested for thermal resistance, isolation and contact. The thermal test applies a known loss and measures the temperature rise at a defined point. The isolation test applies a high voltage between the live parts and the earthed sink, at a defined voltage and time. The contact check confirms the mounting pressure or the clamp height.
The first article should include the sink flatness, the interface thickness, the clamp force or torque record, the thermal result and the isolation result. A production test may be a sample thermal check plus a full isolation test, because isolation is a safety function and a thermal test is slow.
The records should identify the interface material, the lot and the mounting hardware. A module that passes on the bench and fails in the field often had the right parts and the wrong pressure, or a pad that was reused after a trial fit. A single use interface and a controlled torque sequence remove two common causes.
Frequently asked questions
What thermal resistance does an IGBT heat sink need?
It depends on the module loss, the maximum junction temperature, the ambient and the other layers. Subtract the junction to case and interface resistances from the allowable total, and the remainder is the sink budget. A 250 W example with a 150 degree limit and a 45 degree ambient leaves about 0.20 K/W.
Why does an IGBT module fail after thermal cycling?
The materials expand by different amounts, so each cycle strains the die attach, the substrate joint, the baseplate and the interface. Fatigue starts at the weakest layer and raises the thermal resistance, which raises the temperature and speeds the next cycle.
Can I mount an IGBT module on an anodised heat sink?
Only if the isolation and earthing design allows it. The anodised layer is insulating, so it can help isolation but blocks an earth bond. Mask the bonding points or use a separate earth strap, and check the bond resistance after assembly.
When should I use liquid cooling?
When the loss density is too high for air in the available space, or when a sealed enclosure cannot reject the heat from its surface. Liquid cooling adds a pump, fluid, hoses and maintenance, so it is a system decision.
Sources and further reading
- IEC 60747-15:2010, Semiconductor devices. Discrete devices. Isolated power semiconductor devices
- JESD51-1, Integrated circuit thermal measurement method. Electrical test method
- IEC 60529:1989+AMD1:1999+AMD2:2013, Degrees of protection provided by enclosures (IP code)
Qumek Engineering Desk
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