PCIM: Heat spreader keeps shape despite thermal cycling

Aiming at IGBT modules, Japanese heat spreader maker ALMT has developed a thermal base plate alloy that improves heatsink contact.

ALMT heat spreader

IGBT base plates are subtly convex (see diagram) so that when they are mounted on a flat base plate, the screw holes have a gap beneath them (~200μm). When the screws are tightened, the curve means that the centre of the base plate, where the hot components are, will make good thermal contact with the heatsink.

ALMT argues that aluminium silicon carbide – the incumbent material – may start out with an appropriate convex shape, but relaxes under thermal cycling, loosing the centre pressure.

Its new material, magnesium silicon carbide, maintains its curvature, stabilising to over 80%of its initial value after a thousand cycles, while AlSiC has already dropped below 20% after 100 cycles (-55 to 175°C).

To match different packaging techniques and materials, MgSiC baseplates can be made with coefficient of thermal expansion anywhere between 5.9 and 7.5ppm/K.

Thermal conductivity is 230W/mK, compared with 180 for AlSiC.

Diamond spreaders

At the same event it the PCIM power exhibition in Nuremberg last week, ALMT announced a silver-diamond heat spreader material for high-end ceramic packages – in aircraft radar and satellites, for example.

Thermal conductivity is 640W/mK, and it is suitable for silver brazing at 780°C.

An infiltration technique is used to make the material – forcing liquid silver into powdered diamond – resulting in a thermal conductivity drop of 3% over the first 1,000 cycles compared with 12% (starting at ~520W/mK) for silver-diamond made by scintering (combining powdered diamond with powdered silver with heat and pressure).

ALMT heat spreader graphOne more thing

The firm also described a layered Cu-CuMo-Cu heat spreader material.

Called CPC-300 or CPC111(50), it uses equal-thickness layers through the sandwich, which offeres 300W/mK conductivity compared with more conventional Cu-CuMo-Cu spreaders, where the CuMo is four times thicker than either of the pure copper layers (known as CPC141 (30), which offers 220W/mK.

ALMT is part of Sumitomo


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