Draft of Thermal Standards
for Power Modules
Deriving one number for thermal resistance of a power
module is difficult for the following reasons:
- Different dice (in a multiple die module) are at
different temperatures. In larger modules, paralleling is
used to increase current ratings. Even among the same die
type, different locations of the dice cause differing
- Module baseplates are so large that the bottom of the
baseplate is typically not isothermal due to lateral heat
- Mounting the module to a heatsink can significantly
change the pattern of heat flux within the module. E.g.
If the module is more tightly secured to a heatsink on
one side, that side will have a lower thermal resistance
to the sink. Thermal grease application is another major
factor is heat flux and temperature patterns. Further,
the heatsink geometry itself strongly influences the flux
and temperature pattern within the module.
- Use a heatsink that is nearly isothermal across the
module surface. This will almost certainly require liquid
cooling, but the only mandate is that heatsink
temperature not vary by more than 1 °
C under the module.
- The measurement of thermal resistance shall be a junction
to sink measurement. The highest temperature on any die
will be used for the high temperature measurement, and
any temperature on the heatsink under the module may be
used for the low temperature measurement. There must be
at least a 10 ° C temperature
difference from higher to lower temperature. Measurements
of power dissipation and temperature must be accurate to
three significant digits.
- If there are two separate types of switch (e.g. diodes
and IGBTs), the two types shall be characterized
- The mounting method of module to heatsink must be fully
disclosed (e.g. thermal grease, solder (!), etc.).
- The heatsink temperature for the test must be disclosed
(to avoid someone improving module performance by cooling
to cryogenic temperatures where thermal conductivities
- How to measure transient thermal impedance? Recommend
- Temperature measurement on dice: an IR camera has
emissivity and accuracy issues, wire bonds get very hot
and may skew measurement of die temperature for
- Will the separate measurements for diodes and IGBTs
(point 3 in proposed solution) cause problems in that the
total temperature will be higher when both are running?
The heat from one affects the other. Superposition may