What is Thermal Management - and Why do I need it?

Most materials expand and contract when they are heated or cooled respectively.


A materials ability to expand and contract is usually expressed in terms of thermal expansion sometime expressed as coefficient of thermal expansion (CTE).  The CTE is a measure of the expansion difference as a function of temperature (expansion rate).  Most commonly CTE is expressed as a value at a given temperature value.   It should be more carefully expressed over a temperature range as an "Average CTE".  If you are really careful you want to look at instantaneous CTE (RATE) as a function of temperature.  I will discuss CTE in greater detail later.

If the combined materials are not matched in thermal expansion, stresses can develop that can cause the assembly to

• warp
• delaminate at the attachment interface
• cause the weaker of the assembled materials to crack

as a consequence these behaviors threaten the reliability of a product.  

In electronics, warping through many thermal cycles can cause the device to fail by breaking electrical connections.  Delamination failures disrupt the thermal heat dissipation path, causing the devices to heat beyond the design limits and also causing greater thermally induced thermal expansion stresses.  In this case the electronics will eventually fail thermally, if no other failure occurs.  The case where the thermal stresses cause the die to crack ultimately results in failure of the device.

So when designing a device that generates a lot of power, which needs to dissipate heat you will need to consider the assembly of the components and make sure that the thermal stresses does not introduce one of the failures described above.  

I will discuss the various failure mechanisms in greater detail with typical product applications: IGBT baseplates; power module coolers for HEV applications, and Microprocessor Lids.

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