Thermal Design in Electronics Packaging

Thermal Design

Electronics packaging is one of the major disciplines in the field of mechanical engineering and it incorporates many engineering subdisciplines including thermal design. This is an overview with some useful strategies for anticipating and addressing thermal challenges in electronics packaging.

Steady State Cooling 

We usually base the thermal design of electronic devices on a steady-state assumption.  We define some worst-case ambient air temperature at which we will test the device, then run the device at that temperature for several hours. A common, though conservative, specification is 55℃. That’s only a couple of degrees above the highest temperature ever recorded on earth.  But consider: temperature difference drives heat transfer, or “Delta-T.”  If you must limit internal temperature to 70℃, then a 55℃ test temperature yields only 15℃ of Delta T to drive heat-dissipation.  Reducing the test temperature to 50℃, which is still unbearably hot, increases the delta-T to 20℃. That is a 33% improvement in heat-rejection.  If you get nothing else from this article, remember this:  Don’t over-specify the ambient temperature!  A more realistic target for your thermal design will result in a more compact and cost-effective product.

The steady-state scenario serves as a baseline against which to discuss more interesting challenges involved in electronics packaging.

Transient Performance

We intend some devices to operate for short periods of time at extreme temperatures.  As an example, the “black box” found on a commercial aircraft is rated to operate for 30 minutes at a temperature of 1,100℃.  That feat is achieved with a rather primitive strategy: a double-walled enclosure and a lot of insulation.  Other, more compact devices use a different strategy.  A tactical radio, for example, uses a housing of insulating plastic and an internal thermal mass that “banks up” heat, in much the same way that a flywheel absorbs kinetic energy.  There is a critical balance to this strategy.  A device that banks up heat as part of the high-temperature strategy must be able to reject that heat to ambient at the conclusion of the test.  The interplay of external heating, insulation, and internal heat capacity requires careful Product Engineering; often including CFD simulation.

Solar Heat Gain

Solar Heat Gain, or insolation, must be considered when engineering products for outdoor use.  Consider a device mounted to an external wall.  Mount that enclosure to a north-facing wall in Death Valley and it will perform surprisingly well.  Mount it to a south-facing wall in much cooler Denver and it will overheat very quickly.  A rigidly attached but thermally isolated sun-shade is one effective solution.  There are many others.

System Engineering

Certain applications require continuous, not transient, performance in a hot environment.  Whether transient or steady-state, System Engineering should explicitly address thermal design challenges, and do so early in the product development process. Numerous strategies are employed to deal with situations that cannot be addressed with purely mechanical strategies.  A few examples:

  • MIL-STD-810 specifies 3 states: Storage, Operation, and Tactical Standby to Operational.  Each state specified by MIL-STD-810 generates a different amount of heat.  It is important to understand what states and durations are expected in a high-temperature environment; perhaps automatically switching between them to control heat generation.
  • Some devices can limit their performance (e.g., clock speed) when they begin to overheat.  They control temperature by generating less heat in hot environments.
  • More expensive electronic components may be specified to increase the Delta-T.  Commercial semiconductors are typically rated to 70℃, Industrial to 85℃, and Military to 125℃.  Military-rated components are rather expensive.
  • Rarely, active cooling is the only practical solution.  Thermoelectric Cooling (TEC) is one example of a technology that can keep electronics cool and reliable even in an extremely hot environment.  This is complex and should not be undertaken by an inexperienced engineer.

Thermal design challenges are rarely the only ones that an Electronics Packaging engineer must overcome.  It is in your best interest in the thermal design stages to thoughtfully and reasonably specify how the device will be tested, enumerate and rank your goals, and retain an experienced Product Engineering team to execute the product.

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