Thermatrons Must Leave
Talk to a mathematician about thermodynamics and heat transfer and before long they’ll be showing you lots of weird symbols in a rather condensed vector notation. Talk to a physicist and such concepts of electron spin states will lead the discussion. Talk to a mechanical engineer and likely analogies will be used that take a well known system or process and apply that to the concept of heat flow. OK, so maybe analogies take certain liberties, they do not reflect the truth underlying the process being described. They are though a good vehicle for communication that leads to appreciation and an intuitive understanding.
In the world of electronics thermal analysis the most memorable analogy I’ve seen describing the concepts of heat flow is in Tony Kordyban’s superb book “Hot air Rises and Heat Sinks (Everything You Know About Electronics Cooling is Wrong)”. There he introduces the concept of Joule Monkeys, always clambering away from each other due to their aversion of crowdedness, having to pass through areas of differing klamberability. Joule Monkeys being the analogy to heat and their crowdedness being a measure of temperature.
Taking a much drier approach one could consider the concept of a thermatron. [WARNING: from this point on this blog is no longer bound by scientific proof or physical definition]. In solids, thermatons are the particles that are driven by a temperature gradient, analogous to electrons being driven by an voltage drop. This is conduction. In air thermatrons hitch a lift with the moving air stream. This is convection. When thermatrons get to the surface of a solid object, some of them are lucky enough to be able to use their teleporting capability to whisk them away at the speed of light to be deposited on another line of sight solid surface. This is radiation.
Thermatons are created en-mass in the silicon in the middle of a powered IC package. Right from their birth they have one goal, go find some place cooler to live (in this respect maybe they’re more like penguinatrons?). Like rain drops running down a window, or lava flooding down a volcano, the thermatrons take various paths to reach their cool nirvana. How many thermatrons are seeking their way out and/or how difficult they find it to get out will determine the temperature they leave behind in their wake.
The nirvana temperature is low and essentially fixed. The backed-up source temperature is analogous to the water level building up behind a dam.
If the resistance the thermatrons experience is reduced, the backed-up source temperature reduces as well.
If less thermatrons are trying to muscle their way out this will further reduce the source (junction) temperature.
The real world is 3D. There are multiple paths, conductive, convective and radiative, that heat can follow on its way out.
Think of each arrow as a part of the electronics system, close to the heat source you’ll have leadframe, die flag, then arrows that represent the various resistances in the PCB. Sometimes the heat splits down two paths then joins back up again. Arrows that represent convection will have a very low resistance (thermatrons are hitching a lift on the air bus remember, easy going, no temperature back-up penalty).
Here’s what it looks like from a typical FloTHERM simulation of a TO220 sitting on a PCB (same example as Kelly Cordell-Morris used in her recent blog):
The arrows indicate direction of thermatron travel, their size indicating how many. OK, analogy over, the arrows are heat flux (Watts/m^2). Knowing where the heat goes and how difficult it finds it to leave is central to subsequent good electronics thermal design.
If you’ve got enough energy to read this far why not gop the whole hog and check out this ‘Beat the Heat’ article on the use of heat flow analysis in electronics thermal design.
19th August, Hampton Court
