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"All models are wrong, but some are useful" Part I

The original business case that David Tatchell and Harvey Rosten put forward for the formation of (what was to become) Flomerics had the following quote on its front page:

All models are wrong, but some are useful

G. E. Box

This sentiment has underpinned the approach my colleagues and I have always taken in the packaging up of computational fluid dynamics technology for the thermal simulation of electronic systems. It still does so as we now continue to work as the ‘Mechanical Analysis’ division within Mentor Graphics. The quote is our pragmatic dogma.  This and the following few blog entries will explore the issues associated with the usefulness, accuracy and cost of CFD based electronics cooling simulations.

Back in the day (and sometimes even now) the issue of the predictive accuracy of FloTHERM was front and center of each sale. “How accurate is FoTHERM?” was answered in a number of ways:

“Accurate enough to be useful”

“It depends on the accuracy of your input data”

“It depends on the accuracy of your experimental measurements”

And finally, when push came to shove:

“Usually ~10% on dT predictions”

With the following response: “O, really, err, ok then”.  Such a response I put down to expectations that are more often than not laid down and set in academia. A wonderful environment, free and unfettered from the restrictions of time (=$$) and singularly minded in the pursuit of accuracy at all costs.

Should we ever be happy with 10%? Yes, and in addition we should be somewhat surprised with <5% and unbelieving of <1%.

Why? Why be accepting of something that could be smaller, could allow FloTHERM users to have an even greater confidence of the thermal compliance of their yet to be made electronics? There’s a long nasty line up of suspects, the ring leader of which is power dissipation.

Power, Q (Watts -> J/s ->Nm/s (speed, hmm, more on that some other time…))

Power, it’s why you need tools like FloTHERM, that and the fact that solid materials tend to grumble by getting hot when you first dissipate heat in, then force heat to pass through, them.

From_hot_to_cold

Electronics cooling is getting the cold in to quench where the power is being dissipated as quickly/easily as possible. Conversely it is trying to get the heat out as quickly/easily as possible to stop the temperature building up at the power source. Either way, power’s a main culprit.

So what form do power dissipation errors take?…

Usually, in an electronics cooling simulation, you’ll be assuming a steady state of your system, meaning in this case that the power dissipation in the die is unchanging in time and therefore an time averaged value should be sought and used. Alternatively if you are performing a transient simulation you might be assuming that the time averaged value changes from one value to another. For many packages and operating behaviours such assumptions themselves will contribute to the overall simulation error.

There was a study once (whose reference alludes me) of taking two ‘identical’ packages from a production line, powering them up and measuring their actual consumption. The consumption that should have been identical varied by ~10% between the two.

If you are using a detailed model of a package in your simulation, where the important internal 3D geometric structures are explicitly represented (e.g. lead frame, tie bars, heat slug etc.) likely you’ll be assuming that the die is a single piece of Si with the power dissipation uniformly spread over it. This assumption can also carry an error. (Note that in the V8.1 of FloTHERM you can use the Die SmartPart to specify a non-uniform collection of discrete areas with differing power dissipation over the die, this goes a long way to shed the inaccurate assumption of overall die power dissipation uniformity).

So what is the relationship between power inaccuracy and resulting thermal predictive accuracy? Hey, good question. I did a little study in FloTHERM, took a detailed model of an 84 lead PLCC, stuck it on a 2S2P test board in the standard JEDEC still air test environment, set the ambient temperature to 45degC and solved at 1W, 0.9W and 1.1W (~10 minutes start to finish, man, I love FloTHERM!). Looking at the resulting Tj values I found that if power dissipation is known to within +/- 10% then the resulting dTj (Tj rise above ambient) is also +/-10% accurate. (Hey, thermal experts, under what situations would it not be a proportional relationship when Q+/-10%?).

plcc84_power_vs_tj_and_graph1

The point about this power inaccuracy issue is that there will be inaccuracies that you will have to carry and suffer not because you are a poor modeler, but because of assumptions you have to make due to the utter lack of good quality thermal data. Getting even the average power dissipation for the package as a whole is difficult enough, let alone  knowing how to account for manufacturing variations in the package construction! Experience has shown that in a design world of unknowns and unknowables a +/- 10% accuracy of an electronics thermal simulation is to be expected, not feared.  If you believe your design requires predictions of even tighter margins then I’d be more worried about your design itself rather than its simulation.

There are suspects in the line up of modeling errors that you as a modeler will have control over. Whereas power dissipation inaccuracy is something you have to suffer, there are others that you could be responsible for….. more on that in Part II

Robin Bornoff

Hampton Court

May 12 2009

Accuracy, Electronics Cooling, Power Dissipation

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About Robin Bornoff Follow on Twitter

Robin BornoffRobin Bornoff achieved a Mechanical Engineering Degree from Brunel University in 1992 followed by a PhD in 1995 for CFD research. He then joined Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd) as an application and support engineer, specializing in the application of CFD to electronics cooling and the design of the built environment. He is now the Product Marketing Manager responsible for the FloTHERM and FloVENT softwares. Visit Robin Bornoff's blog

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Comments 6

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[...] of a package. A 3D object representing the die, with the correct material properties, with the power being set to dissipate on its surface. All the other important internal objects such as die attach, bond wires or [...]
If your ring-leader for model inaccuracy is power dissipation, then I would like to offer "material properties" as a close-run second. Sure, for common materials such as copper the thermal conductivity, specific heat, density, etc., have been tested to death and are well known and readily available. But what about, say, the plastic encapsulant used in that Far-Eastern QFP package? If you're lucky you *might* get a figure for thermal conductivity - but specific heat? Good luck with that... Also, most leadframe materials are not pure copper anyway and usually have some small percentage of other metals, which can have a profound effect on thermal conductivity. I would say that material properties often provide me with the biggest headache because many of the other factors are at least partially under my control.

Chris Hill
12:15 PM Jun 23, 2009

http://www.matweb.com/ is the most extensive materials database. It's not free (well, not all of it) but is the best source for troublesome rho and Cp.

Robin Bornoff
12:50 PM Jun 24, 2009

Indeed I was fortunate enough to discover matweb some years ago and have used it often. I particularly like the ability to search for alloys based on percentage composition. However, my experience is that even matweb often cannot help me with density and SHC figures.

Chris Hill
6:13 AM Jun 29, 2009

[...] the fact that all models are wrong a detailed model representation of a package is least wrong and comparatively the best. Each [...]
[...] them to his/her advantage. A about a year ago to the day I ran a series of blogs based on the quote “All models are wrong, but some are useful”. In the same way that art is never reality, just an interpretation of it, then a computer model will [...]

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