So, you want to predict component temperatures do you? Part I
Prediction of component temperatures is central to electronics cooling simulation, the management of their temperatures is central to electronics cooling design. Either way, heat is dissipated inside a component, component gets hot, if component gets too hot, component stops working. When creating an electronics cooling simulation model the question of how to thermally represent the components is key.
The vast majority of electronics cooling simulations performed today use a technology called CFD, computational fluid dynamics. A 3D CAD type definition of the components, PCBs, heatsinks, chassis, fans etc. are input. The equations that predict temperatures and air flow distribution are solved by clever number crunching executables. Finally the predicted temperatures and air flow can be examined in both graphical (CFD: color for directors) or tabular formats.
Why is there any question about how to model components, surely you would just define them as they exist in reality? It all comes down to data availability. Components (packaged silicon normally) are quite complex, incorporating many different parts, with different materials, from bond wires to lead frame, from a spreader to the die itself. Having to manually create all that for EACH component would be madness.
So, why not get such a description from the component manufacturer? Well, they tend to be quite secretive about the internals of their packages. There’s a lot of IP bound up in there, last thing they want to do is to provide a full 3D physical description of it to all and sundry.
Where does this leave the poor thermal engineer? With little data and pressing needs, a number of different component modelling methodologies have emerged over the last 20 or so years.
For this first part of this blog series we’ll cover the most common, a lumped block representation. A 3d homogeneous block with a single material.
In terms of data, the one thing that is available is the package footprint size, this is required for routing and manufacturing design (as is often the case with the more unique disciplines, thermal eats the crumbs that fall from the giants table). If you’ve got a footprint shape then the only other thing you’ll need to make a 3D model is the height. The component height can itself be quite rare, maybe less nowadays as component libraries become more refined and the need for mechanical interference checks becomes much more common place.
In reality a component will have a range of temperatures throughout it. The die being the hottest, the peripheral corners being the coldest. Components will be specified to work up to a maximum junction (die) or case (usually middle+top) temperature. How on earth can we get a range of temperatures out of a component model that is just one lump? Well, you can’t. Why not at least dissipate the heat in that block where the die actually is? Well, don’t bother, even if you did know how big the die was there is still no advantage in accuracy as you’re not modelling all the other heat flow paths in any detail. You can’t be half accurate when trying to be accurate. You’re just going to have to lump it, literally, and spread all the heat throughout your block representation.
Last thing required is a material definition. A 3D thermal simulation requires that all solids have a thermal conductivity defined (to enable a steady state thermal prediction) and density and specific heat (to obtain the thermal capacitance) in addition if a transient thermal simulation is required. Well fear not young thermal engineer. We, your preferred vendor, have created a library of ‘typical lumped packages’ materials with values that will result in typical case temperature predictions when modelling your components as lumped blocks. If you buy the best you get the best, even when it helps you create a more accurate block.
Next time you see some nice piccies of an electronics cooling model, check out how the components are modelled. More than likely they’ll be blocks. Not the best approach by far, in fact it is the least accurate but easiest to define. How inaccurate? Hard to say as it varies a lot. On average I’d say between 10-30% error on case temperature rise with no indication of junction temperature at all. Not bad considering what you’re not representing.
9th October 2009, Ross-on-Wye
More Blog Posts
Add Your Comment
About Robin Bornoff's blog
Views and insights into the concepts behind electronics cooling with a specific focus on the application of FloTHERM to the thermal simulation of electronic systems. Investigations into the application of FloVENT to HVAC simulation. Plus the odd foray into CFD, non-linear dynamic systems and cider making.
- Why Not Just Shove a Heatsink on Top of it? Part 2: Heat Flow Budgets
- Why Not Just Shove a Heatsink on Top of it? Part 1
- Experiment vs. Simulation, Part 5: Detailed IC Package Model Calibration Methodology
- CFD – Colourful Friday Distractions
- Experiment vs. Simulation, Part 4: Compact Thermal Models
- Experiment vs. Simulation, Part 3: JESD51-14
- May 2013 (2)
- April 2013 (3)
- February 2013 (1)
- January 2013 (2)
- September 2012 (1)
- August 2012 (2)
- July 2012 (3)
- May 2012 (2)
- April 2012 (2)
- February 2012 (1)
- January 2012 (5)
- December 2011 (1)
- November 2011 (1)
- October 2011 (3)
- August 2011 (2)
- June 2011 (3)
- May 2011 (1)
- April 2011 (4)
- March 2011 (1)
- February 2011 (1)
- January 2011 (4)
- December 2010 (1)
- November 2010 (3)
- October 2010 (2)
- August 2010 (2)
- July 2010 (4)
- June 2010 (2)
- May 2010 (4)
- April 2010 (2)
- March 2010 (3)
- February 2010 (3)
- January 2010 (8)
- How much do ‘U-Value’ good thermal insulation? Part I
- Keeping the caveman warm – HVAC blog
- FloVIZ, the free FloTHERM/FloVENT CFD results viewer, try it, it’s free
- ‘Heat Trees’ – taking a leaf out of natures book
- The Most Extreme CFD Model Ever Ever – Explained
- FloTHERM and its new XML neutral file format
- The Most Extreme CFD Model Ever Ever
- So, you want to predict component temperatures do you? Part VII
- December 2009 (2)
- November 2009 (3)
- October 2009 (3)
- September 2009 (3)
- August 2009 (3)
- July 2009 (9)
- At the Speed of Heat
- A Load of HVAC TLAs
- How-to: Invert your thermal model to good effect
- Clogged cooling fins, a cautionary tale
- Invert your thermal model to good effect
- “I work with computers”
- Fractals: Gods Artwork, Part II
- Fractals: Gods Artwork, Part I
- “All models are wrong, but some are useful” Part V
- June 2009 (5)
- May 2009 (3)