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JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011
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Strange results.

255 Views, 17 Replies
01-28-2014 08:46 AM

I modeled a simplified version of a PCBA we're already testing in the lab.  With ambient conditions at 26C and a power input of 5.4W with a 100% duty cycle, the thermal image indicates a chip temp of about 131C.

 

I modeled this in the software, applied the same boundary conditions, and the first 100 iterations produced a chip temp of about 500C.  I used an air volume aspect ratio using the Autodesk suggested info.  I thought maybe it didn't have enough iterations to converge properly, so I hit run again, and this time the temp shot up to 5500C  (yes, that's 5500C).  Here's a link to the analysis and the results (it's almost 10MB, so I can't upload it here):

 

http://a360.co/1jEvjMc

 

 

What's the deal?  I am REALLY struggling to replicate what we're seeing in the lab, and I'm on the hot seat to create a predictive analysis of the entire board assembly.  I must deliver this within 2 days (no, not this simplified version).  I'm finding small alterations in just the air volume changes the results significantly, so I'm losing confidence we can use this tool in a predictive scenario.  Now I ran into this "result", which is completely out of range of any reasonable estimate of what we should be getting from the software.

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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: Strange results.

01-28-2014 09:10 AM in reply to: JohnTomasik6493

Oh, I noticed my materials were wrong for the air (don't know how air emissivity was the "default"), but even changing it to "variable air" still results in unreal numbers (+2000C).  

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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: Strange results.

01-28-2014 12:34 PM in reply to: JohnTomasik6493

And two more studies.  I noticed I didn't have the variable air environment set.  Regardless, I'm still getting high temps (coolest was around the 205C range).  

 

I noticed if I oriented the board vertically, my temps doubled, which isn't what we see in the lab.

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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: Strange results.

01-28-2014 03:20 PM in reply to: JohnTomasik6493

I went ahead and created the assembly, even though I have concerns about the results I was seeing and spoke to in this thread already.  I've attached the CFZ.  At ambient (22C), that one chip (which reached 131C in real life, and was off the map in the above studies) only got to about 120C.  When I elevated the ambient temp to 85C, that chip only reached 115C, which is 5C less at the much higher ambient temp?  How is that possible?

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wildej
Posts: 757
Registered: ‎08-25-2011

Re: Strange results.

01-29-2014 12:07 AM in reply to: JohnTomasik6493

Hi John,

 

I have been looking at the vertical model only. Ideally we need to change the box size, it needs to be wider.:

 

Vertical.png

 

Right now the mesh is too coarse, at the outlet we only have 2 elements across the width, with a wider box also we should aim for at least 10 or so. Ideally we do not want to see any strange velocity profiles when we view the results on a cut-plane, if the mesh is too coarse, this is exactly what we will see.

We also need at least 2 elements through the thickness of each solid part.

 

You had gravity set as components, when it needs to be 'earth' with a 1 value (and I did the exact same on a different model today, very easy mistake to make).

I would run with the Mixing Length turbulence model with natural convection.

 

With these changes (I did not change the box, for some reason it made the model hard to manipulate, easier to draw it in CAD) I got max values of 89C:

 

Temp.png

 

As an aside, the horizontal box must also change size, we need a larger space above the unit:

 

Horizontal.png

 

I hope this helps.

 

Kind regards,

Jon



Jon Wilde
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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: Strange results.

01-31-2014 09:26 AM in reply to: wildej

Jon,

 

That helps, although in real life, that chip's temp in those conditions is 131C.  If you have any thoughts of why the inaccuracy, let me know.  

 

My concern remains that as I alter the air space beyond your recommendations, I'm increasing uncertainty in accuracy of the results.  I've been playing around with different air spaces beyond your recommendations, and the results can be quite different.  Short of my having to show up here begging for guidance, and then relying on your remote input, is 10 elements the magic number that'll provide an accurate study?

 

When I'm in the setup mode, and I've autosized the mesh, do I assume that each teal dot is a node of the mesh?

 

 

Last, attached is the share file from the assembly that diverged.  I ran this same study with an ambient of 26C, and the study converged.  I then changed only the ambient temp to 75C, and it diverged.  I checked to ensure that intelligent solution control was enabled, as instructed by the error box I found when I came in this morning.  Any idea what I can do to resolve this?  Please note that this study is to simulate a condition of an enclosure sitting on a table.  I used the recommended air volume sizing for that scenario as instructed by your web site.

 

 

John T.

 

 

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Product Support
wildej
Posts: 757
Registered: ‎08-25-2011

Re: Strange results.

02-03-2014 05:07 AM in reply to: JohnTomasik6493

Hi John,

 

I would say that any innaccuracy is likely down to model setup here.

If the unit is on a table, we should not be using the 'chimney' approach (top and bottom both open) where the unit needs to be a good distance from the bottom. We should be using the 'bucket' approach. Where only the top is open with a P=0 and also a film coefficient (as per the guide here).

Model the bench thickness in CAD and also the air domain, the size you have it but sitting on the bench. The unit should also be on the bench and not floating a little.

 

Let's see what the flow regime looks like on a cut-plane first. Looking at the flow and whether it is seperate to the lines of the elements is the first check we can make to understand if we have sufficient mesh - if the flow look jagged, we know we need more mesh (and that will bring increased accuracy).

The key to accuracy is now not domain sizing (as it is nearly OK) but meshing, the results should all be the same as you increase the domain size but you need a very good mesh to ensure they are accurate. I often run mesh sensitivity sutdies. Refining the mesh to 0.7 and seeing if the results change. If they change by more than 5%, I know that my results are not yet independent of the mesh sizing and that I should refine futher.

 

When I'm in the setup mode, and I've autosized the mesh, do I assume that each teal dot is a node of the mesh?

Yes, exactly that. 

 

I hope that helps.

 

Kind regards,

Jon



Jon Wilde
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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: Strange results.

02-03-2014 09:37 AM in reply to: wildej

Jon,

 

The first set of boards (development boards) and their associated studies were held in a vice, elevated off a table.  So, they were more of the chimney effect, which is what we were first discussing (where you said my air volume was too small).  When I went to the desktop assembly, I modified my simulation model to reflect the bench mounted scenario, as suggested on your site.  That's just for clarification.  Noted about using the film coefficient.  I'll modify the model to use that.

 

Much thanks about the meshing input.  I'll be sure to look at that from here on out.

 

 

John T.

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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: Strange results.

02-12-2014 04:00 PM in reply to: JohnTomasik6493

I have a somewhat unrelated question, but I didn't want to start a new thread for this relatively small item.  I want to present some key IC chip temps to my client from an electronics thermal study that has far more IC's than I want to discuss.  I started using the decision center to pick parts and include them in a summary.  When I went to the decision center, I couldn't select any radio buttons or retrieve an values for those chips.  The documentation I've looked at online mention critical values.  I tried figuring out where to enter these numbers, but I ran out of time.  Where would I enter those values?  Are no selections available in the decision center because I don't have critical values assigned?

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Product Support
wildej
Posts: 757
Registered: ‎08-25-2011

Re: Strange results.

02-13-2014 01:54 AM in reply to: JohnTomasik6493

Hi John,

 

You should be able to do this post run. Try going to 'Part' within results and selecting just one chip. Then tick the small box.

Then within the Decision Centre you should be able to see the part. You will need to right click it to update values (CFD will grab the value from each design/scenario) and then you should have the temps listed. Does that work?

From there, where to type in the critical value should be straightforward as it's on the same page.

 

Kind regards,

Jon



Jon Wilde
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