9 Replies Latest reply on Jul 30, 2013 6:04 PM by Jared Conway

    What am I doing wrong with this thermal simulation?

    Justin Broughton

      Ran two simulations. One was without a heat spreader. One was with a heat spreader. The temperatures can go up to 85 degrees Celsius before the integrated circuits are permanently damaged.

      (Before I start on the next part I want to say I should do hand calculations. I would do hand calculations but I am a rising sophomore in college and still have not had thermodynamics. I was told by my summer boss that these numbers are higher than they should be. So before I go and tell him he is wrong I want to make sure that there is nothing wrong with my simulation.  Also, there are many small features that I am sure are not in the model but I was told to do my best with what I was given.)

      Attached are two pictures- one from the first simulation without a spreader and one with a spreader. What I am trying to do is take the heat from the ICs (the hottest parts) and spread them through the copper to the top half of the enclosure. It is not supposed to be a heat sink (where it has fins, I believe) as I tried to model one and my boss said not to.  The strange thing is two simulations before, I had numbers ranging from upper 20 degrees Celsius to 92 degrees.  I am not aware of anything that changed, but I reset the simulation because for some odd reason the “run” button would not respond. 

       

      I know I have posted something similar, but I feel like I did not elaborate enough or provide the file for anyone to actually give specific advice.  

       

      I will attach file with the assembly/simulation as well.

       

      By the way, the difference between the two simulations is I did not have the parts heatsink1, heatsinku1, heatsinku2and3(1), heatsinku2and3(2), and heatsinku6 in the simulation. 

       

      TL;DR-  Added a heat spreader but the result is similar and both simulations are way too high according to the overlords

       

      *attach two pictures of temperatures and the file itself

        • Re: What am I doing wrong with this thermal simulation?
          Jared Conway

          Just to make sure I understand, your boss wants absolute results, or doesn't want the chips to be over 85 deg c? Rather than relative results between with and without heat spreaders?

           

          If so, I could see where your boss is concerned. But this also means you need to spend a lot of time limiting your problems assumptions. Have you thought about writing them down? There may be a big one that he can help you with knowing that it needs to be eliminated to get good results.

           

          Similarly, do you have physical results that you know the chips aren't at 276deg c in config one? If not, how do you or your boss not know that is the right answer? Without anything to compare against, physical or hand, I'd go with a relative approach. And front the results, it looks like the spreader is doing something.

            • Re: What am I doing wrong with this thermal simulation?
              Justin Broughton

              The boss wants to make sure that we need a heat spreader before going through the trouble of putting one in. And then afterwards, he wants to make sure that the heat spreader will do the job and prevent the chips from overheating assuming they overheat in the simulation without the heat spreader.


              And unfortunately, I cannot ask the boss for help because no one in this division is familiar with Solidworks and no one is a mechanical engineer (all double Es).  I asked for some help from some people down the road but they have very little time and do not have the updated software to access the files (2013 versus 2011).

               

              For assumptions - I was told by one of the people from down the road that I only needed four things to run the simulation: Ambient temperature (for the time being just around 20-25 degrees celsius), materials the assembly and the parts are made of, the 9 power outputs, and the thermal conductivitie.  I thought a problem might be that I was using incorrect materials with similar thermal conductivities but different specific heats, but I think someone on the forum said I do not need specific heats unless the thermal simulation is for transient thermal simulations. 

               

              Like you said, I saw a lot of relative results that proved the heat spreader was working.  The problem is that these results do not support the heat spreader actually lowering the max temperature.  Also, my results are never consistent whenever I recreate the assembly.  The simulation before this one showed the results to be 92 degrees with the heat spreader as opposed to 100+ degrees celsius.  So I think I did it correctly that one time.

              with_copper.png

              I will ask one of the people if they have any physical test results, but I hear the production of these chips is time consuming and expensive so they want to ruin as few as possible.

                • Re: What am I doing wrong with this thermal simulation?
                  Bill McEachern

                  If you can get some dough you could hire me to do this for you. I have done lots of these analysis on very complicated pieces of kit from Llidars to muxponders. Obviously, you would rather do it in house. Another option is mentoring - we do lots of this over the webs as well. We just hook up over the web for an hour at a time typically and we get you on the right track - gives you a place to go for advice and gut checks. We require that you buy about a days worht of time that you can consume over say 6 months or less.

                  • Re: What am I doing wrong with this thermal simulation?
                    Jared Conway

                    Justin, email me at jared@hawkridgesys.com. Send the model and a description of the environment that this will be used in and I'll take a look at the model on Monday when I'm back in the office.

                     

                    If you can, send he models that were behaving before. Running a sim twice shouldn't result in different answers. If you have the results, even better because we can dig into those. (Rerunning may result in the new answer which won't help)

                      • Re: What am I doing wrong with this thermal simulation?
                        Jared Conway

                        Hi Justin, back in the office today and went to take a look but you only uploaded the sldasm file. YOu need to use pack and go to package the assembly and all the requisite parts.

                          • Re: What am I doing wrong with this thermal simulation?
                            Justin Broughton

                            Hey Jared, sorry about not sending the email yesterday. I'll send the file to your email. I was running some simulations just to make sure I don't know what I'm doing, haha.
                            Here's the simulation I ran overnight (swtiched from flow simulation to just simulation to see if the strange results were consistent.

                             

                            Also, an update: my metor said he would give me new power values (lab-tested maximum values as opposed to the highest possible values the packaging said for each IC unit) sometime later todaY

                             


                            Also a further note: I did not let all of the simulations converge. When they hit values of smaller than .1 degrees celsius in change (by that point over 200 celsius typically) I stopped the simulation.

                             

                            Thanks for all the help! Static Thermal Analysis.png

                              • Re: What am I doing wrong with this thermal simulation?
                                Jared Conway

                                "Also, an update: my metor said he would give me new power values (lab-tested maximum values as opposed to the highest possible values the packaging said for each IC unit) sometime later todaY"

                                 

                                this could make a huge difference. did you get more information about how it is actually tested and confirm that the environment you've chosen is the right one for the testing?

                                  • Re: What am I doing wrong with this thermal simulation?
                                    Justin Broughton

                                    Sorry for late response, just got ahold of him a few minutes ago.

                                     

                                    The only value that changes is "Heat Generation Rate 8" and it changes to 1.5 W instead of 3. 

                                     

                                    I do not know what you mean by confirming environment I've chosen is the correct one, but I'll give it a try. He's testing it in a room temperature environment right now without the enclosure around it. The chips purpose is to be used outside in any circumstances such as a lot of sun and high temperatures to cloudy at room temperature.  I've been asked to test 4 different scenarios: 1) sun, high temp, no heat spreader 2) room temperature, no heat spreader 3) heat spreader, high temp, sun 4) heat spreader, room temp

                                     

                                     

                                    If he changes any of the values again I will notify you ASAP