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riscy (Geotechnical) (OP)
22 Dec 10 7:14
Hi

Has anyone have thermal capacitance and resistance model for the QFN and TSSOP package (QFN, 4x4, 7x7 and 9x9 with exposed pad) and similar size for TSSOP with exposed pad.

I actually have thermal resistance, derived from IC website but they do not have thermal capacitance, I need this to estimate heat flow from IC (ie 3W) into PCB.

Thanks   
tkordyban (Mechanical)
23 Dec 10 17:12
You did not say whether your calculation is for steady state or for a transient condition.  In steady state you do not need the thermal capacitance values.

In general, very few multi-resistor models are publicly available for component packages.  Models with thermal capacitance are even more rare.  There is also the problem that if you do find one, how do you know it is correct?  These models must be verified against test data, and that is not easy to do.

Sorry to seems so negative, but I want to save you the time of searching for something you probably will not find.
riscy (Geotechnical) (OP)
23 Dec 10 23:48
Thank for the email, it is very interesting note. Which is the reason I posting this message.

Does this mean the thermal capacitance modelling method for transient is not a working model as published say. I'm curious for reason why designer prefer not to use it or publish their finding, I wonder if this is good method for 1st order approximation.

The capacitance is quite simply based on PCB volume, specific heat capacity and density. I just calculate a few but nice to confirm if the result is similar what I have calculated.    

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