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LTM4646: Thermal Resistance and Allowed Mechanical Pressure when mounting Heatsink


We use LTM4646 for 3.3V/10A design. Estimated power dissipation on LTM4646 is about 4W. Two question.

(1) We noticed there is three thermal resistance parameter: θJCtop = 12.6°C/W, θJCbottom = 1.8°C/W, θJB = 2.3°C/W, What the difference of θJCbottom and θJB? Since θJCbottom  is much smaller than θJCtop, does it mean that most therm will disspated from the bottom the IC ?

If we design top side mounted heatsink, should we just use θJCtop ? But 12.6 is relatively large, so how could we take advantage of small thermal resistance of θJCbottom?

(2) We may use compressible thermal pad for interfacing material placed between case top of IC and metal heatsink.According to supplier of thermal pad , some pressure should be applied to keep the thermal pad contact with IC surface tightly to ensure the good thermal conductivity. So what is the recommended Mechanical Pressure for LTM4646 chip (unit is MPa or PSI),  and what is the pressure limit so as to keep IC from being destroyed by over stressed, as well as ensure long term reliability of BGA balls under consistive mechanical load.

Any comments on thsi thermal design are welcome. 

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[edited by: XYanalog at 5:56 AM (GMT 0) on 21 Jun 2019]