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.
θJCbottom is thermal resistance from junction to bottom case. θJB is thermal resistance from junction to board. It is true that majority heat is dissipated from the bottom of the IC.
Could you explain a little more about the following question: 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?
The max mechanical force to apply to LTM4646 is around 60lbs.
1. Got it. I think 60lbs this is strong enough.
2. Further explanation:
As θJCtop is larger than θJCbottom, majority heat of the 4 Watt power is dissipated from the BOTTOM of IC and finnally flows through BGA balls and PCB board. So we need to know , is it really correct or necessary to design heatsink on TOP of IC, after all there is little heat go through the IC TOP ?
And we are some kind of confues on how to use θJCtop and of θJCbottom to design a effective heatsink for LTM4646.
hi yanliang, are you still there?
Putting heatsink on top can improve heat dissipation from the top, so more losses will be dissipated from the top.
Please refer to this article. It explains the thermal model for uModule products and will help you in designing heatsink for LTM4646.