Reduce thermal resistnace on Board

[rakeshm55]

Hi,
 I am using LTM4613 SMPS in one of my board. The device has a low thermal resistance from Junction to Board (2.3^C/W).
 Effective thermal resistance is mentioned as 8^C/Wfrom junction to ambient  for BGA heat sink operation. It also come with a suggested application note AN110 for Heat sink or Metal plate.
Here what worries me is that the solutions offered for head dissipation are to remove the heat from top. But as I understand device dumps down all the heat to Board/bottom.
In the recommended layout no thermal vias are suggested. If that being the case what are the options available to remove heat off the PCB . If I add thermal vias then i guess heat will pass to GND planes quickly, from there what are the options available, removing heat via screws for mounting PCB are they good methods.
Please suggest some techniques to remove off heat from PCB. Should i expose pads to bottom layer so that I can connect unmasked area to shield using thermal pads
My aim is to reduce the thermal resistance from board to ambient.

[T3sl4co1l]

Note that the ambient spec is as given in the notes, whatever board and copper they used for the test.

0. Do you know that you need improved thermals?  Typically, if you need higher ambient temperature operation, you would.  Or maybe operating at (or beyond?) ratings, if it's derated a bit to begin with.

1. Typical builds on multilayer boards have good thermal performance already, given you've used inner layer planes, which spread the heat out.  Heat conducts reasonably well between top and inner layers, where the laminate is thin (a typical proto board has 5-10 mils of prepreg here).

2. Thermal vias are fine, and can also be filled with solder to approximately double the thermal conductivity; beware of solderability issues with the BGA/LGA device however.  You don't want the vias thieving solder from the pads.  Place them between pads if possible, and tent with soldermask.

3. Screws?  Even with copper screws (which you probably can't get -- aluminum, brass and bronze, yes, but they also conduct significantly worse than pure copper), you have the problem of getting heat over to them; lateral heat spreading is not very good in PCBs.  Consider using a heat spreader at least, but even better, a thermal pad or something.

4. Don't worry about soldermask, by the way.  It's thin, a tiny fraction of the total thermal resistance.  It hardly matters, even if you're using a thermal pad to sink heat out directly by conduction.  It likely improves performance in a passive convection scenario (the slight edge coming from the higher IR emissivity of the coating).

Tim