I have a power mosfet part number APT20M45SVFR from Advanced Power
Technology. The initial design was done on FR4 substrate. Although the
board is tested functionally OK, we have a problem of heat dissipation
from 6 of these poer mosfet.

These 6pcs of power mosfet are D3PAK package. Presently, the heat is
conducted through the surface by contacting with the chassis box with
help from thermal tape. However, this method does not seem sufficient.
We improvised it by providing a wide exposed trace to the drain and
attached copper block to dissipate the heat onto the chassis. However,
our thermal group said that this method is still not sifficient to get
rid of the heat.

The component supplier said that this D3PAK has to be used with alumina
substrate to provide efficient heat dissipation. My questions are :

1. Is alumina/ alumina nitrate substrate really necessary?
2. If no, what other cheaper and effective method can we use to
dissipate the heat. Don't suggest heat sink as space is a constraint!
3. Although each Mosfet can handle up to 300W, we only expect to reach
120W. How do you determine the heat dissipation on this board if it is
enclosed within a chassis box?
4. We are using a 28V with 4Amp to drive these 6 mosfets in two groups
of 3. Is 120W too high or too low?

Hope someone out there can help me out of this HOT situation : (
Thanking you in advance.