Hi John,
Most of my projects are related to propulsion systems for fishing kayaks.
About a year ago I did some heatsink testing with the 18v25.
I was using an off the shelf 30-pound thrust trolling motor. Brushed motor.
I discovered that getting even close to the 25A max rating was not realistic without a heatsink.
I tested out some heatsink ideas and they worked very well, allowing me to operated to the 25 amp rated max.
Attached are some links to a couple of YouTube videos I made related to this topic as well as a couple of files related to my results and 18v25 setup. The videos are nothing special. I made them for a buddy of mine that was going to make me some heatsinks on his CNC setup.
I used MX-4 Thermal Compound on the heatsink. It has a Thermal Conductivity = 8.5 W/(mK)
My whole fishing kayak propulsion project changed when I moved from a brushed motor setup to a brushless motor (thruster) setup.
In my opinion, the 18v25 board is not the easiest thing to mount to a heatsink.
Also, I am not sure your 40 amp goal is realistic. One might be able to remove the heat for the FETs but I am not sure the connections and other associated high current components can tolerate sure high currents. You might get away with 40 amps for a very short period of time, maybe measured in seconds. Best I can remember there is no max current setting but you can set the max temp limit, via the Simple Motor Control Center.
Pololu will know the answer to this question.
As you probably know, Pololu sells a range of brushed motor controllers,
maybe some of these can have their outputs connected in parallel.
A quick check of these controllers did not indicate if they could but Pololu will know.
Phil
Pololu Simple High-Power Motor Controller 18v25 - Temp Test
Brushed DC Motor Speed Controller Heatsink
HEATSINKTESTRESULTS001.pdf (38.0 KB)
smc_settings.txt (3.0 KB)