Attaching the Power Transistors to Heatsink
Put a light coating of thermal paste on the heat-sink where the transistors are to be located. Gently place one isolation film pad for the transistors on top of the thermal paste over each of the holes that have been drilled.
Put a thin coating of thermal paste on the transistor pads of the PC board.
Now turn the PCB over and place an isolation ring into each of the transistor holes. This can be done after placing the board assembly on the heat-sink but I think that this is helps to "eye-ball" the positioning of the assembly over the holes better.
Before placing the PCB assembly onto the isolation pads, carefully hold the PCB assembly over the isolation pads and align the transistor holes over the drilled heat-sink holes.
Next, gently lower the PCB assembly onto the isolation pads. Take care and try not to move the pads as this might cause a short between the transistor and the heat-sink.
NOTE: The center picture shows the a first revision PCB that has a "Blue Wire" that corrects the missing ground trace mentioned elsewhere on this site that has been corrected in the current revision.
If you noticed a pad moving, then a small center punch can be gently inserted into that transistors (and regulators) mounting hole. Moving the center punch in a circular motion will usually move the pad back into position.
Finally, insert the 4-40 screws into each screw hole. Do not tighten any of the screws until all of the screws have been installed and a few threads have been engaged. This allows the PCB to move slightly, making starting all of the screws easier.
Once all screws have been installed, snug up all of the screws.
DO NOT OVER TIGHTEN THE SCREWS AS THIS MIGHT CRACK THE TRANSISTOR ISOLATION RINGS!
Use a Q-Tip or tissue paper to clean up any excess thermal paste that will have squeezed out from between the transistors and the heat-sink.
Once the screws have all been snugged down, use an Ohm meter and check that each transistors (and regulators) metal mounting pad has not been shorted to the heat-sink.
If you find nothing shorted then you are good to move on the the unit assembly.
If you find a short then unfortunately, you need to take it back apart, clean up the thermal paste and figure out why you have a short and try again.