Audio Engineering

Noise and Grounding 405

sink is not normally essential, as the PCB need not make any connection to the

heat sink fi xing pads.

● TO3P power transistor installation. These large fl at plastic devices are usually

mounted on to the main heat sink with spring clips, which are not only rapid to

install, but also generate less mechanical stress in the package than bolting the

device down by its mounting hole. They also give a more uniform pressure onto

the thermal washer material.

● TO3 power transistor installation. The TO3 package is extremely effi cient at heat

transfer, but notably more awkward to mount.

My preference is for TO3s to be mounted on an aluminium thermal coupler bolted against
the component side of the PCB. The TO3 pins may then be soldered directly on the PCB
solder side. The thermal coupler is drilled with suitable holes to allow M3.5 fi xing bolts
to pass through the TO3 fl ange holes, through the fl ange, and then be secured on the other
side of the PCB by nuts and crinkle washers, which will ensure good contact with the PCB
mounting pads. For reliability, the crinkle washers must cut through the solder tinning into
the underlying copper; a solder contact alone will creep under pressure and the contact
force will decay over time.

Insulating sleeves are essential around the fi xing bolts where they pass through the
thermal coupler; nylon is a good material for these as it has a good high-temperature
capability. Depending on the size of the holes drilled in the thermal coupler for the two
TO3 package pins (and this should be as small as practicable to maximize the area for
heat transfer), these are also likely to require insulation; silicone rubber sleeving carefully
cut to length is very suitable.

An insulating thermal washer must be used between TO3 and fl ange; these tend to be
delicate and the bolts must not be overtightened. If you have a torque wrench, then
10 Nw/m is an approximate upper limit for M3.5 fi xing bolts. Do not solder the two
transistor pins to the PCB until the TO3 is mounted fi rmly and correctly, fully bolted
down, and checked for electrical isolation from the heat sink. Soldering these pins and
then tightening the fi xing bolts is likely to force the pads from the PCB. If this should
happen, then it is quite in order to repair the relevant track or pad with a small length of
stranded wire to the pin; 7/02 size is suitable for a very short run.