Experimenting with a 1976 Kenwood KA-3500 (40 watts per channel into 8Ω (stereo)) integrated amplifier.
I replaced all 4 original NEC 2SB618+ 2SD588 "batwing" [TB33] pwr trans with new Toshiba 2SC5200 / 2SA1943 which are TO3PL (TO-264) pkg. So they cannot be screwed into orig heatsink w/o modification.
I don't want to go that route just yet.
It may be more educational to experiment with coupling the TO3 to the heatsink w/o extensive mods.
And then, test it out with FLIR thermal camera, finger test, wattmeter (Killawatt), o'scope waveform analysis. The thermal camera is a very useful device, and can give a lot of insight.
What I've done is use double-sided (insulated) thermal tape to adhere TO3 (TO-264) devices to heatsink. Then I am using several women's long hair pins to compress the TO3 (TO-264) to heatsink (emulating the compression of the single screw that's not being used) . The amount of tensile compressing of each hair pin increases as it spread further apart. So this gives me an opportunity to add smaller heatsink units to the FRONT of the TO3 (TO-264) devices. I'm still playing around. I have not used any thermal compound or thermal tape for the smaller, front heatsinks. I have maximized the number of hairpins I can fit on the width of the TO3.
This approach to dissipating heat seems to be doing okay, but I have not done the usual 10-min full pwr test (just below clipping).
The specs for KA-3500 are 280w from wall at full 40W into 8 ohms.
Idle pwr draw of the KA-3500 is about 13W (120 VAC). I have taken the volume up, with 1k sine wave (into 6-ohm) dummy load , with the Killawatt reading about 38W at 120AC mains. The thermal camera can see parts of the TO3 (TO-264) that the front heat sink does not touch. And that is about 160F at the 38W draw.
Perhaps the most interesting thing I've learned from this experiment is how warm the FRONT heat sinks become. (Yes, the the large rear OEM heat sinks get warm, but they are very large.

Questions:
Is the thermal coupling of these pwr transistors meant for ONLY rear heat transfer? Meaning internal die couples ONLY to rear for heat conduction? If so, why does the FRONT of device still get hot (to finger touch or probe/FLIR measurement)?
How tight is the usual screw/bolt (only one is needed to TO3) supposed to be? I assume: It can't be too tight to avoid mechanical damage to stress to silicon wafer.

 

When you use a thermal camera, just remember that different materials have different thermal emissivity. From the top of my head, brushed natural aluminum is a poor thermal emitter. The thermal readings will be inconsistent. If you can, substitute those with black anodized heat sinks.

 

You can use large binder clips such as this:

As long as you don't use too much force. Excess force can cause thermal grease to be squeezed out and reduce the amount of heat transfer.

why does the FRONT of device still get hot?

Same reason why the base of a lightbulb gets hot. Thermal energy moves from the higher energy to areas of lower energy. The heatsink allows that energy to radiate to atmosphere. But the whole device still gets hot. Surface, leads, solder, circuit board - etc. Heat always moves from the higher level to the lower level. Using a clip like what's shown will also act as a heat sink to some degree.

This type of heatsink is designed to go on TOP of an IC. As for the internal construction of an IC, I've seen many, some may have the chip mounted on the top of the IC internally.

Nevertheless, if a chip gets hot - a heatsink can draw it away and radiate it to atmosphere. Other sorts of heatsinks exist that are oil filled and piped away to a fan to further accelerate dissipation of heat.

 

As long as you don't use too much force. Excess force can cause thermal grease to be squeezed out and reduce the amount of heat transfer.

Not true.
You want a minimum layer of thermal grease for maximum heat transfer, since the grease has a much higher thermal resistance than the metal (but lower than air).
The grease is just to fill in any small air gaps between the transistor and the heatsink.
Any grease more than the minimum to fill those gaps, will reduce the heat transfer.

 

You can use large binder clips such as this:
View attachment 371523
As long as you don't use too much force. Excess force can cause thermal grease to be squeezed out and reduce the amount of heat transfer.

Same reason why the base of a lightbulb gets hot. Thermal energy moves from the higher energy to areas of lower energy. The heatsink allows that energy to radiate to atmosphere. But the whole device still gets hot. Surface, leads, solder, circuit board - etc. Heat always moves from the higher level to the lower level. Using a clip like what's shown will also act as a heat sink to some degree.

This type of heatsink is designed to go on TOP of an IC. As for the internal construction of an IC, I've seen many, some may have the chip mounted on the top of the IC internally.
View attachment 371527
Nevertheless, if a chip gets hot - a heatsink can draw it away and radiate it to atmosphere. Other sorts of heatsinks exist that are oil filled and piped away to a fan to further accelerate dissipation of heat.

The large binder clip would be good, but that large and tall orig heatsink does not allow for much clearance for anything other than the hair clips.
BTW:
Some photos above are not showing so trying again:

 

So what about the FRONT of the device, be it a transistor, regulator, etc? As I asked in the OP:
Is the thermal coupling of these pwr transistors meant for ONLY rear heat transfer? Meaning internal die couples ONLY to rear for heat conduction? If so, why does the FRONT of device still get hot (to finger touch or probe/FLIR measurement)?

 

Hello,

The images still do not show.
Are you using an external picture server?
Please upload the pictures to the forum.

Bertus

 

Hello,

The images still do not show.
Are you using an external picture server?
Please upload the pictures to the forum.

Bertus

Not sure why some show and some don't. But in PREVIEW mode, they show just fine.
BTW: No way. I'm not uploading any of my IP to this greedy forum so they can claim it as their own.

 

The views all show on my HP win7 computer!
Devices intended to have a heatsink certainly have a PRIMARY heat-transfer surface, but all of the other surfaces can also provide heat transfer out.