I have always wondered about what kind of super material circut boards are made out of...how could the aluminum body get so hot that I can barely hold it, yet the thermal paste and circut board remain normal.
Second part...
how thick and durable is that little gold contact area where the body tube makes contact with the board? It seems really thin to me like a thin strip of gold plating. Is it resilient enough to withstand many many twist-on's and off's?
Circuit boards are usually fabricated from a material called FR-4. This is glass reinforced epoxy laminate, with added fire retardant agents - brominated compounds.
I'll give you a simple example. All of this is variable, but in my experience these numbers probably apply to 90% of the commercial circuit boards I have seen in the past 30 years. For a "two-sided" (no internal layers) the most generic laminates would be about 0.060" thick, made from seven fabric layers, and would have copper foil on the outer surfaces that is "one ounce" (1 oz / ft2) or about 0.0014" thick. The pattern is etched in the copper, and a gold finish is plated over a barrier layer of nickel plate. The gold itself is only about 30 to 50 microinches (one micron) thick.
In practice, a PCB (printed circuit board) that needs a lot of power/heat dissipation is fabricated as a multi-layer board, with internal power and ground layers of copper foil connecting to the plated through holes (the pattern of the internal layers has etched clearance around all but the desired connecting holes, so that the Vcc and the GND holes connect to these "solid" internal planes. They are typically thicker, up to say 0.003" thick. The layers can dissipate considerable heat, especially if they are thermally connected to heat sinks, mounting rails, etc.
FR-4 material withstands high heats and thermal cycling, the newer materials are based on advanced epoxy resins with excellent properties.
But...only the copper in the PCB carries any significant heat. There are advanced types of PCB's with metal cores, these are reserved for applications like avionics and are very expensive.
With the advent of cellphone and other similar "high density interconnect" circuitry, all of this geometry has shrunk in the past decade or two. Now the plated holes are drilled by lasers, and can be down to about 0.006" diameter. The total thickness of the PCB is down to 0.030" or so, and there are probably 4-6 layers of circuitry in the laminate stack. The copper plating in those holes is sometimes the current and heat carrying limit, as it is only about 0.001" thick.
The green color you see on the PCB surface is really only the "soldermask" which is applied as a final step to insulate and protect the copper pattern.
So, the contact area you see (gold plating) is generally built up with the one or two ounce copper foil (0.001-0.003 in. thick) then a nickel barrier plate layer about 100-200 microinches, and finally nearly pure gold say 40 microinches thick.
The gold itself is very soft, and not durable. The nickel provides the strength and wear resistance, it is much harder.
I have often thought that the mechanical friction of the top of a battery button on a gold plated contact will quickly wear the gold through. The life of this electromechanical contact surface will be a function of the exact mating surfaces, the normal force applied, and the number of cycles.
Of course, no battery is gold plated, so the contact resistance of the circuit board is not really a limitation. But my advice is to apply a lubricant. It will certainly extend the life of the contact surface.
I am somewhat amazed that the little contact on the board in a Fenix L0D can withstand the wear it does. I know my one year old L0D has thousands of cycles and still works fine. I will have to take a look under a scope at work and see if there is any gold left, I doubt it.
A good reason to take a look at the exact surface of the button on your AA or AAA (I try to polish my Eneloops a bit on my jeans, and be sure there is no grit), and apply some contact lubricant like the Caig DeoxIT.
