I can't tell for every single CPU model made, but generally speaking, and
within a certain manufacturing process, heat dissipation depends on the
actual clock input, no matter what the
certified speed is. The latter parameter defines the
propagation delays, which in turn increase with temperature, thus it makes some sense to build the fastest CPUs with revised technology in order to reduce power consumption (and thus heat) for a certain speed -- in other words, get the same temperature at higher speeds.
Not sure about the 68030, but
some revisions of the 68040 (
HRC) do dissipate less heat at the same speed, waiving the use of a heatsink @ 25 MHz.
Actual performance will vary from sample to sample... for instance, I have a Q840AV mobo
without CPU, and tried it with three 68040's -- none of them rated 40 MHz
1) The
XC68040RC
25B from my old, trusty Quadra 700: does chime and the desktop pattern appears. I didn't want to fry this one (was my only '040 at the time) so I shut it down immediately without any further testing.
2) One MC68040RC
33 does nothing -- no chimes, no video >
But it works perfect on the Quadra 700 @ 25 MHz.
3) Another 33 MHz piece (same mask than
2) does chime and gets all the way to the blinking question mark floppy. But when it heats up (all three samples do get really warm }
) and the propagation delays increase, it's no longer able to chime or show video... but if I turn it off and let it cool down, it can chime again -- somehow expected behaviour.
Back to your original question, I
believe a later revision could reduce heat dissipation or fix bugs, but they all
should be 100% compatible -- as long as you keep it cool enough. Just downloaded the 68030's datasheet (3rd edition) and has no mention at all about revisions... but it says that the
50 MHz part specs are
guaranteed up to 80ºC case temperature only, which confirms what I was taught in College
