Dmitry Kiryashov wrote:
>
> Matt Bonner wrote:
>
> > > I would do tests on time related items, such as USARTS (as you have said),
> > > I2C, pwm etc. Also, running the outputs at their rated currents on several
> > > pins to led's.
> > >
> > Not possible on the last point: Ohm's Law has to come into play at some
> > point.  At 25mA per pin that's 1/8 W - pretty hard to dissipate at 150
> > deg C.  Anyway, the only way you can get high temp out of a part is to
> > run it at pretty close to "storage temperature" currents (i.e.: zero).
> > High loads should be buffered with a VFET (VN10KE) or whatever.  BTW, at
> > what temperatures do LEDs melt? <g>
>
> Why not possible ? It's more easy (and cheaper too) to glue PIC to some
>  radiator and not apply any additional switches like VFET or so.
> You should use well temperature conducting glue and according radiator
>  and you will dissipate your 1/8 W at 150*C without any troubles.
> Another way - to make a tight contact PIC with radiator through
>  termo-conducting paste .

Dmitry, sometimes it's not possible to use any sort of heat sink (space
constraints etc.).  Anyway, the 74A works great in my design at 150 deg
- the point I was making was that I plan to go _much_ higher (at least
to 200 deg).  Also, the 1/8 W was only _one_ pin.  It's been years since
I've done any proper thermal transfer equations, but common sense tells
me that, at an ambient temperature of 150 deg, it is impossible to
dissipate any amount of power and still maintain the IC die at its rated
storage temperature of 150 deg.  What I'm counting on (but not making
any bets on) is that, with minimal operating current, the 74A will
function much hotter.  I cannot believe that long term operation out of
spec is possible if I overly "abuse" the part.

When destructive testing the 74A, I want to test _only_ the 74A.  If it
survives, I'll add more parts one at a time to find the weak link -
memory, analog components, etc.

BTW, the reason I can't use a heat sink is that my board is 0.95" wide,
14" long, and fits it a stainless steel tube.

--Matt