Hello from San Diego:
Kevin Harris asks some questions about temperature
measurements.
> 1. Is this a valid temperature measurement for the PCB? I'm of two minds
> on this. It could said that I'm really measuring the diodes temperature
> and not the PCB. On the other hand the diode pad does touch the PCB .
For safety purposes, we are concerned with the performance
of insulation. Insulation performance is a function of its
temperature.
(We may also -- for safety purposes -- be concerned with the
physical support of the conductors by a plastic material,
which may sag as a function of temperature.)
Most electrical insulators are poor thermal conductors. To
get an accurate measurement of insulation temperature, it is
better to measure the temperature of the conductor in contact
with the insulator. This MAY give a slightly pessimistic
value, but that is better than a lesser value. So, measuring
the diode pad gives a reasonable measurement of the insulation
temperature.
> 2.Would it be more reasonable to measure the temperature in the same
> neighborhood as the pad but make sure that the probe does not touch the
> PCB pad? Would a notified body be of the same opinion?
Because the insulator is a poor thermal conductor, no matter
how close you get your thermocouple (or probe) to the PCB pad,
there will be a thermal drop between the pad and the thermocouple,
and the measured temperature will be less than the temperature at
the pad.
> 3.If you feel that the first method is a valid measurement technique
> then do you know of any ways to work around the problem? There are many
> power devices that can easily and safely exceed a PCB material spec of
> only 110 C.
There are several ways of reducing pad temperature:
1. Use long leads which increases the thermal resistance
from the diode junction to the pad (previous mentioned).
2. Mount the diode away from the PCB to reduce radiated
heat (previous mentioned).
3. Use maximum pad size (i.e., don't worry about keeping
it circular) on BOTH sides of the board to increase radiation
from the pad to the surroundings. This (heat-sinking) is
probably the single most effective way of reducing temperature
at the PCB.
(Of course, this makes it harder to get a good solder joint
because it takes more heat to reach solder temperature!)
4. Change to a Schotky diode (to reduce the forward resistance,
which reduces the I*I*R power dissipation).
(Changing to a higher power diode will not change the power
dissipation and therefore will not change the temperature.
Changing to a physically larger diode will increase the
radiation, thus reducing temperature.)
5. Change to a higher temperature rated PCB (previously
mentioned).
One more point. Often, running a PCB at a temperature slightly
above its rating will result in permanant discoloration. This
shows that a chemical change has occured in the PCB material.
The discolored PCB now no longer has the same electrical or
physical characteristics as a normal colored board. You should
not expect a discolored board to have the same dielectric
properties as a normal colored PCB.
Best regards,
Rich
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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