Hi Tom,
You'll probably get quite a few responses to this one but this is what
I've done for quite a few years:
1. Review the PCB layout, layer by layer, and determine the area(s) of
the board that are large thermal masses.  You'll want to put
thermocouples at these potential hot spots.  If these "hot spots are
located near the edge of the PCB, I utilize this edge as the trailing
edge (it's less likely for this thermal mas to starve the adjacent areas
when on the trailing edge vs the leading edge).  NOTE:  The ideal would
be to review the board layout prior to fabrication and provide feedback
to the designer regarding conductor balance for local and global heat
transfer control.
2. Place thermocouples on the leading and trailing edges of the PCB.
3. Place thermocouples at the largest component and the smallest
component and under components with underside terminations (i.e. BGA).
 NOTE:  Have you run out of thermocouples yet?  Pick and choose these
locations based on your past experiences.

I have created a lot of profiles with just the PCB and others with the
smallest and largest components.  To populate or not to populate depends
on the board layout, construction, component mix, and your oven
(IR/Convection or Convection).  Start profiling boards with and without
components and judge for yourself, there is not an exact recipe on how to
develop a profile for every board.  Experience through trial and error
will provide a much better approach than any text book (how do you think
the basis for the text books were attained).

Good luck.

Steve Sauer

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