Hi Iain,
The responses so far do not answer your question--they address only measuring
the profile of the PWB, which is always following the highest temperature
profile. However, as your questions indicate, you recognize the fact that the
temperature profiles at the solder joints, particularly of larger components,
run lower profiles.
During actual reflow soldering, a solder joint for a larger leaded component
forms as follows: paste liquifies on pad (it is hotter than lead which is
heat-sinked by component) and wets the pad-the liquid solder adds to the heat
transfer to the lead-the lead gets hot enough to affect wetting-the surface
tension of the liquid solder results in the final SJ geometry. Using already
soldered test assemblies does not catch this effect unless a component lead
is left unsoldered and the T/C is directly attached to this unsoldered lead.
The hotter gas temperature in the oven is not a factor IF insulated (glass
weave) and thin enough (5 mils or less) T/Cs are used.
>1)  T/c tip bonded to the lead/pad with something like a 2 pack epoxy quick
>drying, covering the tip.
>- -Easy to apply and as the bonding agent isn't very thermally conductive
perhaps >less likely to be influenced by the higher air temperature in the
chamber.
The problem with this method is that you do not know exactly what temperature
you are measuring.
>2)  T/c tip touching lead/pad bedded onto a thermal compound paste with it
>covering it.
>- -Easy to apply but perhaps influenced more with the air temperature
although >being sunk by the board.
The board is not a sink, but a heat source, since it heats up faster than
components. Also it is not quite clear what temperature you will be measuring.
>3)  T/c tip soldered to lead/pad.
>- -Difficult to achieve as the tip isn't very receptive to SnPb solder.
>4)  T/c embedded through board directly adjacent to lead/pad with compound
>covering tip.
>- -Time consuming, destructive but perhaps less influenced than the others
with >air temperature.
You should have a profiling assembly that will never be shipped to the
customer.
Werner Engelmaier

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