Question: For reflow profiling, where's the best place for me to attach my thermocouples?
Phil Zarrow: To get a real good indication of what your delta-T at peak temperature during profiling is, you need to attach at least one thermocouple to what you consider to be one of the hottest spots, joints, and one at one of the coldest joints. Typically, your hottest joint is going be a small-mass component, like a passive component resistor, chip capacitor, something typically near the edge of the board.
Your colder joint is going be a large component, typically an IC and typically located toward the center of the board.
Question: What about if I have a BGA?
Phil Zarrow: BGAs are a little bit more challenging. To really do the proper methodology, best practice is to drill a hole underneath one of the locations of the balls of the BGA, run your thermocouple up there, attach it with thermally-conductive epoxy, fill in the hole with a non-conductive epoxy, and now you've done your due diligence and you'll get a good indication of that joint temperature.