Marc, Topside temperature is one of the most critical parameters for a successful solder process. Temp stickers are OK to use but won�t tell you the temperature in tight areas. The temp gun like Jay suggested is what I use as a quick indicator. (Raytek MT is the one I have and cost me about $50)
The temp that the flux manufacturer is referencing is a reading taken just before the board makes contact with the solder. So your 110-120 degrees is just before the solder contact and the reading is on the top side.
As Patrick said, the best tool to use is a profiler. It works by attaching thermocouples to your board at multiple areas and then gives you a picture of temp over time for each location. The following is a link that shows the profile output from a profiler:
http://www.ecd.com/emfg/instruments/mole/mole_scan.asp
(or you can get an idea as to delta t by using the 2 readings like Jay does and then doing the math. If you have SMT, get a profiler though.) In our training classes we recommend the use of a profiler regularly and the temp gun for quick checks. And I can�t stress enough how important the temperature is to a successful process; especially for a no clean process!
Good Luck, Vince Whipple, Sono-Tek.
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