Be aware that IR thermometers don't work well on aluminum, at least not without adjusting. I'd put an empty PCB on the hotplate, and measure that instead.
A PCB with lots of little vias would be more interesting. FR4 is not a great heat conductor, so vias play an important role in heat transfer from the hotplate up to the balls. I even read in some design guide a recommendation to place the dogbone pattern on unused pins to increase the heat mass of the upper level for even heating. It might even be a good idea to put vias on unused pins to move more heat up.
On a different note, I believe that solder balls are 63% tin and 37% xxxsolderxxx lead. According to wikipedia
https://en.wikipedia.org/wiki/Soldering#Solders, this mix is
eutectic, meaning that it has a melting point of 183C (not a range) - it transitions straight to liquid from solid.
There is a slight heat gradient between the heating elements (my heaters are 80mm apart). This will require setting the temperature a little higher. If there is a next time, I will probably put more heating elements spaced closer together. The current set up is workable, but I have to figure out the temperatures - right now I am just guessing.
In my later, successful experiments, I jammed the plate temperature way up, to 250C. This melts the balls in around 15-20 seconds (I will time it next time). The board then has to be removed quickly (and carefully) to avoid smoking the FR4.
I would like to lower the temperature a little. I know that at around 200 it seemed to take forever to heat it. I had no luck until I set it to 220 or so.
Looking at some profiles on the Internet... I see wave soldering going up to 260C, IR heating to 230C, etc... Perhaps I am not that far off.
Given the complete lack of calibration of any kind, this is not very helpful. I should spring for a thermometer, or at least verify with another thermocouple setup (I am thinking about a PID-controlled toaster oven next).
Oh, I still have to drill the TC hole...
