120 C is well over the boiling point of water (Keep that in mind for mental reference.)
Your electronics start out within some sort of enclosure (metal? Non-conducting Plastic? Thermally-conductive plastic? Wood?) at room temperature. That assembly has some mass - which you have not told us. It has some shape - WHich you have not described.
The "container" is then surrounded completely by the molten silicon (whose mass and external shape you have not defined!), which then MUST cool down to room temperature, right?
You now know (or can calculate) the thermal mass (inertia) of this undefined silicon-enclosed assembly. First, find the missing data. Determine "exactly" what is the maximum temperature your electronics inside the glob can tolerate, and for how long they can tolerate that temperature. Set a margin below that temperature.
The maximum temperature on the interior of the "glob" can be controlled by cooling the outside of the glob. To reduce the interior temperature with least effort and contamination, blow cool air on the outside of the silicon as soon as possible after the silicon sets. Theoretically, you could even dunk it in water, but that's too difficult to dry, is messy, allows spills and probably will kill the electronics.
Run several test cases before committing to the process in the production line. Trust the test results, ANY transient heat exchange calculation of the interior of the glob will lucky to be within 5-10%.
