I realize that soldering at such a low temperature isn't the norm. However, I'd think you wouldn't have to crank it up so much higher than the JBC? It certainly has enough power to deliver. If the tip remains under load, why doesn't it continue to deliver power until it's up to temp?
As heat transfers from the tip to the work, the tip cools, and the sensor then detects the cooling, and the MCU then sends more power to the heater.
But, if there is no solder bridge because the temperature of the work is below reflow, heat can't move out of the tip, so the sensor stays hot, and power doesn't increase. It's the same reason that you can't solder with a dry tip and need to touch the solder to the tip before feeding it onto a pad.
The tip geometry, composition, and plating determine the thermal resistance, R
T in °C/W: the temperature difference required per watt of heat transfer. On a workpiece with a heavy ground, more heat transfer is needed to keep the solder molten, so the tip must be hotter.
One of Pace's comments indicates that their tips are higher R
T compared to JBC, because they have heavier plating.
In other words, it's a false belief that you can keep a low tip temperature on any kind of work, just because you have the ability to supply high power. The R
T means that the minimum usable tip temperature is
always higher for heavier workpieces.