Introduction
Some users of wave soldering equipment use different solder alloys for different products. In that case it is necessary to exchange the solderpot. To keep the down time as short as possible on can heat up the pot that should come inside the machine while cooling down the pot that was in use. During the exchange operation the solder in both solderpot should however be in a solid state. This must be ascertained to prevent accidents that might happen when during the exchange procedure the pot is suddenly stopped in his movement for whatever reason.
Exchanging the solderpot
A solder pot can be safely moved when all solder in the pot is solid, using all necessary protective aids, such as spectacles, gloves etc. The solder is solid if the temperature is well below the melting temperature of the solder alloy involved. It should be well under the melting temperature for the following reason:
Solder that is solidifying is during cooling for a relative long time on solidification/melting temperature. During that time part of the solder is solidified while a remaining part is still liquid at which the temperature is almost constant. Only when all solder is solidified the temperature will drop further. During the changing of the state of aggregation a lot of energy is transferred at a constant temperature. This is comparable by the changing the state of aggregation from water to ice at 0°C.
The reverse situation is true for heating the solder from a lower temperature to molten solder. To be sure that the solder is still in a solid state the heating should in that case be stopped well below the melting temperature.
Depending on the alloy composition this temperature of changing of the state of aggregation is different.
E.g. For tin-lead alloys this temperature is 183°C.
For SAC alloys this temperature is 217°C.
For Sn100c alloys this temperature is 221°C.
The outside of the solder, like the solder surface, will cool faster so that one can get the idea that all solder is solidified. As long as the temperature is not well below the mentioned temperature for the given alloy, part of the solder can still be molten. Since the temperature is only measured at one position in the solderpot one should for safety reasons at least let the solder cool down for another 10°C before the pot is moved. Moving the pot should be always smoothly since we have to handle a large mass at still relatively high temperature, so take care.
For more information about cooling and heating solder see Soldering in Electronics Second Edition, Chapter 4.2.2 Figure 4.6.