Problem
The original problem was to make arrangements to replace an old RF soldering rig, which worked, with a new one that could process many more items much faster. Unfortunately it didn't quite work out. The devices being operated on were dangerous, containing a bit of (pyrotechnic) explosive material. If they were heated too far, or rather if the heat got to the wrong parts, they would go off. There wasn't any problem containing such an event. For these people that was a way of life. The problem was that it upset production schedules.

Can we explain behaviour?
The measurement of internal temperature gave us a means to check a model of the heating process. We had good data on the geometry and construction of the device but no real knowledge of where exactly the heat was going within the device. There were several complete electrical loops in the vicinity of the highest RF fields, made of different materials (copper, steel etc). I made a judgement (the ring of solder was pretty conductive itself) as to where the heat input really took place and continued.
Given a rate of heat input, we could see its dynamic propagation through the device (in the model, that is) and see if the resulting time-temperature response at the 'thermocouple' matched up with the reality. When that made a reasonable match, we could look at what might happen with a different RF source.

Will the new source work?
Our assessment of the new, proposed RF heating devices showed that the electrical field would be a bit different in shape, but we could adapt that a bit. The peak power was lower. On the basis of simply raising the temperature of the devices, we could meet the cycle time objectives. However, our model showed that getting the temperature up high enough for the solder to flow, caused the internal temperature, where it mattered, to get too high. So, back to the drawing board!

Was it a failure?
In the sense that that particular design was defective, yes. But we saved a lot of money that would have been spent had we relied on the simple assessment made by the purchasing department and the equipment vendors.