As I mentioned in my previous post, the goal of preheating, regardless of method used, is to increase the metallic structure of the joint to a specified temperature. It would be ideal if the board laminate and the component remain at ambient temperature; however, this would require the metal to absorb the heat energy while the insulating material reflected it. Unfortunately, this is rarely the case because most of the joints formed are shiny/reflective metal surfaces. To ensure adequate heating, heat is transferred from the body of the component or the base laminate rather than direct heating of the metals to be joined.
Lamp-style preheaters generally exhibit the worst case scenario due to their short wavelength emissions. Their epoxy glass surface can burn, but there is not enough time for the copper or solder-coated metals to reach the same temperature. However, implementing the correct parameters can correct this issue.
As the wave length increases, there is more time for the insulating base and the metallic circuitry to reach an even temperature, but the ability to focus energy and achieve a fast response time are lost. The ability for any surface to absorb or radiate the energy of a particular wavelength depends on factors such as color, texture, and the nature of the material. The main point is that a material cannot be adequately judged by a visual metric.