My last post talked about the pros and cons of CalRod® style preheaters. This post will be devoted to hot air style preheaters.
It makes sense that this was, most likely, the first form of preheating which would have been used to dry rosin-based fluxes that were popular in the past. Hot air removed the solvents (mainly alcohol), but didn’t heat the actual PWB very well. The heat transfer efficiency between the air and the board was low when open to the surroundings and, thus, preheated poorly.
The outcome is different if you place this same system in an enclosure. The energy is able to heat the board. Correctly implemented, this form of preheating can produce more even heating than most of the direct radiation systems alone. Therefore, it is common to couple the hot air preheater with another heating method.
I will be discussing quartz plate preheaters in the next installment.