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Preheating - An Introduction: Wave Soldering (A Segmented Synopsis) Post #9

Many of my posts have been discussing the different types of wave soldering fluxers: their capabilities and their shortcomings. This section will be similar, except preheaters will be discussed. But first, a quick introduction.

The definition of preheating is the practice of applying heat to a PCB after flux deposition and immediately before soldering. Wave soldering can be performed without preheating, but the soldering speed will be slow. The age-old saying that “time is money” emphasizes the need for preheating. In The Handbook of Machine Soldering SMT and TH, Woodgate mentions, “During some experimental work with preheating, a board was soldered at 12 ft. per minute with the normal preheat applied. Without preheat it could not be soldered faster than 2.5 ft. per minute.” Just because a joint can be formed without this process doesn’t mean that it should.

The reasons given for preheating are almost always:

  • To “activate” the flux by heating it
  • To prevent solder balls by drying the flux solvent that would otherwise spatter if only contacting the solder wave
  • To reduce thermal shock to the PCB from the solder wave, thus decreasing warping and other defects associated with an abrupt increase in temperature

The most important reason to use preheat is to “heat the board, components, terminations, and other parts of the joint so that the final heating and soldering can proceed more quickly.” By minimizing the exposure to the molten solder, damage to the components can be minimized as well. 

*Stay tuned for the next installment: Preheating Part I - Types of Preheaters. This begins the second mini-series, which is part of the larger program entitled Wave Soldering (A Segmented Synopsis).