Introduction
In wave soldering the preheating is necessary to evaporate the solvent from the flux and to prepare the flux for its function during the soldering operation.
For the evaporating of the solvent the board temperature during the preheating phase must be over the evaporation (boiling) temperature of the solvent. For most fluxes it will be sufficient to preheat just over this temperature.
There are fluxes however that needs a defined temperature that is well over the boiling temperature of the solvent. This higher temperature is often necessary to prepare the flux activator for its job.
In some cases preheating is also used to reduce the thermal shock on SMD components.
For multilayer boards a high preheating temperature is sometimes used to promote a better solder penetration to the topside of the board in case the board lacks a good thermal solderability.
Preheating can be established with different systems, like forced air convection, a combination of middle wave IR radiation with natural convection and a system with short wave IR lamps.
Preheating systems
The forced air convection circulation system has the benefit that the maximum preheat temperature on the PCB is limited by the temperature setting of the system. The capacity of this unit is sufficient to cope with the maximum machine load during production. Due to its large thermal mass it may take several minutes, depending on the temperature difference that must be bridged, before a change in the temperature setting will result in the newly set system temperature.
The forced air promotes a quick evaporation of the flux solvent vapors that will be emitted during the preheating stage. On relatively wet flux layers however the use of this unit is not advised due to the fact that the airflow from the heater might displace the flux from its position.
For relatively thick wet flux layers a preheating station with middle wave IR radiation with natural convection is advised as first preheater unit. This preheating system will evaporate the first part of the solvent, making the flux less fluid and more viscous without moving the flux. Next the heating system with forced convection can be used as a second station for its best energy transfer regarding the flux solvent evaporation.
The middle wave IR preheater with natural convection emits relatively more heat into the PCB material than the forced convection preheater. The main energy source is the radiation coming from the heating elements. As a result of the high temperature of these elements the air around these elements will be heated and will cause a natural convective flow that will assist in flux vapor evaporation. The correct setting of this preheater must be determined by measuring on the PCB to be soldered.
A simple practical method that can be used from the old days where there was lack of measuring systems is to touch shortly the underside of the PCB with a finger after preheating. If the flux is not wet or feels tacky, the preheater setting is good for most applications.
If a mix of boards have to be soldered, where different boards might need different preheating temperatures or preheater settings, on needs a preheater that can act quickly on these different demands. Here the use of a short wave IR system is recommended. Due to its fast reaction time and immediate radiation in accordance with its settings, this system can be used to boost up the preheat temperature of the board. Due to the short wave radiation most of the energy will be emitted in the board material and not so much in the 'transparent' flux layer.
As the effect of preheating is also related to the type of board and the conveyor speed, it is recommended to measure the effect of the different settings on the PCB itself.
Finally the soldering quality might give a good indication on how the machine settings will affect the total result.
If the flux is not predried well it might be washed partly from the board surface as the board enters the wave. Flux can adhere to board material, but can not adhere to a vapor layer that will be formed if a wet flux enters the solder wave. Good preheating may assist in solder penetration in joint gaps. It can however never compensate a poor thermal joint design.
Too much preheating might have a detrimental effect, due to the fact that the flux activity might be exposed too early.
One should also take into consideration that the component temperature during the soldering operation should in general be kept as low as possible. Some components can only withstand a limited temperature for a limited time. The component data sheets can be used to get the necessary information regarding the allowable soldering conditions for a specific component.