Wave soldering preheating

For a lead-free wave soldering process, a three zone (length about 1800 mm) preheat region is sufficient to heat the boards and remove flux solvents. A typical wave solder preheat configuration for lead-free solders would consist of calrods for the first zone, followed by forced convection in zones two and three. 

Most commercial wave soldering machines have a preheating section before the PCB or assembly is immersed into the solder wave, the thermal shock is reduced As the ambient temperature has already been raised before the solder wave, the thermal gradient in the reed switch seal is reduced. 

The most commonly used process for the soldering of through-hole and mixed (through-hole and surface-mount) circuit boards is wave soldering. The wave-soldering process is shown schematically in Fig. 40.7. The populated (or stuffed ) circuit board is secured to a conveyor belt. The conveyor belt carries the board through the fluxer, then the preheat stage, and lastly on to the molten solder wave. 

Wave solder pot temp. 250°C-> 260°C 260°C 270°C (with higher preheats and longer soldering dwells)... 

One attribute of this process is the speed at which solder joints are formed. Much quicker than oven reflow soldering, the wave-solder process allows little time for preheating, fluxing, and solder-joint formation, which also explains the variabiUty of this process. 

There are five basic subsystems of a wave soldering machine, including conveyors, fluxers, preheaters, solder pots with pumps and heaters, and ventilation. If alcohol-based fluxes are being used, it is wise to install a fire suppression system also. 

Flux quantity may also be of concern in the wave-soldering process for yet another reason fire hazard. Flux-laden boards are preheated going into the wave. If the flux application is too heavy, the flux may drip onto preheater elements. This may cause the flux to volatilize rapidly, combine with oxygen in the atmosphere, and provide the right conditions for flame initiation. Even if there is not direct exposure of the liquid flux to preheaters, if the quantity of volatile, flammable components is high enough to be an ignition source, then an explosive condition may develop. With the advent of more eco-friendly, water-based fluxes, fire hazard is less of a concern. 

Preheating the PWB and components serves three purposes in the wave-soldering process ... 

PTH components are generally repaired by solder fountain, which is akin to a wave-soldering system with a vertically pumped jet of solder. It is significantly smaller than a wave-soldering machine and generally lacks a conveyor, a fluxer, and preheaters. Since the equipment is much smaller, often benchtop-mounted, it permits an operator to manually remove parts and replace them. The classic solder fountain is a manually controlled system. Nozzles are chosen to shape the molten solder jet to match closely the size of the area to be soldered. Nonetheless, a shield or pallet is often used to localize the soldering and to prevent contact of the molten solder with previously soldered components on the board. 

Decreasing or eliminating thermal shocks by preheating the board before hot-air leveling, wave soldering, rework with a solder pot, etc. 

The preheat temperature for lead-free wave soldering is not very different than that used with tin/lead soldering. The typical top-side temperature for lead-free wave soldering ranges from 180° to 225°C, depending on the flux in use. 

FIG. 11 Comparison of wave soldering profile for eutectic Sn-Pb and lead-free solders. The main differences are increased preheat and soldering temperatures. (From Ref 53.)... 

VOC-free, water-based spray and foam fluxes for wave soldering and VOC-free solder pastes for reflow soldering have been developed. The high latent heat of vaporization of water puts high demands on the preheating section of wave soldering machines. 

A new category of low solids fluxes was introduced in the early 1990s to meet the needs of localities where volatile organic compounds (VOCs) are regulated. These fluxes are marketed as VOC-free or low-VOC fluxes. The solvent in this case is 100 percent water or at least greater than 50 percent water. Use of these fluxes requires special care in the preheat step where the water (solvent) must evaporate before the assembly reaches the solder wave. Failure to do this will result in excessive solder ball formation. 

The circuit board is placed on a motorized, edge-hold conveyor where it is fluxed, then preheated both to activate the flux and give the PWB a thermal boost.The board is next skimmed over the crest of the molten solder wave. Only the bottom of the circuit board is exposed to the molten solder (see Fig. 47.22). 

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