Since the introduction of automated PCB assembly and component lead soldering, wave soldering has been the mainstream method of applying solder automatically to the bottom side of PCBs. Wave soldering utilises a stainless steel bath filled with molten solder, a solder heating element and an agitator to produce a stationary wave across the surface of the liquid solder. Considerable expertise is required to maintain the solder in prime condition, at the correct temperature and wave dynamics to achieve the best soldering results. Figure 8-8, shows the wave shape as the PCB passes over it.
In the process, there are four stages; fluxing, preheat, solder wave and cool down. A fifth washing stage may follow if that is the quality assurance policy of the PCB assembler.
The main purpose of the flux is to clean and prepare the component leads, PCB plated through holes (PTH) and pads by removing oxide and compound films that inhibit the solder wetting process. Some fluxes are highly active and corrosive whereas others are very mild or mildly active. Grade “R” has very low activity, and grade “SRA” is very strongly active. Grade RMA is the most common and classed as mildly active. Strong fluxes tend to leave corrosive residues on the component and PCB, requiring costly washing and cleaning operations. Long term, these corrosive residues damage components and introduce unreliability of the product in later life. Flux can be applied in several ways, but spray fluxing is most common. Figure 8-9 illustrates a typical spray fluxing process.
Many systems incorporate an air knife after the flux application to remove excess flux and force flux inside the platted through holes. The preheat stage in wave soldering is very similar to that in surface mount production apart from the arrangement of the heaters, as shown in Figure 8-10. Preheating is achieved by blowing air through perforated heaters onto the underside of the board. The temperature of the PCB and components increases slowly until at the end of the preheat tunnel, the topside of the PCB and components are between 100 and 150ºC. This reduces thermal shock when the PCB and components make contact with the molten solder wave. Thermal shock to the PCB in wave soldering is a serious concern. If the differential between the topside and bottom side is too great, the board will delaminate and warp, causing high mechanical stress on the components when the solder has solidified. The preheat stage also activates the flux, so that all metal parts are clean prior to reaching the hot solder wave.
The temperature profile through the complete process is shown Figure 8-11. The temperature drop as the board exits the preheat stage is caused, on some machines, by the difficulty in placing the wave immediately at the exit of the preheat tunnel. The temperature fall can be 20 - 40°C, which may be significant for large integrated circuits. Attention to thermal profiling or any temperature excursion merits consideration for the component and its long term reliability. As described in Chapter 2, all physical dimensions of a ceramic component will expand or contract around 6 ppm per degree centigrade, whereas an FR4 board material will change by 14 ppm per degree centigrade. Due to these differences, the component terminations will be under tension when the solder solidifies and the completed assembly cools down. Components are usually designed for this but great care must be taken with the use of large components and the choice of PCB laminates, with respect to matching TCE. Such failure modes are associated with termination tear and solder joint fractures. The cool down stage in wave soldering systems maybe based on natural cooling to ambient or controlled.
