There are still components that are not available in surface mount configuration (for example, some connectors and capacitors). Therefore, some assemblies have through-hole and surface mount components mounted on the same PCB. Figure 8-13 shows a typical mixed technology board. The surface mount component is glued to the PCB with heat curable adhesive. Generally, the conventional components are inserted first and then the surface mount component adhesive applied before the board is passed through the pick and place line. The adhesive holds the component until cured.
No solder paste is used in this type of assembly. The surface mount components pass through the solder wave with the leaded-component leads, all surface mount components and protruding leaded component leads being soldered simultaneously (Figure 8-14).
After components have been inserted and glued to the PCB, the assembly passes through spray fluxing and preheat stages prior to entering the solder wave. Where components are mounted on the bottom side of the PCB, a dual-wave system is utilised to avoid unsoldered joints due to air pockets. The dual-wave system has a short wave, which circulates more violently than the main wave, dispelling air and trapped gases from the connections. The short wave is approximately 10 mm wide and the second main wave 30 to 40 mm wide. The component specification will identify maximum soldering temperature and duration for one pass only, as apposed to two passes in a reflow process. This requirement is often violated by PCB manufacturers, when a second pass is required, as in the case for a rework situation. Ceramic chip resistors are sensitive to additional solder passes, because the internal silver-palladium inner terminations will evaporate, causing partial or total open circuit.
The dynamics of wave soldering are important. For example, to achieve the best solder joint and minimise solder shorts between component leads and PCB tracks, the relative speed between the wave and the component as it leaves the wave must be zero.
Also, the board is inclined at about 10º to reduce the formation of solder icicles by using gravity to overcome the surface tension of the solder web or peel back point. Wave soldering of mixed technology boards is not ideal, particularly if there is a component mix on the board as shown in Figure 8-15. Surface mount and through-hole components are located topside and surface mount on the bottom side.
With this arrangement, the topside surface mount components are solder pasted and the plated through holes for the leaded components are masked off to avoid solder blockage. Surface mount components are then placed on the solder paste and the PCB passed through the reflow process. The sequence to mount the other components varies depending on circumstances but generally the board is inverted, adhesive applied to the surface mount positions and components placed and adhesive cured. Through-hole components are then inserted in the topside through holes, and the bottom side of the board passed through the wave soldering process, as described previously. With very complex mixtures of components, or during rework, the PCB may be passed through the solder wave for a second time, which may compromise many components and long-term unreliability.
