In PCB assembly process the PCB components are placed and soldered over the exact positions of components matching pins of component and footprint. Generally, this is performed manually but due to advanced techniques there are automatic pick and place machines available that reduces the human effort and performs all manual steps automatically. The through hole or large components are assembled manually whereas the small components like SMT are assembled by machines. However, sometimes the automated pick and place machines are not feasible all the time specifically for small production volumes.
For through hole components after inserting and fixing the components over the footprint position the PCBs are forwarded for soldering. For larger PCBs with more than 100 components a team of workers participates for components placement or fixing. Each worker places his designated components within allocated time over the PCB and forwards it to the next worker. For PCB movement motorized conveyer belt is installed. Finally, after a little inspection the PCB with components is dipped into a hot molten solder reservoir for soldering. This method, called wave soldering, is preferred for large scale soldering of PCBs. After soldering once more the inspection is carried out for unsoldered components and wrong placed components and then proceeds for power up. At the last and final stage a PCB becomes fault free and qualifies to be assembled in the mechanical jig. In figure 1, PCB components fixing line is shown.
Figure 1: PCB components fixing and assembly line
In figure 2, wave soldering method for through hole components is depicted.
Figure 2: Wave soldering method for through hole components soldering
In SMT/ SMD assembly, at first step, a small quantity of paste is applied by using stencil. Stencil is a metallic / plastic sheet with holes on the components soldering pads. The paste which is applied prior to the components placement holds them during PCB movement. After placement the PCB is soldered by reflow method. The mixed sort of components PCBs, i.e. DIP and SMD, are wave-soldered and reflow soldered. The hand soldering method is extensively applied in rework, troubleshooting and small scale productions but that is not suitable for large scale PCB productions. In figure 3, SMT components pick and place machine is shown. The complex components such as BGA and QFN packages are soldered and inspected with specialized machines and are even more costly.
Figure 3: SMT/ SMD auto pick and place machine