The Electronics hobbyist may choose to do the PCB assembly on his own, in which case he/she should be well versed in the art of soldering. Since the electronic components have been shrinking in size over the ages, soldering is indeed an art in itself and can only be mastered through training and experience. The minimum equipment required for PCB assembly are as follows:
A soldering iron with temperature control is preferred because there are many components which would be destroyed if soldered with too much heat. Flux is a solution that is used to improve the wetting characteristics of the liquid solder and it also prevents and removes oxidation. Nowadays, flux cored solder wire is available which negates the requirement to use flux separately. Isopropyl Alcohol or IP for short is used to clean the residue of flux after the soldering is complete. Another option is to use no-clean flux, which negates the requirement for using an IP solution.
Before beginning the soldering process, the soldering iron tip is rubbed on the piece of sponge soaked in water to remove the remnants of solder sticking on to the iron tip from the previous soldering. The soldering iron tip is covered with a uniform layer of solder, by melting the solder wire and running the wire all along the surface of the tip of the soldering iron, so that there is a better transfer of heat from the iron to the tip and to the solder used to solder the component on board. This process is called tinning the iron tip.
The legend silkscreen printed on board is meant to serve as a guide to help the PCB assembly process by enabling identification of the component on board, so that the right component is assembled at the right location. Since majority of the PCB assembly today takes place through an automated process, many PCB designers avoid printing a legend on the surface of the PCB especially when it is a densely packed PCB. They keep a separate layer with the legend marking alone, which can be printed out and used as reference if required.
While manually assembling the components on the board, it recommended to assemble the components in the order of increasing sizes. If the taller components are assembled first, it may become very difficult to assemble the smaller components, which may be even be placed between two tall components, like an electrolytic capacitor for example.
Through hole components are pressed in to the holes and the leads are bent to hold the component in place. While placing SMD components a tool, like a tweezer would have be used to hold the component in place while soldering. This procedure has become very critical in the soldering process, since the fine pitch components like QFP, QFN etc, have to be placed very precisely with perfect alignment of pins and the pads and the soldering is to be done. Even if there is a slight shift in the alignment, it would result in a failure, since inadvertently, it would cause a short between adjacent pads. Unless the board contains very few components, the components are to be placed one by one on the board and then soldered. It is not possible to place all the components on the board and then solder it in the manual process. It is only in the automated process that all the components are placed on the board first. The board is exposed to a layer of glue before component placement, so that the component stay in place.
Once the component is place at the corresponding location on board, place the solder wire on to the junction between the component pad and the lead. Place the tip of the iron at the end of the solder wire, just long enough to melt the solder and create a bond between the pad and the lead. The soldering iron can be removed when a bond in the form of solid concave fillet has been formed. If sufficient flux has been used, the fillet will be formed with a smooth finish. If not more flux is to be applied and the soldering iron tip is applied again to get a smooth finish. A dull irregular finish is indicative of cold solder and should be reworked to get a smooth shiny finish. A cold solder if left as it is, can in the future result in oxidation and ultimate failure of the solder joint.
Once all the components are soldered in this manner, it is time to clean the board of all the flux residue. The board can either be placed in an IP bath, or IP can be applied to the board and small brush used to clean the surface of the PCB.
If the BOM contains components with packages like BGA and QFN, the it is better to leave the component assembly job to the experts. It is almost impossible to solder packages like BGA, manually. There are many professional component assemblers available, who uses automated methods for placing and soldering the components, in which case there is a very little chance of the solder joint to go bad. Professional PCB assemblers charge you at the rate of number of soldering points.
Over the ages a manual visual inspection technique has been used for inspection of the assembly for quality control purposes. Since the board densities have been increasing and component sizes have been decreasing, it is critical for the assemblers to have a fast, accurate and efficient inspection process. A growing number of assemblers have integrated Automated Optical Inspection (AOI) and X-Ray inspection into their inspection processes. In comparison to manual inspection, these two inspection technologies offer distinct advantages. The production yields have been known to increase when highly accurate inspection processes such as AOI and X-ray inspection is used.