Hand soldering is the process of manually joining two metals through the use of soldering alloy, a heat source along with flux. This metal joining process has been around for several thousand years and has been adapted for modern electronic components with the tool of choice is a soldering iron. This writing explains several hand soldering fundamentals along with some aspects of PCB rework techniques.
The solders in most common use for the hand soldering of electronics are divided into “lead-free” and “tin-lead” alloy types. Their liquidus temperatures range from the mid 150 °C to 300 °C range.
Reliable solder connections can only be accomplished with cleaned surfaces with the cleaning agent being a flux. Flux materials are made from a variety of materials and are either not cleaned up post soldering (“no-clean” fluxes) or are cleaned with water (“water-soluble” fluxes) or in some cases with other cleaning agents. The fluxes are formulated such that they provide consistent cleaning action during the soldering process while also serving as a wetting agent in order to get the solder to “flow” during the reflow process. Both of these actions lead to strong intermetallic bonds between the component lead and PCB surface to form a firm mechanical bond. These fluxes need to be “activated” at the right temperature and if proper hand soldering techniques are used little to no unspent flux residue is left on the PCB surface post hand soldering.
Soldering irons come in a variety of sizes and shapes to provide the heat necessary in order to get the solder to flow in a liquidus state. The soldering iron tip should be cleaned by wiping it on a wet sponge prior to soldering. The soldering iron is there to control the rate at which the work gets hot, how hot the tip gets, as well as how long it stays hot in order for a reliable solder joint to be formed. The soldering iron tip forms the heat bridge between the two surfaces which are soldered together. Its size should be matched to the pad/lead surface area as well as thermal mass (i.e. the thermal energy required to maintain a soldering temperature). The tip is in contact with both the pad and component with a tinned (i.e. solder on the tip) tip improving the thermal bridge. Solder on the tip of the soldering iron will make sure that there is a rapid transfer of heat into the component-PCB interface.
Before applying solder, the surface temperature of the parts being soldered must be raised to a point above the liquidus temperature of the solder.
Solder applied to a surface which has been cleaned, fluxed and heated to the right temperature based on the solder alloy will melt and flow. It should provide a smooth, even surface, feathering out to the edges of the structures being soldered. For an appropriate solder joint to be formed, the parts being soldered together should be held in place until the solder solidifies.
Depending on the flux being used, cleaning the solder joint area may be required. Cleaning of the solder joint area requiring cleaning will ensure the reliability of the solder interconnection. Post cleaning the PCBs should be dried using a nonparticulate-generating cloth or air source.
One of the common soldering joint anomalies is a disturbed solder joint. A disturbed joint has a rough, irregular appearance and looks dull instead of bright and shiny. It is the result of there be movement of the elements being soldered together during the time the solder is on the liquidus state. A disturbed solder joint may be unreliable and may require “touching up” or rework. A disturbed solder joint will usually require only reheating and reflowing of the solder with the addition of the same flux type used during the original assembly process. If this “touch up” does not correct the condition, the solder should be removed and the joints re-soldered.
It always best to not touch up any solder joints. However, when re-soldering is required, quality standards for the re-soldered connection should be the same as for the original connection. IPC standards for inspection criteria for such solder joints based on the class of the assembly are well documented.
If parts need to swap out for new this is considered PCB rework. The standard methods for reworking different component styles are outlined in IPC 7711 “Rework of Electronic Assemblies”.