A tube of TW solder electronics solder used for manual soldering.
All solder pads and device terminals must be clean for good wetting and heat transfer. The soldering iron or gun must be clean and pre tinned with solder, otherwise components may heat up excessively due to poor heat transfer. The devices must then be mounted on the circuit board properly.
One technique is to elevate the components from the board surface (a few millimetres) to prevent heating of the circuit board during circuit operation. After device insertion, the excess leads can be cut leaving only a length equal to the radius of the pad. You may use plastic mounting clips or holders for large devices to reduce mounting stresses.
Heat sink the leads of sensitive devices to prevent heat damage. Apply soldering iron or gun to both terminal lead and copper pad to equally heat both. Apply solder to both lead and pad but never directly to the tip of soldering iron or gun. Direct contact will cause the molten solder to flow over the gun and not over the joint. The moment the solder melts and begins to flow, remove the solder supply immediately. Do not remove the iron yet.
The remaining solder will then flow over the junction of the lead and pad, assuming both are free of dirt. Let the iron heat the junction until the solder flows and then remove the iron tip. This will ensure a good solid junction. Remove the iron from the junction and let the junction cool. Solder flux will remain and should be removed.
Be sure not to move the joint while it is cooling. Doing so will result in a fractured joint. Do not blow air onto the joint while it is cooling; Instead, let it cool naturally, which will occur fairly rapidly. A good solder joint is smooth and shiny. The lead outline should be clearly visible. Clean the soldering iron tip before you begin on a new joint. It is absolutely important that the iron tip be free of residual flux. Excess solder should be removed from the tip. This solder on the tip is known as keeping the tip tinned. It aids in heat transfer to the joint.
After finishing all of the joints, remove excess flux residue from the board using alcohol, acetone, or other organic solvents. Individual joints can be cleaned mechanically. The flux film fractures easily with a small pick and can be blown away with canned air. In solder formulations with water-soluble fluxes, sometimes pressurized carbon dioxide or distilled water are used to remove flux. Traditional solder for electronic joints is a 60/40 Tin/Lead mixture with a rosinbased flux that requires solvents to clean the boards of flux.
Environmental legislation in many countries, and the whole of the European Community area, have led to a change in formulation. Water soluble non-rosin based fluxes have been increasingly used since the 1980's so that soldered boards can be cleaned with water or water based cleaners. This eliminates hazardous solvents from the production environment, and effluent.
