Soldering - how it works
Soldering is a well known and widely used process where two or more metal items are joined together using a fusible alloy with a melting temperature that is lower than their own. The most commonly used solder is a fusible alloy consisting essentially of a tin and lead mixture.
The solder actually dissolves a small amount of the metal’s surface, at a temperature that is well below its melting point and joins with it. It is this solvent action of the solder alloy that causes it to fuse with and attach to the surface of the metal items being joined.
The solvent action that takes place, between the solder and the metal, makes the joint chemical (not just physical) in nature and causes the properties of the joint to differ from the original solder’s properties and from those of the surface of the metal items being joined. When metal parts are joined by solder, a metallic continuity is established as a result of the interfaces where the solder is bonded to the metallic surfaces.
Soldering vs. Welding
The metal joining process that is generally referred to as soldering (or soft soldering) requires temperatures between 183 to 445°C. The joining of metals at temperatures above 445°C (and below the melting point of the metals being joined) is more commonly referred to as brazing (or hard soldering). The actual melting and fusing of the metal items that are being joined together is considered welding. There are, of course overlapping situations that may occur when classifying a process.
The actual joining characteristics that take place are physically different in each of these processes. Soft solders attach to metals by what is referred to as a solvent action that takes place at relatively low temperatures. Hard solders, or brazing alloys contain metals that require higher temperatures to cause the solvent action to take place and fuse the alloy with the metal being joined. Because welding involves actually melting and fusing the surface of the metals that are being joined together, a filler, or fusible material is not always used.
Creating a Quality Solder Joint
Soldering is the process that uses solder (a metal alloy usually consisting of tin mixed with other metals) for the metallurgical joining of metal components to form an electrical, mechanical or hermetically sealed bond at temperatures (less than 449°C) that are well below the melting temperature of the individual components that are being joined. The soldering equipment used to create the required heat and other materials (solder, fluxes, heat sinks, fixtures, etc.) should always be properly matched to the intended soldering application. The equipment and materials used may vary, but the basic soldering techniques that are required will usually remain the same.
One of the most important rules to remember about soldering is "keep it clean". This includes, not only the items being soldered, but also the materials used. Choose quality solders and fluxes without unnecessary impurities. Surface oxidation, contaminants and other impurities are some of the most common reasons for poor quality solder joints. The use of fluxes does not eliminate the need for pre-cleaning the surfaces you are joining, especially if heavy oxidation or large amounts of grease, oil or dirt are present. The stages of soldering are:
Clean: Thoroughly clean all surfaces to be joined, removing any dirt, grease, oil, oxidation, paint, coatings or other impurities that may exist before attempting to solder. Proper wetting can only occur when the intended solder joint area has been properly cleaned. Soldering should be performed as soon as possible after cleaning to eliminate the possibility of re oxidation or contamination of the items being soldered. [So leaving pieces fluxed overnight is not good practice. Flux only the area that can be soldered in the next few minutes.]
Flux: Apply flux sparingly to each of the intended joint surfaces. Flux is primarily used for the removal of light oxidation and to protect against re-oxidation during the actual soldering process. Make sure you have the right flux for the application being performed.
Heat: Apply heat directly to the intended joint area. The correct application of heat is important and should be consistent with the operating requirements determined by the type of equipment being used. Fast and accurate heating will minimize the risk of thermal damage.
Solder: Add solder to the heated surfaces you are joining (do not apply solder directly to the tip, or other heat source being used). The solder should flow uniformly over all of the surfaces that are being connected. Stop feeding solder as soon as you have applied an adequate amount and then remove the heat source. The amount of solder is important because too much will create unnecessary waste, while too little can affect the mechanical strength and conductivity of the finished solder joint.
Cool: Allow the finished solder joint to remain undisturbed until it has completely cooled. You should never attempt to speed up the cooling process by blowing on the solder joint. Even minor vibrations or disturbances during cooling, can cause micro fractures or other types of damage that may severely weaken the solder joint.
Inspect: Check all finished joints for proper wetting, the right amount of solder, a good physical appearance, and the required mechanical strength.
A quality solder joint is not achieved solely by the equipment and techniques being used, but also by the operator being trained to use them properly. An operator should know how the physical appearance of a finished solder joint helps to determine possible flaws that may exist.
A quality solder joint appears bright, shiny and smooth with all components appearing well soldered. The surface of a finished joint should never look rough, grainy, dull, or flaky (these are signs of what is commonly referred to as a cold solder joint). Problems with proper wetting (solder balling up and not adhering to the components surface) are sometimes associated with too much heat, but are more often related to cleanliness issues.
Exhausting Soldering Fumes
Making a fan
Exhausting fumes while soldering is a safety issue of some importance. If you happen to have an outdoor screened-in studio a simple fix can be had with a computer fan You can scavenge such a fan from an older used computer ready for disposal. Simply cut four timbers 50mm square or 25mm x 100mm to fit around it as a box. Attach a long electrical cord to it with an approved plug;. Attach a screen to both sides. Plug in. An additional feature is to attach an activated carbon filter (as used for cooker hoods) to the front of the fan. This removes particles and some fumes.
Always set it to draw fumes away you, generally pointing it so that it is blowing the fumes in the same direction as the larger air flow in the studio. In general a very large fan doesn't always do the job alone, since the fumes always seem to rise up and find your nose. However, with the additional tiny computer fan sitting right next to where you are currently soldering, the fumes just move away.