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Soldering Basics

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Soldering is a basic skill that is required for many electronics projects, troubleshooting, and household repairs. Soldering takes some practice to master, especially soldering more complex surface mount components. No matter what you are soldering, the basic idea remains the same - create an electrical connection by melting a low melting temperature alloy (solder) to connect two conductors with a soldering iron.

Soldering Irons

Soldering beings with the soldering iron, the most basic soldering tool. Soldering irons are available with a wide range of specifications and features from analog and digitally temperature control, different wattage ratings, and changeable tips. The most basic soldering irons plug directly in to the wall and are not suitable for most electronic soldering due to the lack of temperature control and no ESD protection. Temperature controlled soldering stations that are ESD safe are available starting as low as $40 and are a great tool for the more than occasional soldering. More expensive hot air soldering rework stations are available starting around $100 and often include a temperature and ESD safe soldering iron as well as a hot air rework wand that puts out air hot enough to melt solder.

The wattage rating of soldering stations or soldering irons is typically not very critical, as the wattage does not directly refer to the temperature of the soldering iron, but to how quickly it can heat up. Having more wattage available, lets the soldering iron heat up a work piece quicker and raise the temperature of components to the melting point of the solder quicker, which is very useful when working on larger components or soldering to the chassis of equipment.

Soldering stations and higher quality soldering irons will have replaceable tips. Tips come in a range of sizes and shapes to fit the work being performed. Larger tips store more heat and can transfer heat quicker through their larger contact area. Fine point tips are available as well, but should be used only when access prevents a larger tip from being used.

Solder

Soldering requires one of several low melting temperature metal alloys, collectively called solders. Solders are generally available in two types, either with a flux in the core of the solder or solid solder. Flux is a wetting agent that removes contaminants and oxides from the surface of the materials that will be soldered that allows the solder to bond and wet the surface of the metal well. The most common solder alloys still use a combination of lead and tin, typically ranging from 40-60% lead with the balance being tin. The most common electronics solder are 60/40 and 63/37 tin to lead. Lead free solders and solder with silver are also available and may be required depending on the application and materials being soldered. Smaller diameter solder, around 1mm in diameter, is best for most applications. The small diameter takes longer to solder larger joints, but helps to prevent solder bridges and undesired connections.

Flux

Getting solder to fully wet a surface and make a good physical and electrical bond often requires the application of flux. Flux is a key element in soldering that has the effect of helping the solder flow quickly to any metal joint and create a good bond. Flux does this by using a gentle acid to remove the oxide (rust) from the metal surfaces and helping to flow away any surface contaminants. Depending on the flux used, the solder joint may need to be cleaned after soldering to remove any remaining flux and acid. If the flux residue is left on the solder joint, the acid will weaken the solder joint and may end up destroying the joint and breaking the connection. Each type of flux requires a different cleaning method. Some fluxes can be cleaned with water while some require alcohol. Be sure to check the label of the flux you are using and clean the surface appropriately. Alternatively, cleaning can be avoided by using a no-clean flux that does not require cleaning. The solder mask on a PCB combined with flux helps solder to flow only to the pads and contacts between the PCB and the components, making surface mount soldering with flux much easier. Accidentally removing the solder mask or using too much solder will still cause solder bridges.

Technique

Soldering begins with preparing the surfaces to be soldered and turning the temperature of the soldering station or soldering iron is set to an appropriate level depending on the work piece and solder being used.

  • Make sure the surfaced are clean and free of debris. Once the surfaces are clean, the two surfaces to be soldered should be positioned together.
  • It is recommended that the components be secured to prevent movement during soldering, either through bending leads, twisting wires together, using clamps, or manually holding components with tweezers.
  • Apply a small amount of solder to the tip of the soldering iron to wet the surface of the soldering iron. Place the tinned soldering iron tip on the joint to be soldered to warm the joint.
  • Apply solder between the soldering iron and the joint to be soldered. Do not apply the solder directly to the tip of the soldering iron, but rather on the joint to be soldered. This allows the flux in the solder to flow where it is needed to create a good solder connection.
  • Once the solder has completely wetted the surface to be joined, stop applying solder and remove the soldering iron.
  • The joint should not be moved for several seconds to prevent a cold solder joint, which can be identified by dull and grainy appearance of the solder joint. If you have a cold solder joint, simply reheat the joint and apply a small about of solder or flux and allow the joint to cool without being disturbed.
  • Finally, clean the solder joint to remove the flux residue as required.
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