Soldering is the joining of two metal surfaces mechanically and electrically, with the use of metal called solder (pronounced “sodder”). Solder secures the connection so it doesn’t break loose from vibration, other mechanical forces and provides electrical continuity, so the electronic signal travels through the connection without interruption. The solder is melted using a soldering iron. Flux is used to clean and prep the surfaces, which allows the melted solder to flow (or “wet”) and bond with the metal surfaces.
Hand soldering is the process of soldering one connection (called “solder joint”) at a time, compared to more automated soldering processes in wave soldering or reflow oven equipment.
What do I need to solder electronics?
When soldering an electronic connector to a contact point (often called a “pad”), you generally need the following:
- A soldering iron capable of reaching the melting point of the solder
- Wire solder, with or without a flux core
- Flux if wire solder does not include a flux core or if the additional flux is needed
What is a soldering iron?
A soldering iron is a hand tool used to solder two metal surfaces together. At its simplest form, it consists of a metal tip, a heating element that brings the tip up to soldering temperature, an insulated handle to allow safe holding of the soldering iron, and a plug for either a wall outlet or the soldering station.
The soldering tip’s job is to transfer heat from the heating element to the work. It has a copper interior, with is an effective and efficient thermal conductor, iron plating to protect the soft, corrosive-prone copper from flux and solder, and chrome-nickel plating to keep the flux from wetting up the tip.
Beyond that, there are options that provide better control over the soldering iron temperature and heat response (time it takes to heat up again after soldering). Some soldering tips are metal slugs that rest against the heating element, and others are integrated with the heating element in a cartridge.
What is the difference between a soldering iron and a soldering station?
On the low end, most appropriate for hobbyists, a soldering iron may plug directly into the electrical wall outlet, which provides no control over soldering iron temperature. Just on or off. With a soldering station, the soldering iron plugs into the station for greater control over temperature, and other features like set-temperature memory, lock-out, etc.
What type of solder should I use?
While there is a large variety of different types of solder, at the most basic, you need to choose between lead or lead-free, the diameter of the wire, flux core or solid wire, and the type of flux.
Lead or lead-free
Solder is generally a combination of metals, which are chosen for the best reliability and conductivity. Lead, often combined with tin, has been the mainstay of electronic soldering since there has been electronic soldering. Lead has a relatively low melting point and readily wets and flows, which makes the process faster, easier, and more fool-proof. Because of environmental and health concerns around lead, there has been pressure to move to lead-free solder, which is often a combination of tin and silver. Lead-free solders have a higher melting point and generally require more active or more concentrated fluxes (higher solids content) to achieve the same soldering performance as leaded solders. For typical manual soldering, if done properly, reliability should be about the same. For high-end electronics used in extreme environments (e.g. aerospace electronics), there are concerns with the tendency of bright tin to crystallize and form tin whiskers, thin wires of tin that can grow from the solder joints.
If you are repairing or assembling electronics for use in the US, lead solder is the easiest to work with and provides the most reliable solder joints. The lower heat also generates less thermal stress on the rest of the PCB. If the end product is being shipped outside of the US, especially to Europe, you should consider lead-free solder. An exception might be for high-reliability electronics like used in aerospace. In that case, check with the specifications and requirements of the end-user of the electronics. Lead-free may still be required, or there may be exemptions in place that allow for the use of lead solder.
Should I use lead or lead-free solder?
If you are repairing or assembling electronics for use in the US, lead solder is the easiest to work with and provides the most reliable solder joints. The lower heat also generates less thermal stress on the rest of the PCB. If the end product is being shipped outside of the US, especially to Europe, you should consider lead-free solder. An exception might be for high-reliability electronics like used in aerospace. In that case, check with the specifications and requirements of the end-user of the electronics. Lead-free may still be required, or there may be exemptions in place that allow for the use of lead solder.
Diameter of solder wire
Make sure you don’t confuse solder wire intended for plumbing with electronics solder wire. Wire for plumbing will be much thicker, 2mm diameter or higher. Electronics solder wire will be thinner, 1.5mm or smaller, down to 1/2mm or less. Match the diameter to the size of the connectors and contacts you are soldering. Too small and you will be going through too much solder, and too large could be hard to maneuver around a dense PCB, and could increase the chance of thermally stressing or even soldering other components that are unrelated to your repair.
Flux core or solid wire
Most wire solder comes with a flux core, so the flux automatically activates and flows over the soldering area when the solder is melted. It is more convenient and efficient to work with. Solid wire can be used, with flux added with a brush, a bottle dispenser, or a pen dispenser. Unless a very specific flux is required that is not available as a wire solder core, flux core wire solder is generally recommended.
Type of flux
No-clean flux is a good choice for soldering where cleaning is to be avoided. The light residue can be left on the board, or removed with a flux remover. Rosin activated flux (RA) provide excellent solderability in a wide variety of applications. It is best to remove the residue after soldering for aesthetics and avoid corrosion down-the-line. Rosin flux (R) or mildly activated rosin flux (RMA) can generally be left on the PCB after soldering unless the aesthetics are a problem. Water soluble flux (OA) is a very active flux engineered to be removed easily with DI water, like in a batch or in-line system. It can also be removed with isopropyl alcohol (IPA). It is very important to clean off water soluble flux residues because they are highly corrosive.
