Solder plays a central role in assembling everything from smartphones to industrial controls, forming strong electrical and mechanical connections between components and printed circuit boards (PCBs).
This guide offers a simple overview for beginners, including what solder is, what flux does, and the most common forms of solder. Learn how technicians use these materials in electronics assembly processes like wave soldering, reflow soldering, and hand soldering.
What is Solder?
Solder is a metal alloy used to join two or more metal surfaces together. It usually consists of a base metal like tin, combined with silver, copper, bismuth, lead, or other elemental metals.
Designed to melt at a relatively low temperature, solder enables the joining of different metal surfaces without melting those surfaces or harming any other materials nearby, such as circuit board components. When cooled, the solder then forms a strong electrical and mechanical bond.
In electronics manufacturing, technicians most often use solder to connect components to a printed circuit board (PCB). This can happen via wave soldering, hand or robotic soldering, or by printing solder paste.
Solder in Electronics Manufacturing
Solder for electronics manufacturing is available in several forms, each designed for specific processes. The most common forms include:
- Solder Bar: A solid, rectangular or ingot-shaped piece of alloy primarily for wave soldering applications.
- Drut lutowniczy: A spool of wire which often contains a flux core for hand soldering and robotic soldering applications.
- Pasta lutownicza: A mixture of powdered solder alloy and a flux medium with a paste-like consistency for surface mount technology (SMT) applications.
What is Flux?
We’ve mentioned the word flux in the previous section. Flux is a substance that goes hand in hand with solder alloys in the soldering process. It is a chemical cleaning agent used to improve the quality and reliability of the solder joint.
Flux removes oxides from metal surfaces and prevents further oxidation during heating by creating a protective barrier. It also allows molten solder to flow smoothly and bond effectively.
There are three primary types of flux chemistries:
- Rosin-based fluxes are most commonly for military and aerospace applications. Technicians must usually clean the residue left by these fluxes after soldering off the boards using special cleaners.
- Water soluble flux residues also need cleaning, but they are designed to be cleaned with a simple water rinse.
- No clean fluxes are the most popular in the industry. As the name implies, the residue left by these fluxes does not need cleaning. It is designed so that it can be left on without causing problems with electrical reliability. However, some users still choose to clean no clean flux residues, and there are special cleaning chemicals designed for this process as well.
Flux also comes in several forms. The most common forms include liquid flux, core flux, and paste or tacky flux. Liquid flux usually comes in jugs and is clear in appearance. It is used primarily in wave soldering where it is foamed or sprayed onto a circuit board before soldering.
Core flux is a thick, paste-like flux that runs along the center of cored solder wire. Technicians use it primarily in hand soldering and robotic soldering applications
Paste flux or tacky flux is a thick paste-like flux that may range from whitish to dark amber/brown in appearance. Flux forms the base for solder paste, but is also available on its own for holding components in place during other processes.
Solder Bar and Liquid Flux in Wave Soldering
Wave soldering is a high-speed process for through-hole assemblies, where the wave solders component leads (inserted into holes in the PCB) from the bottom side. In this method, solid solder bar is melted in a heated pot to create a standing wave of molten solder. Machines first coat assemblies with liquid flux, then preheated and passed over the wave, which touches only the underside of the board.
The molten solder does not damage the board or components because of tight process control. Heaters gradually warm the board before it reaches the wave, and exposure time is brief. The solder adheres only to exposed metal pads and component leads because of surface tension and flux activity. Technicians primarily use wave soldering for large production runs of PCBs with many through-hole components, such as power supplies or industrial controls.
Soldering with Wire: Hand and Automated Processes
Solder wire, often cored with flux, is a versatile form that technicians use for manual assembly, rework, or selective automation. In hand soldering, a technician applies the wire directly to the joint with a heated solder iron, for precise control over where and how much solder is deposited. The built-in flux cleans the surfaces as the solder melts, helping it bond with the metal pads and leads.
Automated wire soldering uses programmable equipment such as robotic arms with soldering tips to perform consistent, repeatable joints for high-mix or medium-volume production. This method is ideal when only certain points on a board require soldering or when through-hole components are used alongside surface-mount parts.
Wire soldering offers flexibility and control, making it the preferred method for prototypes, repairs, and complex or low-volume builds.
Soldering with Solder Paste
Surface mount technology (SMT) is the most common method of assembling modern electronics. Instead of inserting component leads into holes, SMT components are placed directly onto pads on the surface of the PCB. To create electrical and mechanical connections, manufacturers use solder paste, a thick mixture of powdered alloy and flux.
Technicians apply the paste using a stencil printing process. They place a stainless-steel stencil with cutouts aligned to the board on top, and a squeegee pushes the paste across, filling the openings. When the stencil is removed, precise deposits of solder paste remain on the pads.
A pick and place machine then places components onto the paste, and the paste’s tackiness holds them in position. A technician then sends the board through a reflow oven. This oven gradually heats the assembly, melting the solder and forming permanent joints as it cools. SMT soldering enables compact, high-density assemblies with excellent repeatability and is essential for most modern consumer and industrial electronics.
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If you’d like to learn more about solder and the soldering process, check out additional AIM Solder blog posts as well as our collection of technical papers.
