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Solder is a low-melting conductive alloy used to create reliable electrical connections between electronic components and printed circuit boards during soldering and PCB assembly. For most electronics work, the main choice is between traditional leaded solder such as eutectic Sn63/Pb37 and lead free alloys such as SAC305. Leaded solder remains popular for prototyping, repair, education, and hobby electronics because it melts at a lower temperature, flows smoothly, and is generally easier for beginners to use with less risk of cold joints. Lead free solder is widely used in commercial and RoHS-compliant products but typically requires higher soldering temperatures and more precise technique. For most makers and students using a soldering iron or soldering station, rosin core solder wire in the 0.5 mm to 0.8 mm range offers the best balance of control and feed rate for general through-hole and PCB work, while thinner solder works better for fine-pitch SMD components and thicker wire is useful for connectors, power wires, and larger pads. Typical soldering temperatures range from approximately 315–350°C for leaded solder and 350–380°C for common lead free alloys.

Flux type also plays an important role in solder performance, wetting, oxidation control, and cleanup. Rosin core solder is widely used for hand soldering because it provides good flow characteristics and manageable residue, while no-clean flux minimizes post-solder cleaning and water-soluble flux offers stronger oxide removal for difficult surfaces but requires thorough cleaning after assembly. Solder paste is primarily used for surface mount (SMT) assembly and combines powdered solder alloy with flux for stencil printing, syringe dispensing, and reflow soldering. Type 3 solder paste is commonly used for standard SMT work, while Type 4 paste contains finer particles suited for smaller SMD components and tighter pad spacing. When selecting solder or solder paste, factors such as alloy type, wire diameter, flux chemistry, melting temperature, and assembly method all affect ease of use and final joint reliability. A properly formed solder joint should appear smooth and evenly wetted across both surfaces, while dull, grainy, or cracked joints may indicate insufficient heating, contamination, or poor soldering technique.