Solder is a low-melting-point metal alloy designed to bond metal surfaces together through fusion, typically used in electronics assembly, plumbing, and metalwork. It melts at temperatures between 180°C and 300°C, depending on formulation, and solidifies to form a durable electrical and mechanical connection.
The most common solder is a tin-lead alloy, historically composed of 63% tin and 37% lead, known as eutectic solder. This specific ratio melts cleanly at 183°C with no plastic phase, making it ideal for precise work. Modern electronics increasingly use lead-free alternatives due to environmental regulations like RoHS (Restriction of Hazardous Substances), which limit lead content in electronic products.
Lead-free solders typically consist of tin combined with silver, copper, or bismuth. A common formulation is SAC305 (96.5% tin, 3% silver, 0.5% copper), which melts at around 217°C. These alloys offer comparable strength and reliability but require higher temperatures during application, affecting equipment settings and process control.
Solder functions by wetting the surface of metals to be joined. Wetting occurs when molten solder spreads evenly across clean, oxide-free surfaces. Oxides on metal surfaces prevent bonding, so flux is applied to remove oxides and promote adhesion. Without flux, solder beads up and fails to form a proper joint.
In electronics, solder is applied via a soldering iron or reflow oven. During hand soldering, a technician heats the joint-typically a copper pad and component lead-until the solder melts and flows into place. Once cooled, the solder forms a solid, conductive bridge between components and the circuit board.
A real-world example: repairing a broken connection on a smartphone motherboard. A technician uses a 60/40 tin-lead solder (60% tin, 40% lead) with rosin-core flux. The iron is set to 320°C. After cleaning the area with isopropyl alcohol, the solder is applied to the joint. Within seconds, the solder melts, wets the copper traces, and solidifies into a reliable connection. The repair restores power to a dead charging port.
There are four primary solder types based on composition and use case:
| Type | Composition | Melt Point | Use Case |
|---|---|---|---|
| Eutectic Sn-Pb | 63% tin, 37% lead | 183°C | Historical electronics, prototyping |
| Lead-Free SAC305 | 96.5% tin, 3% silver, 0.5% copper | 217°C | Modern consumer electronics, industrial devices |
| Sn-Ag-Cu (SAC) | Variable tin/silver/copper ratios | 215-225°C | High-reliability applications, aerospace |
| High-Bismuth Alloys | Tin-bismuth (e.g., 58% tin, 42% bismuth) | 138°C | Low-temperature repairs, heat-sensitive components |
Each type balances melting point, strength, conductivity, and cost. For example, high-bismuth alloys melt below 150°C, enabling repair of temperature-sensitive parts like LED arrays or thin-film sensors. However, they are less mechanically solid and prone to cracking under thermal stress.
Flux types also vary. Rosin-based fluxes leave minimal residue and are suitable for precision work. Water-soluble fluxes clean easily but can corrode if not removed. No-clean fluxes leave behind non-conductive residue that doesn't interfere with function but may cause issues in high-voltage environments.
Many assume solder is pure metal. It is not. Solder is always an alloy-mixtures of metals designed to achieve specific physical properties. Pure tin melts at 232°C and tends to form whiskers over time, leading to short circuits. Pure lead melts at 327°C and is too soft for structural joints. Combining them creates a material with lower melting point and better flow characteristics.
Another misconception: solder conducts electricity because it's metallic. While true, the electrical performance depends on joint quality. A cold joint-where solder doesn't fully wet the surface-can have high resistance, causing signal loss or intermittent failure. A properly formed solder joint has low resistance, often below 1 milliohm.
A non-obvious fact: solder's density varies by alloy. Lead has a density of 11.3 g/cm³. Tin is 7.3 g/cm³. A 63/37 Sn-Pb solder has a density of approximately 8.7 g/cm³. In contrast, the densest known metal is osmium, at 22.59 g/cm³-over twice as dense as lead. This difference matters in weight-critical applications like aerospace or portable devices.
I once repaired a vintage radio using 60/40 tin-lead solder. The original circuit had no silkscreen labels. I traced connections manually, then replaced a cracked solder joint on a capacitor lead. The joint failed initially due to insufficient flux and overheating. After cleaning with a brass brush and adding fresh flux, the second attempt succeeded. The radio powered on immediately. That moment confirmed: correct soldering technique and material choice matter more than tools.
When selecting solder, consider the environment. High vibration or thermal cycling demands stronger alloys. Moisture-prone areas require flux removal to prevent corrosion. Always match the alloy to the application's temperature profile, mechanical stress, and regulatory requirements.