Solder is a fusible metal alloy engineered to create a permanent, electrically conductive, and mechanically sound bond between metallic workpieces. The fundamental requirement for any solder is that its melting point must be significantly lower than the melting temperature of the components being joined. This is achieved by combining different metals into an alloy, which results in predictable melting and flowing characteristics necessary for soldering. The alloy composition is controlled to deliver the optimal balance of a low melting point, strong adhesion, and reliable electrical conductivity.
The Classic Lead-Tin Alloy
For decades, the standard material for solder was an alloy of tin and lead. This traditional composition was favored for its ease of use and superior wetting properties, which is the molten material’s ability to flow smoothly and adhere to the base metal. The most common ratio historically used in electronics was 60% tin and 40% lead, which melts over a narrow temperature range between 183°C and 188°C.
The most precise blend is the eutectic alloy, consisting of 63% tin and 37% lead. This specific mixture has a single, sharp melting point of 183°C, transitioning instantly from a solid to a liquid without a semi-solid, or “pasty,” phase. This characteristic prevents the formation of weak connections, known as “cold joints,” which can occur if the joint is disturbed while the solder is partially solidified. The low melting point of the lead-tin mixture also reduced the thermal stress placed on sensitive electronic components during assembly.
Modern Lead-Free Alternatives
A shift in solder composition was necessitated by environmental and health concerns surrounding lead. Regulatory initiatives, such as the European Union’s Restriction of Hazardous Substances (RoHS) directives, mandated the removal of lead from most consumer electronic products. This compelled the electronics industry to develop and adopt new lead-free alloys.
The most prominent modern alternative is the Tin-Silver-Copper family of alloys, commonly designated as SAC. The standard composition, SAC305, is comprised of 96.5% tin, 3.0% silver, and 0.5% copper. The addition of silver and copper helps maintain the necessary mechanical strength and electrical performance of the joint.
SAC305 has a melting point in the range of 217°C to 220°C, which is higher than traditional lead-tin solder. This elevated temperature requires higher processing temperatures, placing greater thermal strain on electronic components and circuit boards. Lead-free solders can also exhibit slower wetting characteristics and may be susceptible to the formation of brittle intermetallic compounds over time. Despite these challenges, SAC alloys have become the global standard for environmentally compliant soldering.
Why Solder Must Be an Alloy
Solder must be an alloy, rather than a pure metal, due to the physics governing material melting behavior. Pure metals like tin (232°C) or lead (327°C) melt at high temperatures and lack the necessary performance characteristics for reliable joint formation. When metals are mixed to form an alloy, the different-sized atoms disrupt the regular crystal lattice structure. This disruption weakens the atomic bonds, significantly lowering the energy required for the alloy to transition from a solid to a liquid state.
This is the metallurgical principle behind the eutectic effect, which yields a melting point lower than any of the pure constituent metals. Alloying is also necessary to achieve good wetting, which is the ability of the molten solder to flow across and chemically bond with the base metal.
The tin component in the alloy is crucial because it chemically reacts with the base metal, typically copper, to form a thin layer of an intermetallic compound. This layer is the metallurgical bond that provides the joint’s mechanical strength and electrical continuity. Without this alloying action, the molten metal would simply sit on the surface, failing to form the strong, permanent connection required.