Solder is a fusible alloy designed to create a permanent, conductive bond between two workpieces. This alloy must have a lower melting point than the metals it joins, allowing it to melt, flow, and solidify without damaging the underlying components. The resulting joint provides both mechanical strength and reliable electrical conductivity, particularly in electronics. The composition has changed significantly over the past few decades due to health and environmental concerns.
The Classic Answer: Tin and Lead
For nearly a century, the classic answer to the question of solder composition was a blend of tin (Sn) and lead (Pb). The most common general-purpose alloy was 60% tin and 40% lead, often referred to as 60/40 solder. This mixture was favored because it provided a low melting temperature, typically becoming fully liquid around 188°C.
A slightly different, and often preferred, mixture was the eutectic alloy, which consists of 63% tin and 37% lead. The eutectic composition is unique because it melts and freezes at a single, sharp temperature of 183°C. This lack of a semi-solid state was a major advantage in electronics manufacturing, as it greatly reduced the risk of disturbed or “cold” solder joints that could result in poor electrical connections. The high reliability and ease of use made tin-lead alloys the standard for electronics and plumbing before modern regulations changed the industry.
The Modern Standard: Lead-Free Solders
The widespread use of lead was restricted due to its toxicity and environmental impact, leading to the creation of lead-free solders. This shift was mandated by global standards, such as the European Union’s Restriction of Hazardous Substances (RoHS) directive, which prohibited lead use in most new electronic equipment. Manufacturers responded by developing new alloy formulations, which are predominantly based on tin.
The most common modern standard is the Tin-Silver-Copper alloy, often abbreviated as SAC. A typical composition is SAC305, which contains 96.5% tin, 3.0% silver (Ag), and 0.5% copper (Cu). Other formulations, such as Tin-Copper (Sn-Cu), are also widely used, especially for applications where cost is a greater factor. These lead-free alternatives generally have a higher melting point, typically around 217°C, which requires changes to manufacturing processes and can put thermal stress on certain electronic components. The high tin content in these alloys also introduces the potential for a phenomenon called tin whiskers, which are small, conductive crystalline growths that can cause electrical shorts over time.
Why These Metals Work Together
Solder alloy selection is based on material science, aiming to balance properties like melting point, strength, and flow. Tin serves as the foundation of nearly all soft solders due to its low melting temperature compared to the metals being joined, and its excellent wetting properties, which allow the liquid alloy to flow smoothly and spread across the surface.
Historically, lead was added to the tin alloy because it dramatically lowered the overall melting point and was an inexpensive way to prevent the formation of tin whiskers. In modern lead-free alternatives, silver and copper are introduced to compensate for the absence of lead. Silver significantly improves mechanical strength and thermal fatigue resistance. Copper, which is less expensive than silver, also enhances mechanical strength and conductivity, making it a common secondary component in the modern tin-based solders.