Solder is the metal material used extensively in both electronics and plumbing to create durable electrical and mechanical connections. It allows tiny electronic parts to be permanently fixed to circuit boards and enables pipes to be sealed together. Solder is not a pure metal; it is accurately defined as a metallic alloy. An alloy is a mixture containing two or more elements, where at least one of the elements is a metal. Pure metals, such as copper or aluminum, consist of only one type of atom, existing as a single element on the periodic table. Solder combines several elements to achieve specific physical traits necessary for joining.
The Composition of Solder
The specific elements chosen for solder alloys dictate their performance in different applications. Historically, the most common type was a mixture of tin and lead, frequently in a ratio of 60% tin to 40% lead (Sn60/Pb40), or the eutectic composition of 63% tin and 37% lead (Sn63/Pb37). This leaded alloy was favored for its low melting point, approximately 183°C (361°F) for the eutectic composition.
Due to environmental and health concerns regarding lead toxicity, the industry has largely shifted toward lead-free alternatives, especially in consumer electronics. These modern solders maintain tin as the primary component but substitute lead with other metals. The most popular lead-free compositions are from the SAC family, which stands for Tin (Sn), Silver (Ag), and Copper (Cu). A common example is SAC305, consisting of 96.5% tin, 3.0% silver, and 0.5% copper.
These lead-free alloys generally have a higher melting point, with SAC305 melting around 217°C (423°F). The choice of specific elements is deliberate, as the resulting mixture must offer a balance of low electrical resistance, good mechanical strength, and resistance to corrosion. The addition of silver and copper modifies the properties of pure tin, making the alloy suitable for electronic assembly. Manufacturers must carefully select these ratios to balance performance characteristics with the processing temperature limitations of the components being joined.
Key Properties and Function
The alloy classification allows solder to perform its primary function: joining metals without melting the underlying components. This is often achieved through the eutectic point, the lowest possible melting temperature for a specific combination of elements.
The Sn63/Pb37 alloy is a classic eutectic mixture because it transitions instantly from a solid to a liquid at a single temperature, 183°C. This temperature is significantly lower than the melting points of pure tin (232°C) or pure lead (327°C), minimizing heat stress on sensitive electronic parts.
Another defining characteristic is the ability of molten solder to “wet” the surfaces being connected. Wetting describes how the liquid solder flows out and adheres to the base metal.
This process involves a slight intermetallic bond between the molten solder and the metal surface, creating a continuous and electrically conductive connection. The alloy’s composition, along with the use of flux, provides the low viscosity needed to fill small gaps and form a reliable joint upon cooling.