Solder is a fusible metal alloy, typically composed of tin combined with other metals like lead, silver, or copper. It is used to create a permanent, electrically conductive, or mechanically strong bond between two metal workpieces. The melting temperature is not a single, fixed value but varies significantly based on the precise alloy used. This temperature is the most important factor guiding its application in electronics manufacturing and plumbing.
Understanding the Melting Range
Most metal alloys, including many solders, melt over a range of temperatures instead of at a single point. This range is defined by two specific thermal points: the solidus and the liquidus temperature. The solidus is the maximum temperature at which the alloy remains completely solid. The liquidus temperature is the minimum temperature at which the alloy becomes fully liquid and can flow freely to form a joint.
Between the solidus and the liquidus lies a “pasty range” where the alloy is a mixture of solid and liquid states. If the solder is disturbed while in this pasty state, it can result in a compromised or “cold” solder joint. Such joints typically have poor mechanical and electrical properties.
An exception is an alloy formulated to its eutectic point, which melts at a single, precise temperature like a pure metal. At this specific composition, the solidus and liquidus temperatures are identical, eliminating the pasty range entirely. This property is highly desirable in electronics assembly because it ensures a rapid and complete transition to a reliable solid joint upon cooling.
Temperature Specifications for Common Solder Types
The composition of the solder directly determines its melting characteristics, leading to two major categories with different thermal profiles. Traditional tin-lead alloys were the industry standard due to their favorable melting properties. The most common composition, 63% tin and 37% lead (Sn63/Pb37), is a eutectic alloy with a single melting point of 183°C (361°F).
Another traditional blend is 60% tin and 40% lead (Sn60/Pb40), which is non-eutectic. It melts over a narrow range, starting at a solidus of 183°C and becoming fully liquid at a liquidus of approximately 190°C. This low melting temperature made tin-lead solders easy to work with and required less heat stress on sensitive components.
Modern electronics primarily use lead-free alloys to comply with environmental regulations, requiring a shift to higher processing temperatures. The most prevalent lead-free formulation is the SAC alloy, typically composed of Tin, Silver, and Copper (e.g., SAC305). This non-eutectic alloy melts over a higher range, with a solidus of approximately 217°C (423°F) and a liquidus up to 221°C (430°F). This increase of 34 to 40 degrees Celsius over the eutectic tin-lead alloy is the main practical difference when transitioning to modern assembly.
How Melting Temperature Guides Tool Selection
The liquidus point is the starting reference for setting the temperature of a soldering iron or reflow oven. The equipment temperature must be set significantly higher than the solder’s liquidus point to effectively create a joint. This higher setting is necessary to overcome the thermal mass of the components and the printed circuit board, ensuring heat transfers quickly to the joint area.
For hand soldering, the iron tip is often set between 315°C and 370°C, substantially above the solder’s melting point. This temperature differential, often 100°C or more, ensures the joint reaches the liquidus temperature rapidly, typically within a few seconds. A quick heat cycle minimizes the time sensitive components are exposed to high temperatures, reducing the risk of heat damage.
Setting the tool temperature too low results in insufficient heat transfer, preventing the solder from reaching its liquidus point and causing a weak, grainy cold joint. Conversely, excessive heat can damage the soldering iron tip or prematurely burn away the flux. Therefore, the melting temperature dictates a controlled, elevated operational temperature to guarantee a high-quality connection.