Lead solder has historically been the standard material for joining electrical components due to its low melting temperature and excellent wetting properties. Understanding the temperature at which this alloy transitions from solid to liquid is fundamental to successful soldering. This knowledge allows technicians to select the appropriate tools and temperature settings for creating reliable connections, as the melting temperature depends directly on the specific ratio of tin and lead used in the alloy.
Melting Points of Standard Lead Alloys
The two most common tin-lead alloys in electronics are 60/40 and 63/37. The 60% tin and 40% lead alloy (Sn60/Pb40) is a widely used general-purpose solder. This alloy melts over a narrow temperature range, exhibiting both a solidus (start of melting) and a liquidus (fully liquid) temperature. The melting process for Sn60/Pb40 begins at 183°C (361°F) and becomes fully liquid at 191°C (374°F).
The 63% tin and 37% lead alloy (Sn63/Pb37) behaves differently because its composition represents the lowest possible melting point for all tin-lead combinations. Unlike the 60/40 blend, the 63/37 alloy transitions directly from a solid to a liquid state at a precise temperature of 183°C (361°F). This single-point melting temperature provides significant advantages in electronics manufacturing, making it a preferred choice for applications requiring precise temperature control.
The difference in melting behavior relates directly to the alloy’s composition relative to the eutectic point. The 60/40 alloy has an 8°C (13°F) temperature window where it is partially solid and partially liquid, classifying it as non-eutectic. Conversely, the 63/37 alloy bypasses this phase entirely and is identified as eutectic. Although the difference in melting points is small, the resulting material properties during the transition are vastly different, which affects the quality of the finished solder joint.
Defining the Eutectic Point
The eutectic point describes the specific alloy composition that yields the lowest possible melting temperature. At this exact ratio, the solidus temperature (where melting begins) and the liquidus temperature (where the alloy is fully molten) coincide. For the tin-lead system, this occurs at 183°C (361°F) with a composition of 63% tin and 37% lead.
Any change in the alloy’s ratio away from the eutectic point introduces a temperature range between the solidus and liquidus states. This temperature window is known as the “pasty range” or “plastic range.” Within this range, such as the 183°C to 191°C span of the 60/40 alloy, the solder exists as a semi-solid, slush-like material. This phase is a mixture of solid crystals and molten metal.
The pasty range in non-eutectic solders poses a significant challenge during manufacturing. If a soldered joint is subjected to movement or vibration while the material is semi-solid, the crystalline structure can be disrupted as it attempts to solidify. This disturbance results in a structural weakness known as a “cold solder joint.” A cold solder joint appears dull and grainy and often leads to electrical failure.
The eutectic 63/37 alloy eliminates this risk by bypassing the pasty range, solidifying instantaneously once the temperature drops below 183°C. This quick transition time ensures the joint is either fully liquid, allowing for excellent wetting and flow, or fully solid. This minimizes the opportunity for structural defects caused by movement during the cooling phase. The absence of a pasty state is the primary reason the eutectic alloy is highly valued for high-reliability electronic assemblies.
Practical Iron Tip Temperatures for Successful Soldering
While the melting point of lead solder is around 183°C (361°F), the iron tip temperature must be set significantly higher to achieve a proper solder connection. The purpose of the soldering iron is to rapidly heat the components and copper traces to the solder’s melting point, not just to melt the solder itself. This quick heat transfer is necessary to ensure the alloy flows and “wets” the surfaces effectively, creating a strong metallurgical bond.
A common operating range for a soldering iron tip when working with leaded solder is between 340°C and 400°C (650°F to 750°F). This higher temperature compensates for the rapid heat loss that occurs when the tip touches the component and the circuit board. If the iron temperature were set too close to the melting point, the slow heat transfer would require the iron to be held on the joint for an extended period.
Prolonged contact time with the heat source can damage sensitive electronic components or cause the copper traces on the printed circuit board to lift and delaminate. Setting the iron temperature higher allows the technician to work quickly, minimizing the overall thermal exposure to the assembly. This elevated temperature also ensures the flux, a chemical agent within the solder, is activated quickly to clean the metal surfaces of oxidation, which is necessary for proper solder adhesion.
The exact temperature chosen within the recommended range depends on the thermal mass of the components being joined. Soldering a large wire or a component connected to a wide ground plane requires a higher heat setting, sometimes exceeding 370°C (700°F). This is because the larger copper area acts as a heat sink, drawing thermal energy away from the joint. Conversely, for small or sensitive surface-mount components, the temperature may be lowered toward the bottom of the range, around 315°C (600°F), to reduce the risk of overheating.