Soldering joins two or more metal workpieces by melting and flowing a filler metal, called solder, into the joint. This creates a permanent connection without melting the base metals themselves. Because the filler metal has a significantly lower melting temperature, soldering is considered a low-temperature joining method, distinguishing it from processes like brazing or welding.
What Solder Is Made Of
Solder is an alloy, a mixture of metals, formulated to have specific melting characteristics. Alloys are often designed to achieve a single, sharp melting point, known as the eutectic temperature, which is lower than the melting points of the constituent metals. For example, traditional solder was a tin-lead (Sn/Pb) alloy (typically 63% tin and 37% lead), which melts sharply at 183°C.
The low melting temperature prevents thermal damage to sensitive components. Since the early 2000s, environmental regulations such as the Restriction of Hazardous Substances (RoHS) directive have mandated a shift away from lead-based solders. This led to the widespread adoption of lead-free alternatives, which are primarily tin-based.
Common lead-free alloys include the Tin-Silver-Copper (SAC) series, such as SAC305 (96.5% tin, 3.0% silver, and 0.5% copper). These modern alloys have a slightly higher melting range (around 217°C to 220°C), requiring adjustments to the soldering process. The alloying elements are balanced to maintain good electrical conductivity and mechanical strength in the finished joint.
Preparing the Surface: The Action of Flux
The integrity of a solder joint begins with surface preparation. All metals, particularly when exposed to heat and air, rapidly form a thin layer of metal oxide. This oxide layer acts as a barrier, preventing the molten solder from making direct contact with the base metal.
To overcome this issue, a chemical agent called flux is applied to the joint area before or during the heating process. Flux contains active chemical ingredients, often organic acids or rosin, that activate when heated. This activation allows the flux to chemically dissolve and remove the metal oxides and other surface contaminants.
The flux also creates a temporary protective barrier over the freshly cleaned metal surface. This barrier shields the metal from the surrounding atmosphere, preventing rapid re-oxidation as the surface is heated. By cleaning the surface, the flux enables the molten solder to flow freely, which is necessary for forming a proper connection.
Without the action of flux, the molten solder would be unable to adhere to the base metal, causing it to bead up into small spheres, a failure known as “dewetting.” The flux residue must sometimes be cleaned away after the joint cools to prevent corrosion or interference with the final product.
Creating the Connection: Wetting and Intermetallic Layers
The physical and chemical processes that occur when molten solder meets a cleaned surface create the bond. The first step is “wetting,” the ability of the liquid solder to flow smoothly and spread across the base metal surface. Good wetting means the solder adheres to the metal rather than pulling away or forming a dome-like shape.
Once the surface is wetted, the molten solder is drawn into small gaps between the components and the base material. This phenomenon, known as capillary action, ensures the solder fills the entire joint area, securing the components mechanically and electrically. The true strength of the solder joint, however, comes from a fundamental chemical reaction at the interface.
Soldering is not merely adhesion, but a metallurgical bond formed by atomic mixing. When the molten tin component touches the base metal (e.g., copper), a small amount of the base metal dissolves into the liquid solder. This reaction facilitates the formation of a new, transitional alloy layer at the boundary called the Intermetallic Compound (IMC).
In the case of tin solder on a copper surface, the IMC layer consists of copper-tin alloys, specifically Cu6Sn5 and Cu3Sn. This IMC layer is the true point of connection, providing strong, lasting electrical and mechanical continuity between the solder and the base metal. A thin, uniform IMC layer indicates a successful bond, but if the layer grows too thick, the joint can become overly brittle, affecting long-term reliability.