The wettability of solder wire is a core physicochemical phenomenon for achieving a strong connection during the soldering process. Essentially, it's the ability of molten solder to spread across the base metal surface and form atomic-level bonds. This characteristic directly determines whether a reliable metallurgical bond can be formed, thus affecting the mechanical strength, electrical performance, and long-term stability of the weld joint.
Wettability relies on the interatomic interactions between the solder and the base metal. When the solder wire melts, tin atoms in the liquid solder diffuse along the microscopic unevenness of the base metal surface under the combined action of surface tension, base metal adhesion, and capillary forces. During this process, if an oxide film, oil, or other contaminants are present on the base metal surface, a physical barrier will form, hindering direct contact between the solder and the base metal. For example, copper-based base metals easily form a thin film of copper oxide in air, with a surface energy much lower than pure copper, causing the solder to shrink into a spherical shape and unable to spread. In this case, the flux removes the oxide layer through chemical reduction, simultaneously reducing the surface tension of the solder and creating conditions for wetting.
The quality of wettability can be visually judged by the contact angle. When the solder completely wets the base material, the contact angle is close to 0°, and the liquid solder forms a thin, uniform layer. If the contact angle is greater than 90°, it is considered non-wetting, and the solder cannot adhere. In actual soldering, a semi-wetting state (contact angle 75°-90°) can cause the solder joint edges to shrink, forming weak areas. For example, in printed circuit board (PCB) soldering, if there are fingerprints or uneven plating on the pad surface, even if the local area is well-wetted, adjacent areas may experience cold solder joints due to excessively large contact angles. This hidden danger can easily lead to solder joint breakage under vibration or thermal cycling conditions. Good wettability is a prerequisite for metallurgical bonding. During the wetting process, the solder and base material atoms diffuse across the contact interface, forming a solid solution or intermetallic compound (IMC) in the crystal lattice. For example, tin-lead solder reacts with the copper substrate to form a Cu₆Sn₅ layer, the thickness of which directly affects the solder joint strength: too thin a layer can easily lead to brittle fracture, while too thick a layer can cause cracks due to stress concentration. Insufficient wettability hinders atomic diffusion, leading to discontinuities in the IMC layer and solder joints exhibiting mechanical bonding rather than chemical bonding. Their tensile strength is only 1/3 to 1/2 that of metallurgical bonding.
The impact of wettability on soldering reliability is also reflected in long-term stability. In high-temperature and high-humidity environments, solder joints must withstand stress cycles caused by differences in thermal expansion coefficients. If initial wetting is poor, voids or cracks may exist in the IMC layer. These defects become stress concentration points, accelerating fatigue crack propagation. For example, in automotive electronic modules, if microcracks form in solder joints due to poor wettability, these cracks may lead to open-circuit failure after thousands of cycles under extreme temperature cycling conditions ranging from -40°C to 125°C.
Process parameters are crucial for controlling wettability. Temperature is the core factor affecting wettability: at excessively low temperatures, the solder viscosity is too high, preventing sufficient flow; at excessively high temperatures, flux decomposition is accelerated, resulting in loss of its cleaning effect. For example, in manual soldering, the soldering iron tip temperature needs to be controlled between 300℃ and 350℃ to melt the solder without damaging the PCB substrate. Furthermore, soldering time, pressure, and ambient humidity also need to be controlled in conjunction—excessive soldering time may lead to overheating and oxidation of the base material, while high humidity will accelerate the formation of a water film on the solder surface, both of which will worsen wettability.
Material compatibility is another key factor in wettability. Different base materials require corresponding solder systems: for example, nickel-based substrates require silver-containing solders to form Ni₃Sn₄ compounds, while aluminum-based substrates require specialized active solders. If the material compatibility is inappropriate, even with optimized process parameters, wettability will still not meet requirements. For example, when directly using tin-lead solder to solder aluminum, the dense oxide film on the aluminum surface is difficult to remove, causing the solder to shrink into a spherical shape and failing to form an effective bond.
