How does the control process of the oxide layer on the solder ball surface affect its melting and spreading performance in reflow soldering?
Publish Time: 2026-02-04
In advanced electronic packaging processes, the solder ball, as a core material for high-density interconnect structures such as BGA and CSP, directly determines the soldering quality due to its surface condition. Although the alloy composition of the solder ball body is highly standardized, the surface oxide layer that forms rapidly upon exposure to air becomes a key variable affecting melting behavior and wetting spreading during reflow soldering. An excessively thick or uneven oxide film hinders the metallurgical bonding between the solder and the pads, leading to defects such as cold solder joints, poor wetting, and increased void ratio. Solder ball manufacturing employs multiple processes, including inert atmosphere protection, surface cleaning, organic coating, and strict storage control, to precisely regulate the oxide layer thickness and chemical state, thereby ensuring rapid melting, good spreading, and reliable connection during reflow soldering.1. The Formation Mechanism of the Oxide Layer and its Impeding Effect on WettingThe main component of the solder ball is tin, which is chemically reactive. With prolonged exposure time, the oxide layer can further transform into a denser SnO₂. Although thin, this oxide film has a high melting point and low surface energy, severely inhibiting the wetting of copper or nickel pads by molten solder. During the reflow soldering heating stage, if the oxide layer is not effectively removed, the molten solder cannot form intermetallic compounds with the base metal, leading to an increased contact angle, reduced spreading area, and even "spheroidization"—the solder shrinks into isolated spheres instead of spreading uniformly. This not only reduces the reliability of the electrical connection but also significantly increases the void ratio, affecting heat conduction and mechanical strength.2. Oxidation Control in Manufacturing: End-to-End Protection from Production to PackagingTo minimize oxidation, high-quality solder balls undergo strict oxygen control measures throughout the entire manufacturing process:Atomization Stage: Inert atmosphere spraying with nitrogen or argon is used to prevent contact between high-temperature molten solder and oxygen;Sieving and Inspection Stage: Operation is carried out in a nitrogen-filled glove box or a low-dew-point dry environment to prevent secondary oxidation;Surface Treatment: Some products undergo micro-cleaning with ultrapure deionized water or a weakly acidic solution to remove initial oxides, followed by the application of an extremely thin organic antioxidant coating. This coating volatilizes during the reflow soldering preheating stage, leaving no residue;Vacuum/Nitrogen-Filled Sealed Packaging: Finished solder balls are vacuum-packed in aluminum foil bags and filled with high-purity nitrogen, along with humidity indicator cards, ensuring that the oxidation rate approaches zero during transportation and storage.3. Oxide Layer Removal and Flux Synergistic Mechanism During Reflow SolderingEven with strict protection, trace amounts of oxidation are still unavoidable. The generated organotin salts dissolve in the flux carrier, exposing a clean metal surface. Subsequently, at the reflow peak temperature, the molten solder rapidly wets the pads, forming IMCs such as Cu₆Sn₅ or Ni₃Sn₄. If the solder ball is excessively oxidized, the flux activity is prematurely depleted, resulting in ineffective cleaning and wetting failure. Therefore, low-oxidation solder balls not only reduce dependence on flux activity but also reduce post-soldering residues, improving cleanliness and long-term reliability.Controlling the oxide layer on the solder ball surface, seemingly a microscopic material issue, actually permeates the entire manufacturing, storage, and soldering process, and is a hidden key determining the yield of high-end packages. Through source suppression, process protection, and end-product collaboration, modern processes have minimized this variable, ensuring that the solder ball melts instantly and spreads like a mirror during reflow soldering. In the future, with the accelerating trends of lead-free and miniaturization, the control of oxidation sensitivity will become even more stringent, driving the continuous evolution of solder balls towards "ultra-clean surfaces and intelligent responses."
