Effects Of Surface Oxidation And Reduction On Wetting Of Tin-Lead (SnPb) Solder

Fluxes are normally used in soldering processes to remove surface oxides on both solder and base metal to improve wetting performance of solder on substrate. After soldering, the fluxes are mostly evaporated into vapor phase, but there are always some residues left on the circuit board that may cause corrosion problems and reduce packaging reliability. Therefore, fluxless soldering technology using protecting atmosphere or high vacuum has being desired. Almost all scientific wetting experiments are conduced using above technology to avoid the introduction of disadvantage factors associated with the use of liquid fluxes. This paper analyzed wetting of Sn63Pb37 solder on copper using fluxless soldering technology.A forming gas (5% H₂ and 95% Ar₂) was utilized to reduce the metal oxides in this experiment to study the wetting of Sn63Pb37 solder on bulk of copper in the absence of flux condition. Surface chemistries of solder and copper which was oxidized for different times were analyzed by X-ray photoelectron spectroscopy (XPS). The effects of the thickness of oxide on Cu and heating rate on wetting temperature were studied. The results showed that: 1. the main oxide on the solder surface is tin oxides, and containing lead oxides. 2. The formation of Cu₂O oxide layer in thickness of 2.0-4.2 nm were found after exposing the milled and polished copper surface in air from 5 min to 5 hours at room temperature. 3. Wetting temperature increases with the increase of the oxidization time of metallic copper (thickness of oxide). The O concentration on copper plays a prominent effect on wetting behavior. 4. The wetting temperature increases with the increase of heating rate. An apparent activation energy (Ea) was therefore calculated to be 96.4±9 kJ/mol, closes with the apparent activation energy for the reduction of Cu₂O(≈115.08 kJ/mol).The wetting of Sn63Pb37 solder on Au was studied in order to analyze wetting mechanism. The results showed that Cu₂O can be reduced by H₂ to form Cu and water vapor effectively, but it cannot effectively reduce the initial oxides of solder during heating. Cu or Au can diffuse into solder fast and cause the breakdown of oxide layers. Molten solder can flow from gaps of oxides by breaking through oxide layers, and the oxides of solder may be greatly even totally dissolved into liquid solder. By analyzing the interface microstructure morphology and the microstructure morphology in solder for quick spreading, slow spreading and no spreading of solder on Cu and for the spreading of solder on Au, we can seen that molten solder is in direct with metallic Cu or Au to form Cu-Sn intermetallic compounds and Au-Sn intermetallic compounds, respectively. It shows good wetting.