Wetting, solder application

However, mainstream SMT soldering applications using eutectic Bi-Sn solder and conventional fluxes and surface finishes have not been reported. This may be due to the fact that the wetting characteristics of the Bi-Sn solder are generally not as favorable as Sn-Pb solder with existing flux materials and surface metallizations. 

Roughness has important implications in wetting applications. While the eutectic solder, SnPb, normally forms a contact angle of 15-20° with copper, it completely wets the surface of rough electroplated copper and forms a fractal spreading front [69]. 

Ceramic materials, including sapphire, have been used extensively in HPLC pumps for more than 20 years as pistons and check valve components. These materials have also been used to construct heads because of their good chemical stability. The use of ceramics is limited, however, because of high cost and brittleness. Although many systems have one material as the primary construction material, the wetted surfaces of a pumping system can contain several other materials. Therefore, for material-sensitive applications, all the materials in the HPLC eluent flow path should be considered. Materials that may be encountered are polymeric materials for pump seals such as fluoropolymers, polypropylene, and Teflon sapphire pump pistons and check valve seats ruby check valve balls Kalrez, KelF, or ceramic washers and spacers polymer-based transducer components and in older systems connections and joints made with silver solder. 

Adhesion to metal is sometimes difficult to achieve. However, due to the development of adhesion promoters industrial applications on metal exist. Electronics applications are important photoresists (both wet and dry film), solder masks, potting compounds, and conformal coatings are products based on UV-curable materials. 

Teme is steel sheet that is hot dip coated with a lead-tin alloy [20]. Because lead does not readily wet steel, 2-25 % tin is added in order to ensure wetting and bonding of the coating to the substrate. Typical coating thicknesses are in the range of 5-15 pm. The product has good weldability, solderability, and formability. Largest applications have been in the automobile industry, particularly for fuel tanks. 

Silver alloys provide the ability to wet and join base metals at low temperatures while providing good strength. Because low temperatures can be used for silver soldering and brazing, the risk of surface oxidation and heat distortion is reduced. Silver alloys usually comprise eutectic mixtures of tin, zinc, or copper, with lead and cadmium being used in some applications. The Ag-Cu alloys have been used with ceramics to produce electrical parts, and they have been used to braze carbide inserts to drills. The carbide inserts can easily be adjusted or replaced by melting the alloy. 

Brighteners. Sometimes Ag is added to solder to improve wetting, making the solder joint smooth and shiny. When not used as a scavenger, the addition of Ag should be avoided as it is not usually needed for most electronic joining applications with Sn-Pb. 

Even though the relative difference of atomic radii of Bi (1.56) and Pb (1.75) amounts to about 12%, the solubility of Bi reaches 23.5 wt% at 184°C and 7.5 wt% at room temperature. While Bi addition has very little influence on mechanical properties, Pb—Bi alloys excellent wetting properties make them valuable as solders for glass-to-metal joints. Their desirable solidification shrinkage characteristics and casting properties (that provide an ability to reproduce surface details) make them useful in printing and prototyping applications. 

Lead-free reflow may be done in air or N2. Typically N2 is not required, and the use of N2 may even increase certain defects (such as tomb-stoiflng) especially for small passive components. In certain situations, N2 may help improve wetting (which in turn may help reduce the amount of voids in the solder joints). For flip chip applications, where flux is used instead of solder paste, N2 becomes necessary to form reliable solder interconnects. An N2 atmosphere with O2 level below 1000 ppm has been found to be effective (Ref 65, 67). 

Particle size has to be chosen to suit the pitch of the structure. The size of the particles in the solder paste for a fine-pitch application has to be correspondingly small in order to faciiitate printing. The smaller the particles, however, the greater the tendency of the solder alloy to oxidize during soldering, and this can detract from the paste s wetting properties. Powder particle size is the feature used for ciassifying solder pastes. [144]... 

Su-Cu Fairly high melt temperature (227 Q Wave solder candidate Only moderate wetting, but sufficient for most applications... 

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