Wetting, solder solderability

FIGURE 51.15 Wetted solder joint. (Courtesy ofIPC.)... 

C. Percentage of land area covered with wetted solder on primary side (solder destination side) The land area does not need to be wetted with solder on the primary side 0 0 0... 

Wetted solder refers to solder applied by the solder process. 

The quinhydrone electrode is of simpler construction than the antimony electrode. A piece of 24-gauge platinum wire about 1/2 in. long is introduced into the capillary, which is then sealed and beaded to the wire leaving a short tip exposed externally. The platinum wire should be cleaned in aqua regia briefly and washed in distilled water before using. After the bead has cooled, self-wetting solder is introduced into the lumen and the assembly is heated until the solder melts. After the solder has bonded to the platinum, but while it is still molten, a piece of clean copper wire is inserted into it to form the electrode connection when the solder cools. 

Touch the swab to the wet solder surface and rub gently for only 10 s or less. 

Another issue with printing lead-free solder paste is stencil aperture design. Traditionally, stencil aperture size is reduced in relation to PCB pad size. This ensures the stencil aperture seals, or gaskets, to the PCB pad. Gasketing reduces solder paste that can get under the stencil and eventually cause wet solder bridges if not cleaned properly. Lead-free solder paste does not spread as well, so... 

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]. 

The BLM layer uses a glue layer of chromium or titanium. These metals stick well to other metals and most dielectrics, but they are not solderable. Copper, nickel, and silver have been used as the solder-wetting layer for BLM in appHcations involving 95% lead—5% tin solders. Gold is commonly used as the oxidation layer on account of its resistance to oxidation and its excellent solderabiUty. 

Alloy selection depends on several factors, including electrical properties, alloy melting range, wetting characteristics, resistance to oxidation, mechanical and thermomechanical properties, formation of intermetaUics, and ionic migration characteristics (26). These properties determine whether a particular solder joint can meet the mechanical, thermal, chemical, and electrical demands placed on it. 

Molten tin wets and adheres readily to clean iron, steel, copper, and copper-base alloys, and the coating is bright. It provides protection against oxidation of the coated metal and aids in subsequent fabrication because it is ductile and solderable. Tin coatings can be appHed to most metals by electro deposition (see Electroplating). 

Fluxes. Fluxes, composed mostly of salts or oxides of metals, serve to protect underlying metal from the air. This prevents the formation of surface oxides that impede fusion and the formation of a strong solder joint. Fluxes may also act to selectively leach elements from the surface of the underlying metal. The result is a surface free of obstacles to fusion, and of a composition readily wetted by the solder. 

Antifluxes are materials used to coat certain surfaces to prevent solder from flowing to those regions. Antifluxes are not wetted by the solder. They should be removed at the time when the flux is removed. 

The contact between the aluminium layers and the ceramic substrate requires a joining material which will wet both metal and ceramic, and solders such as the conventional Pb-Sn alloy have been used which are molten during the annealing process. The contact between the solder and the aluminium layer is frequently unsatisfactoty because of the intervention of the AI2O3 layer, and a practical solution appears to be to place drree layers of metal clrromium in contact widr the aluminium, copper in contact with the clrromium, and gold between the copper layer and the solder. 

The above Sn nitrate ( ) deserves mention because it was thought to be a fire and expin hazard in industrial accidents. For example, at the Spandau plant in Ger, several fires erupted in areas where wet NC came in constant contact with soldered bronze joints. In order to prevent further incidents, all soldered joints were examined and were found to be corroded with a coating contg Sn and nitrate ions. It was also found that the corroded material exploded when removal with a chisel was attempted Ellern (Ref 2) mentions that in the presence of w, cupric nitrate and Sn foil, on prolonged and intimate contact, will produce flaming and sparking... 

This chip version is typically made in glass and has the great advantage that the flow can be directly visualized [40,44—46]. Fabrication is achieved by photolithography and wet-chemical etching followed by thermal bonding of the plates covered with a thin layer of solder [47]. 

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