Paste residual free lead oxidation

With increase of temperature in the curing chamber the rate of lead oxidation increases, irrespective of the RH. However, at RH >81% this increase is insignificant, whereas at RH = 55—75% it is substantial. For 1 h of curing at 50 °C in air atmosphere with RH between 55 and 75%, the content of residual free lead in the paste diminishes from 21 to 11—12%. 

The rate of lead oxidation is influenced also by the moisture content of the paste prior to curing. The amount of water in the paste pores should be reduced so as to open the pores for access of oxygen from the air needed to oxidize the residual free lead in the paste. The rate of water evaporation from the paste pores depends on the RH in the curing chamber. This dependence is illustrated in Fig. 8.17 for plate curing at 25 °C and various relative humidities [14]. 

Even after recrystallization of 3BS and 4BS is completed, the residual free lead in the cured paste is oxidized and a corrosion layer is formed on the grid surface, the structure of the plate is still mechanically weak. A great number of the paste particles are interlocked through their... 

Oxidation rate of residual free metallic lead vs. paste moisture content [2]. 

Washed and dried polypropylene pieces are sent to a plastic recycler, where the chips are melted [12] and extruded to produce plastic pellets for use in the manufacture of battery cases and other plastic components. Great care must be taken to ensure that the mechanical breaker cleans the polypropylene chips free of any residual oxide, because subsequent handling of the chips by operating personnel, especially at the plastic recycling plant, can result in significant levels of lead exposure. Should the wash sprays on the breaker fail to remove all traces of battery paste, consideration should be given to further washing with a dilute sodium hydroxide solution. 

As noted earlier, lead-free alloys require high reflow temperatures compared to eutectic Sn-Pb— with thermal profile plateaus in the range of 180-190°C, which are too high for fluxes formulated for eutectic Sn-Pb solder. The oxidation of rosin in air and the increased polymerization at elevated temperatures result in residues that are very difficult to clean with standard solvents. Flux vehicles selected for use in lead-free solder pastes are dramatically different from those used in eutectic Sn-Pb solder pastes. It was determined that cleaning products that preformed very well with eutectic Sn-Pb technology did not remove some residues left as a result of the new flux vehicle formulated for a lead-free solder [11]. 

---