Everything - or at least a lot - about the addition of phosphorus in lead-free alloys.

The authors of the article do not predict a great career in solders for phosphorus - it is responsible for the erosion of crucibles, hinders the solidification of copper-containing binders and increases the amount of scratches produced by up to 60%.

Phosphorus has long been a 'secret ingredient' in tin-lead alloys, especially recycled metal solders. Phosphorus is a powerful deoxidizer that removes slag and gives the surface of the molten solder a shine. With the shift to high-silver lead-free solders, which have a strong tendency to oxidize and generate large amounts of scuffing, phosphorus has become a widely used but rarely replaced component of the alloy.

The erosion of crucibles, which has forced manufacturers to improve the materials used in the construction of wave soldering machines, is primarily due to the presence of 100 ppm phosphorus, commonly found in the composition of SAC alloys. Nowadays, as solder makers try to promote low or even zero Ag solder, the presence or absence of phosphorus and other antioxidants that have less harmful side effects has become a controversial issue. In The Effects of Phosphorus in Lead-Free Solders, the authors presented a series of experiments that were undertaken to determine the effect of phosphorus additives on the behavior and properties of a widely used lead-free solder.

Control of the amount of skimmings

Tin is the most oxidizable component in lead-free solders and is also the primary component: lead-free solders contain at least 95% of tin compared to 63% for lead solder. The oxidation of the solder, especially when combined with the turbulence associated with the action of the solder wave, causes the formation of skimmings, which is a slime mixture of unoxidized solder trapped in the oxide network. This slag interferes with the efficient operation of the wave soldering machine and must be periodically removed. The scrap can be processed and the metal content recovered and recycled, but slag production wastes expensive solder and increases the operating costs of the line, so production should be kept to a minimum. Phosphorus is a powerful antioxidant that has been widely used to control the amount of slag formed in lead-tin solders and has become a common ingredient in lead-free solders such as the widely used Sn-3.0Ag-0.5Cu alloy, i.e. SAC305.

Influence on solder fluidity

Another issue related to the addition of phosphorus is the presence of a lead-free nickel micro-alloy additive in the solders. It was found that the addition of nickel at the level of about 0.05% has a very strong influence on the behavior of the Sn-O.7Cu alloy with a nominal eutectic composition. The unmodified Sn0.7Cu alloy solidifies in two steps: the tin dendrites grow and solidify first, and the eutectic Sn-Cu6Sn5 alloy eventually solidifies in the spaces between the dendrites. This two-stage solidification results in a dull, cracked surface of the alloy. The smooth and bright surface of the Sn0.7Cu-0.05Ni alloy under the same conditions indicates that two-stage solidification has been replaced by one-stage pseudoutectic solidification, which means that in brazing processes the alloy behaves more like eutectic Sn -37Pb.

However, it was found that when phosphorus was added to Sn0.7Cu-0.05Ni, the new additive reacted with nickel to form nickel phosphide. The nickel bound in this way no longer supports the pseudoutectic solidification process, which in turn has a negative effect on the fluidity of the solder - after adding phosphorus, the Sn-0.7Cu-0.05Ni microstructure indicates a return to the two solidification stages, characteristic of the basic Sn-0 alloy , 7Cu.

The full article you can find here: https://tek.info.pl