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%.

14.10.2021
 

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.

 
 

Erosion of soldering tools

In addition to being a powerful antioxidant, phosphorus can also break down the oxide layer that makes stainless steel resistant to wetting by molten solder. After penetrating through the protective oxide layer, the molten solder wets the underlying Fe-Cr-Ni alloy which then begins to dissolve in the solder. The consequence of this is erosion of parts exposed to molten solder and even perforation of the walls of the soldering pot, which results in leakage of molten solder and creates serious health and safety problems as well as damage to the machine.

One solution to the problem of machine erosion due to the presence of phosphorus added to control the amount of scratch formed is to make the solder crucibles of cast iron that is more resistant to dissolution or the surface finish of titanium. Pumps and nozzles have been made resistant to wetting and erosion by applying ceramic coatings or by finishing the top layer with metal nitrides, which are much more resistant to wetting and dissolving. However, these treatments add significantly to the cost of wave soldering equipment.

Another approach is to use germanium as an antioxidant, which does not attack the protective oxide layer on the stainless steel, and is at least as effective as phosphorus in controlling the formation of slag in the crucible: a solder in which germanium is the antioxidant is already widely used ( Sn-0.7Cu-0.05Ni + <0.01Ge).

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.






Example of erosion of a soldering robot tip by reaction with phosphorus

Source: 'The Effects of Phosphorus in Lead-free Solders' by Keith Sweatman, Takatoshi Nishimura and Takuro Fukami of Nihon Superior Co., Ltd, Osaka, Japan.


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

Changes in the production process are forced by the progressive miniaturization of the equipment, which requires greater precision. An additional issue is a greater concern for the safety of the natural environment,...
2020/04/20
The production of electromechanical components is an extremely difficult and complex process. In the past, the production of small components in a short time...
Sensitive cable harnesses can at any time succumb to minor faults that endanger the main electronic equipment. This applies primarily to...
DFT (Design for Testing) analysis is used in the process of designing and manufacturing electronics circuits. Some of integrated circuits are equipped with additional ...