The Effects of Phosphorus in Lead-Free Solders

Keith Sweatman, Takatoshi Nishimura and Takuro Fukami
Nihon Superior Co., Ltd
Osaka, Japan

Phosphorus has long been a "secret ingredient" in tin-lead solder, particularly solder made with recycled metal, as it is a powerful deoxidant that removes dross and brings a sparkle to the surface of the molten solder. With the change to high-silver lead-free solders, which have a strong tendency to oxidize and generate large volumes of dross, phosphorus became a widely used but seldom mentioned ingredient.

The solder pot erosion that forced manufacturers to upgrade the materials used in the construction of wave soldering machines is primarily the result of the 100ppm phosphorus commonly included in the formulation of SAC alloys. Now, as solder manufacturers try to distinguish their low- and no-Ag solders from those of those offered by other suppliers, the presence or absence of phosphorus, with or without other antioxidants that have less detrimental side effects, has become a controversial issue.

In this paper the authors will report a series of experiments that have been undertaken to determine the effects of phosphorus additions on the behaviour and properties of a widely used lead-free solder.

The results of laboratory tests and evaluation in soldering processes confirm the conclusions of earlier work that phosphorus does not have a place as an alloying addition in lead-free solders, particularly those that rely on nickel to promote eutectic solidification or stabilize the hexagonal form of the Cu6Sn5 intermetallic.

Phosphorus additions to lead-free solders promote the wetting and consequent erosion of stainless steel machine parts.

When added to tin-copper-nickel solders phosphorus reacts with the nickel reducing the beneficial effects that it has on solder fluidity and Cu6Sn5 stability.

Wetting of copper is slowed.

Erosion of iron plated soldering tool tips is accelerated.

The rate of dross production in tin-copper-nickel alloy that already have germanium as their antioxidant is increased. Rather than working with the germanium synergistically to achieve an even lower rate of drossing, the phosphorus appears to act as an accelerant with the rate of dross production increasing by more than 60%.

Germanium is an effective antioxidant that does not cause the detrimental side effects that occur when phosphorus is used as an alloying addition to control oxidation or for any other purpose.