Impurities are harmless unless their level goes too high.

When discussing contaminating elements in solder, we must first make a distinction between the levels in virgin solder delivered by the supplier and solder in use in the solder pot.

In this document, we define “contaminating elements” as those elements not deliberately added to the solder but remain after the solder refining process during production. These contaminating elements are also called impurities. Although the name suggests they are harmful, in fact they are not, unless the level of these elements becomes too high.

For new solder, during the past decades, the level of elements that is not deliberately part of the alloy (impurities) is steadily decreasing to very low values. This is because solder manufacturers are now able to refine solder to a higher purity level.

There was not, however, a direct technical need to reduce these numbers in view of any effect on the solder process or joint reliability. In fact, there are two reasons for better refining: One, new refining technologies make it possible to produce solder with lower impurity levels. Because this might constitute a commercial benefit, these types of solder are promoted to the electronics industries.

The second reason is that, for Pb-free alloys, “impurities” are sometimes deliberately added to give the solder alloy special properties. Since such alloy compositions are in most cases patented, the competition is forced to keep those elements at a very low percentage so as not to infringe the patent.

For these reasons, we find in J-STD-006 for nickel, for instance, a percentage of only 0.01% as maximum impurity in new solder.¹ However, there are patented solders where this element is deliberately added, although only in a low percentage, but well above 0.01%.

A few decades ago, percentages of up to 0.08% of this element were permitted in new solder. Even at that percentage, the solder process could run without difficulty, making perfect and reliable joints.

So much for new solder alloy compositions. What about solder in use in the wave soldering pot?

Regarding the maximum contamination allowable for the solder in use, most suppliers could not provide exact data. If data are provided, then these numbers are always on the safe side. One reason for this is that the solder supplier will not be held responsible for any claim if solder is used with a higher contamination level than what they advise as maximum impurity level. The second reason is that they live by selling solder, so it is also partly in their interest if the solder has to be exchanged.

Exchanging Solder

During use, each solder alloy will, in due time, reach a new equilibrium as a result of dissolving elements introduced by the soldered metals. But at the same time, the joints formed and the dross that is removed also take out this solder. On the other hand, we have the addition with new solder to compensate the solder taken from the pot. In fact, this newly added solder will partly “refresh” the solder in the pot.

In practically all cases, this equilibrium of “contaminated” solder is fine to use without the need to exchange it. Joint formation and reliability will not be affected by minor changes in the solder composition as a result of this “contamination.”

An exception might be the copper content in a SAC alloy. If too much copper is added as a result of dissolved material from leads or the board, it might change the melting characteristics of the alloy. Too much copper in the solder might also promote the formation of CuSn crystals. These needle-shaped crystals may increase the risk of solder bridging.

When beginning with a new process, it is normal that within a few months’ time, virgin solder will reach its contamination equilibrium in a solder pot. Normally there is nothing to worry about as this happens; it is simply part of the process of using solder for wave soldering.

The results of the soldering process itself should be the decisive factor if one should worry about solder composition. Normally, one should analyze the solder only in cases where the quality of the solder alloy is in doubt.

For more detailed information about the effect of different impurity elements in the solder, see R.J. Klein Wassink, Soldering in Electronics, second edition, Chapter 4.4.

References

1. J-STD-006B, Requirements for Electronic Grade Solder Alloys and Fluxed and Non-Fluxed Solid Solders for Electronic Soldering Applications, January 2006.

Gert Schouten is a retired engineer. He was with Vitronics-Soltec. Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. This column appears monthly.

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