Zinc has always been considered a contaminant in solder. As quoted from Manko’s Solders and Soldering, McGraw – Hill, page 68.
“Zinc. This material has small solid solubility in tin and no apparent solubility in lead. No intermetallic compounds are formed with either tin or lead. The addition of zinc is reported to be very detrimental to the solder alloy. As little as 0.005 per cent of zinc is report to cause lack of adhesion, grittiness, and/or susceptibility to failure during solidification. Little zinc is encountered in electronic soldering to endanger the properties of solder pots.”
From R.J. Klein Wassink, Soldering in Electronics, Second Edition, Page 73, Para 2.3.3.3 Contaminated Solder, he states “Zinc contamination of solder causes strong dewetting. Although the dewetting mechanism in this instance had not be unraveled completely, a similarity is found between this cases and that discussed in the preceding text, Figure 2.36 show a dewetted copper plate with zinc-contaminated solder (0,3% by mass). Part of the surface (not shown) is covered by droplets with an appearance similar to that of the droplets in Figure 2.31. On the dewetted region solder is obviously present, and again many non-wetted spots are found . On the other hand the surface show features, such as that present at the top-left of the picture, resulting from broken oxides skins.
On page 194, 4.7.4 Alloys with Higher Strength Values it does mention “The alloy lead90 – cadmium9 – zinc1 has a higher fatigue resistance than eutectic tin-lead solder [Denlinger et al] but this alloy has several technological disadvantages, such as the high melting range of 238-260C”
With the RoHS requirements, this alloy would not be allowed due to constituent of Cadmium.
So from my sources Zinc is not a good thing to have as an element in a soldering alloy. From a search on Wikipedia on Brass, the following appears.
Brass is any alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.[1] Note that in comparison bronze is principally an alloy of copper and tin.[2]
As can be seen, zinc does exist in some products that we may solder to during the process of making electronic products and cable. Brass connectors are not uncommon for coaxial cable, which is why we need to take caution when soldering those products.
I hope this help your understanding of having Zinc in the solder alloy and the problems it may cause.
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee, covering areas of mounting, soldering, solderability and assembly. 