Historically, the joint reliability of Sn-Zn solder on the copper plated substrate was known to be poor.
Other Sn/Zn lead-free soldering materials contain bismuth (Bi) to lower the melting temperature and to improve wetting and spreadability, usually drequires gold plating. When the electrode terminal contains lead, the Bi quickly reacts with the Pb to form a Pb-Bi intermetallic compound layer on a Cu plated substrate. With application of small stress such as warping, its interface will separe easily from the substrate.

By adding very small amount of Aluminum (Al) to Sn/Zn alloy, it eliminate the formation of intermetallic layer without the gold plating. Thus the joint becomes stronger and high reliable.