Development of Sn-Zn-Al Lead-Free Solder Alloys

Fujitsu has implemented a company-wide effort to progressively reduce the use of lead (Pb) and eventually eliminate this environmental pollutant from its products. As part of this effort, we have developed a new lead-free solder that consists of tin (Sn), zinc (Zn), and aluminum (Al) and yet offers superior productivity and joint reliability. The new lead-free solder has a melting point equivalent to that of an Sn-Pb eutectic solder and enables devices to be packaged at a lower temperature than with the increasingly popular Sn, silver (Ag), copper (Cu) solder. The new lead-free solder, therefore, accelerates the elimination of Pb from products. We have already used printed circuit boards containing the new lead-free solder in some products and plan to extend its use to other products. We made a solder composed of Sn, Zn, and Al and investigated its mechanical characteristics and the effects of adding Al. A tension test was conducted in air at 25°C and 60%RH at a tensile speed of 60 mm/min. The results showed favorable elongations in the following order: Sn-9wt%Zn < Sn-7wt%Zn < Sn-7wt%Zn-Al < Sn-9wt%Zn-Al. Al-added specimens showed better stress values than specimens without Al. Observation of the periphery of fractured specimens without Al revealed numerous, deep cracks and the contraction was smaller than in the Al-added specimens. We also joined Sn-Zn-Al solder balls to a Cu/nickel (Ni)/gold (Au) plated substrate at 215°C for Chip Size Package (CSP) applications. After 1000 hours of aging at 150°C, no deterioration was detected in the ball shear strength compared with the initial value. This paper describes a study and the characteristics of the new lead-free solders.