Reaction Diffusion in the Au-Cu-Pb-Sn System at Low Temperature
The microstructure of a Cu 3 Au v. Pb-5wt%Sn diffusion couple was examined in the as-dipped and annealed conditions. The as-dipped couple showed no reaction between the solder and the base metal. The couple developed intermediate layers of two intermetallic phases upon annealing for 289 hours at 169°C. These phases were identified as phi and eta’ which are contained in the Au-Cu-Sn ternary system. No phases containing significant amounts of Pb formed, indicating that even at low concentrations in a Pb-Sn solder, Sn is the active chemical specie in the solder for the Au-Cu-Pb-Sn system. The microhardness of the eta’ phase was found to be equal to that of Cu 3 Sn, which forms in the absence of Au in the Cu base metal. However, the the phi phase was harder, and therefore the presence of Au in the base metal may adversely affect the thermal fatigue resistance of the solder joint.
Solder Connections with a Ni Barrier
Copper-tin intermetallic compounds readily form when tin-bearing solder connections are made to copper surfaces. These compounds continue to grow during the life of solder connections and represent potentially weak interfaces. An investigation was undertaken to determine if a diffusion barrier between the solder and copper would inhibit Cu-Sn intermetallic compound formation and thereby extend joint life. Hybrid integrated circuit solder connections formed between external clip-on terminals and Ti-Pd-Cu-Ni-Au (TPCNA) thin-film terminations were investigated. The terminals had 60wt%Sn-40wt%Pb solder clad on the circuit-gripping surfaces of phosphor-bronze clips. The diffusion barrier consisted of an electroplated Ni layer on the phosphorbronze underneath the solder. The mechanical strength of solder connections formed using terminals with and without a Ni barrier were compared by measuring joint strength both initially and after temperature cycling and accelerated temperature aging. Joint metallographic cross sections and fracture interfaces after pull testing were analyzed to determine how effectively the Ni layer isolated Sn in the solder from Cu in the phosphor-bronze. SEM analysis confirmed that 1-um-thick Ni provides an effective diffusion barrier. Consequently, strength of 60n40Pb solder connections formed between phosphor-bronze clip-on terminals and TPCNA thin-film terminations were markedly increased, particularly at elevated temperatures up to 150°C.
Significant Features of Solder Connections to Gold-Plated Thin Films
An investigation of 60Sn-40Pb solder connections between phosphor-bronze clip-on terminals and Ti-Pd-Cu-Ni-Au and Ti-Pd-Au thin film terminations is described. The investigation consisted of measuring joint strength and analyzing resultant fracture interfaces and metallographic cross sections by scanning electron microscope-energy dispersive X-ray analysis (SEM-EDAX), electronmicroprobe (EMP), and X-ray diffraction, both initially and after accelerated temperature aging. Similarities and differences observed for the different terminals and termination conductors used in this, and previous investigations, are presented. Observations are explained by termination interdiffusion, intermetallic compound formation between terminal, termination, and solder constituents, and joint geometry related to the terminal design.