Reliability of Clip-on Terminals Soldered to Ta-Ta2N-NiCr-Pd-Au Thin Films

Reliability of solder bonds to Ta-Ta
2
N-NiCr-Pd-Au conductors used for resistor-capacitor (RC) hybrid integrated circuits (HIC's) was investigated. The investigation consisted of aging 60 Sn-40 Pb solder bonds at elevated temperatures and measuring the bond strength. The experimental results are similar to those previously measured for Ta
2
N-Ti-Pd-Au conductors used for resistor HIC's. An Arrhenius extrapolation of the aging data shows that bond life based on acceptable mean bond strength exceeds a 50°C 40-year requirement. The effective activation energy for solder bond aging E
A
is slightly greater for Ta-Ta
2
N-NiCr-Pd-Au (1.4-1.6 eV) than for Ta
2
N-Ti-Pd-Au (1.1-1.3 eV). This difference is attributed to a more continuous RC film, which is produced by the additional tantalum layer, and which consequently has a slower dissolution in solder. Analysis of weak bond interfaces and metallographic sections indicates that intermetallic compounds cause solder bond aging. However, bond aging E
A
consistently exceeds activation energies for solid-state diffusion and the formation of the observed intermetallic compounds. The most likely causes for this difference are 1) Pd diffusion into the gold with a consequential reduction in AuSn
4
and PdSn
4
growth rates, and 2) rate-limiting Sn diffusion to the terminal-film contact area.