Thermocompression Bondability of Bare Copper Leads

Thermocompression bondability of bare (unplated) Cu leads to TiPdAu ~ms on alumina substrates was investigated. This is an extension of earlier work to reduce the Au-plating thickness on Cu leads. Initial bondability, prebond shelf life, and long-term reliability were evaluated. The results follow. 1) Initial bondability is satisfactory provided suitable bond deformation is used. 2) Prebond shelf life is described by t(SL) = 7.5 X 10'-10 exp (0.80 + 0.2 eV/kT) where t(SL) is the time in hours to unacceptable bondability. The mechanism causing loss of bondability is surface oxidation of the Cu leads. A prebond shelf life (without recleaning) of about two years is indicated for storage at room temperature. 3) Long-term reliability is described by t(LT) = 9.7 x 10-'10 exp (1.0 0.2 eV/kT) where t(LT) is the time in hours to unacceptable bond strengths. The mechanism causing loss of bond strength with time is believed to be interdiffusion of the Cu lead with the Au metallization. A lifetime of greater than 40 years at 50°C is predicted. Comparisons between the present findings for hare Cu leads and those previously obtained using Au-plated Cu leads are presented. They indicate that the bare Cu leads have less processing and safety margin. But for many applications, bare Cu leads should be acceptable.