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External leads intended for thermocompression (TC) bonding to Au-metallized thin-film circuits are typically Cu-based alloys plated with Au or a combination of Ni and Au. The objective of this work was an evaluation of the Ni/Au system to determine the optimum plating thicknesses for reliable TC bonding. Also investigated was the minimum Au thickness necessary for TC bonding when the Ni diffusion barrier was omitted. Four criteria were evaluated: initial bondability; bond strength after accelerated aging; susceptibility to cracking in a 90° bend test; and fatigue behavior. The test vehicle was a 32-lead dual-in-line package utilizing alumina substrates metallized with Ti/Pd/Au films and Cu lead frames (CDA 102 and 110) electroplated with Ni and Au in the ranges of 0 - 41 µm and 0.4 - 20 µm, respectively (0 - 1630 and 15 - µin, respectively). Optimum strength and reliability with the Ni/Au system were obtained for a Ni plating thickness of 0.25 - 1.3 µm (10 -50 µin) and a minimum Au plating thickness of 2.5 µm (100 µin). For the special case of zero Ni thickness, acceptable TC bonds were obtained with as little as 0.6 µm (25 µin) of Au.