Epitaxy-Like Protective Layers for High-Performance Low-Cost Au/Pd/Ni Preplated Cu Alloy Leadframes

Abstract

In this paper, we report the performance, the crystal microstructure, and defects of Au/Pd/Ni/Cu preplated lead frames (PPF). The wire pull test, the solderability test, and high-resolution transmission electron microscopy (HRTEM) are employed to characterize the PPFs in order to understand the relationship between performance and microstructure. The electroplated PPFs are composed of Au (3 nm)/Pd (5 to 15 nm)/Ni (0.7 mum)/Cu substrate. We optimize the electroplating profile and determine the minimum thickness of the Pd layer with the PPF performance satisfying the industry standards. Further increasing the Pd layer thickness beyond the critical thickness will not enhance the performance more, but increase the product cost. With the optimized electroplating profile, the electroplated Au layer is epitaxially deposited on the Pd layer, and so does the Pd layer on the Ni layer. The interface between the Au and Pd layers is in general coherent in spite of the about 4.8% mismatch strain between the Au and Pd lattices because the Au layer thickness is only 3 nm small. Misfit dislocations and nanotwins are present at the interface between the Pd and Ni layers, which are generated to release the about 10.4% misfit strain between the Pd and Ni lattices