Abstract
The interfacial-adhesion performance between the lead frame and molding compound was studied after temperature cycles and hygrothermal aging, simulating a typical package-assembly process. The hygrothermal aging involved a treatment at 85°C and 85% relative humidity (RH) for 168 h and three cycles of infrared (IR) solder-reflow condition. The interfacial-bond strengths were measured using shear and lead-pull tests. The lead-frame surface finishes studied include a bare Cu, microetched Cu, spot Ag coating, Ni, Pd/Ni, and Au/Ni coatings. Special emphasis was placed on the study of the changes in surface characteristics and the corresponding interfacial adhesion after various manufacturing processes. It was found that moderate thermal cycles enhanced the interfacial adhesion for all coated lead frames, except the Ni coating. Hygrothermal aging was detrimental to the interfacial-bond strength, especially for hydrophilic or polar surfaces, such as bare Cu, Ag, Pd/Ni, and Au/Ni coated lead frames. The introduction of tiny dimples etched on the lead frame was effective in mitigating the reduction in interfacial-bond strength arising from hygrothermal aging. This result confirms the important role of the mechanical-interlocking mechanism provided by dimples in retaining the interfacial adhesion in a humid environment.
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