Mechanical and structural analysis of thermosonic wire bonds

To reach excellent reliability of micro wire bonds, manufacturers must apply materials at high quality. This work discusses the comparison of two kinds of ENEPIG (Electroless Nickel - Electroless Palladium - Immersion Gold) surface finishes on copper sheets of terminals used for thermosonic gold wire bonding. The main difference between the two samples is the thickness of the gold layers on the top. To estimate the thickness of the layers a calculation based on EDS (Energy Dispersive X-ray Spectrometry) method was used. The thicknesses of the checked samples showed perfect match with the specification. The surface roughness was examined by Alpha Step Surface Profiler; and AFM (Atomic Force Microscope) was used to get a picture about the surface in higher resolution. The discovered raggedness of the surface, which certainly originates from the rolling of the copper sheet, may influence negatively the strength of the bonds, because the stitch bond gets in touch with the surface on a smaller area. The reliability was tested by measuring bond shear force after more and more cycles of thermal shock test. We noticed that the shear force increased with the advance of the number of the thermal cycles. According to the examinations the low cost surface finish covered with thinner gold layer can replace the more expensive layer structure with thicker gold.