Mechanical and structural analysis of thermosonic wire bonds
In hybrid circuits, multi-chip modules and sensor-based multifunctional devices the electrical contact between the integrated-circuits and the substrate or between the substrate and the leads of the package are realised by using thermosonic gold micro-wire bonds. A high reliable wire bond needs a clean and bondable surface finish layer that is usually prepared with some sort of multi-layered metallization. In the course of my work I compared two ENEPIG (Electroless Nickel – Electroless Palladium – Immersion Gold) surface finishes on copper sheet. One of them was covered with a very thin gold layer; the other one was covered with a thicker gold layer. The main objectives were to assess the layer thicknesses in the ENEPIG structures and to compare the bondability of the two kinds of pieces. The industrial benefit of the work was the lower cost of the material covered with thinner gold layer.
To assess the layer thicknesses I used several methods, the results of the different approximations did not show any contradiction. We can state that beside the element map analysis and the linear element analysis on the cross-section of the samples, the calculation based on EDS (Energy Dispersive X-ray Analysis) method also provides good estimation. With further considerations we can get more accurate values. The roughness of the surfaces, especially the roughness of the copper core was visualized by using Alpha Step surface profiler and high magnification optical microscope. The approximately 1 μm thickness of the ENEPIG layer does not smooth away the raggedness of the surface which presumably originates from the rolling of the copper sheet. These kinds of surfaces do not favour to the wire bonding. On such a rough surface a stitch bond can have low quality as the stitch bond gets in touch with the surface on a small area. The raggedness influences negatively the adhesion between the materials. A special solution in the manufacturing, a previously touched down bump (called „under bump”) can stuff the surface for the stitch bonding. In this way the stitch bond fixes to the bump instead of the rough surface. The geometrical parameters of the bump have a wide range. The secure adherence is the only requirement against it that can be assured by using higher bonding force, higher ultrasonic energy and a longer duration of the process. Thus placing the „under bump” a less bondable surface can be well-prepared for stitch bonding.
The quality of the bonds is described by the mechanical strength which is determined by measuring the bond shear force. The reliability is tested for example with thermal cycle test. I noticed that the ball shear force increased as we enlarged the number of the thermal cycles. The explanation of the increased ball shear force is based on the pictures of the atomic force microscope. As the effect of the thermal cycles, the gold layer on the top became smoother, diffused into the palladium layer that caused a stronger bond. My examinations and the research data of the literature prove that the low cost surface finish using thinner gold layer can replace the more expensive layer structure with thicker gold, in particular when an enhanced bonding method, like under bump, is applied.