Copper is the most cost-effective alternative to gold which is used in wire bonding. Hardness of bonding wire and FAB (free air ball) is double-edged swords, wire with higher hardness and strength can build more reliable loop, but damage the bounding pad on the chip more easily. Precisely measuring hardness is meaningful for improving the performance and reliability of the bonding process. Nanoindenter is a powerful experimental technique for detecting mechanical properties of materials. In this work, the mechanism of nanoindentation and the selection of hardness test parameters were studied. Hysitron's nanoindenter with Berkovich tip was used to measure hardness of 0.8mil (20μm) 4N(99.99%wt of copper) copper ball bonding wire, FAB and bonded ball. It is confirmed that copper wires get harder mainly in ball formation and ball bond form process.
Recently, copper wire bonding has been given increasing attention due to its low cost and excellent material performance. Compared with pure copper wire, Palladium coated copper (Pd-Cu) wire gets more extensive application partly due to the Pd-coated layer protection for Cu FAB from oxidation in the bonding process. Therefore the research of Pd coverage on free air balls (FAB) is an important aspect for Pd Cu wires. FAB of Pd-Cu wires may have exposed Cu region on top side of F AB where it is to be bonded, this Pd uncovered part could be possibly extended and then exposed to the outside of bonding area. After mold encapsulation, the exposed Cu region on FAB may have higher risk to be subject to corrosion compared with the Pd covered region and lead to failure of fIrst bonding. This paper presents a study for Pd coverage issues on FAB of 0.8mil Pd-Cu wires. A chemical etching method with FeCI3 solution for FAB was adopted to analyze the Pd coverage on FAB. The etching condition had been optimized through careful examinations on the surface morphology of etched FAB. With various FAB forming process parameters, e.g., electronic flame-off (EFO) current intensity and heating, Pd coverage has been studied. The Cu exposure, before and after FAB formation, was also compared and analyzed. This analysis results will provide an important reference for the optimizing FAB process parameters in Cu wire bonding process.
Cu wire is the alternative material to the Au wire in many aspects, such as better electrical and thermal conductivity, higher mechanical strength and its lower cost for the high volume manufacture. Due to the surface oxidation of the Cu wire, the Pdplated Cu wire (Pd-Cu wire) has been adopted quickly in many fine pitch and high density package devices. Cu wire bonding has been well understood in recent years, while Pd-Cu wire is still under intense investigation. Here we present a study on the interface of the stitch bonding by TEM analysis.Our experiments were performed on 1 mil (~25um) Pd-Cu wire bonding with Ag plated leadframes under the 5% Hydrogen forming gases. The Pd coating thickness of the Cu wire was about 100nm, while the thickness of the lead-frame's Ag coating was about 6um. The leadframes were subjected to pre-heating treatments at 200 in normal, 5 minutes extended and 20 minutes extended condition, total 3 samples, in order to investigate whether it will form oxide material on surface of the lead-frames and degenerate the bondability. The research tried to understand the underlying microscopic mechanism of the stitch bonding interface by means of micro-topography and elements distribution analysis. The samples for TEM examination were prepared by specific manual grinding followed by FIB micro-machining.No oxide layer or Oxygen gathering region was found at the interfaces of three samples by TEM topography. The results indicated that the existing Pd coating at the Cu wire surface was able to prevent Cu from being oxidized, and the surface of the lead-frames also showed no oxygen gathering. A thin "dark" Pd layer at the interface of three samples between copper and silver layers was observed by TEM with EDX, and the peak value of the Pd at the interface was about 40 atomic %. The thickness of the Pd layer was about 30nm which was much thinner than the original coating thickness. It indicated Pd coating has been seriously deformed during the bonding process. Each element (Cu, Pd, and Ag) at the interface had low interdiffusion rates under the pre-heating temperature (200 ) from the analysis of the abrupt elements distribution curves. It was observed that the content of the Cu in Ag coating was varying for the three samples. When the pre-heated time of the lead-frame increased at 200 , the content of the Cu in Ag coatings increased from 10% to 20% and then to 30%. The details of Pd layer microstructure and topography need further investigation with improved analytical means.
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