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Improvement in Flip-Chip Bonding by Reduction of Oxides using Hydrogen Radicals
Keywords: lead-free solder, hydeogen radical, flip-chip bonding
Recently, as various electronic equipments such as portable telephones, digital cameras, and personal computers have been miniaturized, the demand for the miniaturization of solder bump pitch in LSI packages and high accuracy interconnection technology has also increased in order to be able to downsize LSI. The flip-chip bonding technique is adopted as the connecting method for being suitable in scaling down the mounted area of LSI and the LSI package. The connecting method is performed by melting the solder. Therefore, when soldering, if there are oxides present on the solder bump surface, they obstruct the interconnection as high-melting point oxides do not melt. Various oxide removal methods are proposed. The existing oxide removal methods are the flux method and the plasma method. Both these methods have their disadvantages. The flux method affects the environment and the equipment because this method uses organic solvents. In addition, the flux residue causes the impedance anomaly. The plasma method, which does not use the flux, is not cost-effective because the plasma supply equipment is expensive, and further, the substrate is damaged by the plasma. In this paper, we propose a novel oxide removal method that uses the hydrogen radical produced on a heated tungsten wire (hot-wire) in order to solve the above-mentioned problems. We have successfully realized that the removal of oxides from the solder bump surface was possible by this treatment. Furthermore, we verified that it is effective to improve the flip-chip bonding. The number of good connections has increased to approximately three times or more by this treatment when compared to the Ar plasma treatment, which is the conventional method.
Tsuyoshi Nakashima, Student
Kyushu Institute of Technology
Kitakyushu-shi, Fukuoka 804-8550,

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