Physical Boundaries of Gold and Copper Ball Formation

摘要

The use of copper ball bonding instead of the traditional gold ball bonding is rapidly increasing within the semiconductor industry. Characterizing the dominant physical parameters that affect ball formation at the tip of a gold or copper wire becomes essential. Two processes dominate the resulting ball characteristics: (a) Shaping of the melt, i.e., ball formation due to the surface tension force; and (b) Solidification of the melt due to the heat loss. A theoretical order-of-magnitude analysis based on assessment of characteristic times of competing dominant processes was conducted. The analysis indicated that the existence of a critical limiting wire diameter under which solidification is expected to occur prior to complete ball shaping, results in distorted and non-spherical balls. Analysis results agree with experimental data. Nevertheless, the agreement of the qualitative trends and the order-of-magnitude of the results are very meaningful and of great significance. A dimensionless number is presented to support different wire material analysis.