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Accelerated Characterization of Bonding Wire Materials
Keywords: Cu wire materials, free-air-ball, online characterization
The common trends in the microelectronics industry are towards miniaturization, higher performance, and low costs. These trends are driving the development of novel materials for microelectronic wire bonding. The wire bonding industry is looking towards extending the use of Cu as a new wire material to replace Au, but the development of Cu wires that overcome oxidation issues is time consuming. One reason for this is that the effects of shielding gas on the electrical flame off (EFO) process and the free air ball (FAB) formation are not fully understood. The consistent formation of round FABs during the EFO process is essential for a reliable bonding process. In this study an online method for characterizing Cu FABs using in-situ thermosonic wire bonding is developed. The effects of shielding gas type and flow rate are presented. The FAB heights before deformation (HFAB) and after deformation (Hdef) are measured using online z-position measurements for various wires. These include a 25 µm prototype Cu wire and commercially available Cu and Au wires. Effects such as large variation in HFAB at a single flow rate, abrupt changes in HFAB with increasing flow rate, or discrepancies between the trends in HFAB and Hdef indicate undesirable EFO processes. The addition of H2 to the shielding gas reduces the oxidation of the FAB and also could possibly provide additional heat input during EFO. The convective cooling effect of the shielding gas was measured and increases with flow rate. Flow rates above 0.7 l/min yield an undesirable EFO process due to an increase in oxidation and drag force on the FAB. Using this online method the EFO process and the performance of novel wires can be determined quickly and efficiently. In this method 90 FABs can be characterized in 2 minutes for a specific set of EFO parameters. A complete characterization of a novel Cu wire with respect to FAB consistency can be done in less than 4 hours and includes determining process windows for different shielding gas flow rates. Traditional sample preparation, optical microscopy and manual measurement methods can take weeks to obtain the same results.
Andrew Pequegnat, Graduate Student
University of Waterloo
Waterloo, ON N2L 3G1,
Canada


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