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Study of Void Formation Mechanism in Electroplated SnAg Solder Bump
Keywords: Void Mechanism, Lead Free Plating Chemical, Solder Bumping
SnAg electroplating method is adopted in the formation of solder bump for flip chip connection. Unlike some methods like solder printing and ball mounting, electroplating is a very promising technology for upcoming finer bump formation. While SnAg electroplating is able to form void free solder bump in a suitable operating condition, void may occur when used in mass production. This study aims at understanding the gas source in the void of a SnAg solder bump and determining the manufacturing process factors which affect the void formation. There are various types of void formation mode in electroplated SnAg solder. One void mode is H2 gas generation on cathode surface during electroplating. Both the numerous cross-sections of solder bumps, as well as an analysis data of the gas in the void taken by the TDS (Thermal Desorption Spectrometry) were evaluated. The cross-section of the solder bump which contains a void due to reflow revealed the existence of some submicron-size pits in the solder before reflow. TDS analysis indicates that the submicron-size pits consisted of mainly H2O, H2 and few organics. A possible void formation mechanism is estimated to be caused from the evaporation of the incorporated plating chemical in the solder followed by the H2 generation during the plating. This small pit in the solder was caused by various process parameters. One of the causes is due to the setting of the current density in the SnAg electroplating process being inappropriate. The current density should be adjusted corresponding to the chemical performance and bump design. The computer simulation demonstrated that a thick PR limits the diffusion of the Sn2+ ion into via hole and having the current density too high causes a lack of Sn2+ ion on the cathode surface and causes H2 gas generation. The other void mode is the Ag displacement of the under bump metallization (UBM) surface in dwell time in the chemical before starting electrolysis. Another process parameter exists not only in the SnAg plating, but pre-treatment (descum and removal of oxide from the UBM surface), and post-treatment (ex. PR stripping, Cu seed etching and reflow) were also evaluated. The adjustment of each process parameter can eliminate the source of the void and achieve a high reliability of SnAg bump formation.
Koji Tatsumi, R&D Engineer
Mitsubishi Materials Corporation
Sanda-shi, Hyogo-ken

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