Thermosonic Flip Chip Interconnection Using Electroplated Copper Column Arrays

A novel flip chip process is reported in which bare dies are thermosonically bonded to arrays of electroplated copper columns formed on a substrate. The new process is intended as a low-cost, lead-free chip-on-board (COB) interconnection method for high-frequency devices. A detailed study has been performed of the electroplating and thermosonic bonding techniques involved. It was found that oxygen plasma treatment of the resist mask could increase the yield of the fine-pitch (<150 mum) column electroplating process, while the flatness of the resulting columns was affected by the plating current density and the sidewall profiles in the resist mold. Under optimal conditions, column arrays with flat tops could be produced with a 100% yield, and with a column height deviation of less than 0.5 mum over an area of 10 mmtimes10 mm. The array thermosonic bonding process was studied with the aid of Box-Behnken design of experiments based on response surface methodology. A second-order relationship between the input bonding variables and the bonding strength was derived and used to determine optimal process conditions. With this optimized process, silicon chips with aluminium metallization were thermosonically flip chip bonded to quartz test boards bumped with gold-capped copper columns. Sixteen prototype assemblies without underfill protection were evaluated by accelerated lifetime tests. The contact resistances of single column connections showed no significant change after 50 thermal shocks of 0 degC to 100 degC, and samples subjected to high-temperature storage remained intact at all bonding interfaces after 1.5 h at 300 degC. Thermal cycling between -55degC and 125 degC produced open-circuit defects in a small number of connections after 70 cycles