Ultrasonic Flipchip Bonding, The Green Low Cost Alternative

ABSTRACT Ultrasonic flipchip bonding is a reliable, lead-free assembly method with great potential for future applications in volume production. But when compared to conventional C4 (Controlled Collapse Chip Connection) flipchip methods, it is not yet commonly recognized. In Ultrasonic flipchip, a chip with either gold stud bumps or plated bumps is bonded to a device by applying ultrasonic energy through the reverse side of the chip. For many applications it could be helpful to apply a low temperature to assist bonding, however cold bonding is possible by ultrasonic flipchip onto any “wirebondable” surface. The advantages are: The mono-metallic connection provides reliable mechanical strength with minimal electrical resistance, and even without underfill, ultrasonic flipchip can be a reliable assembly method. A significant reduction of thermal stress as compared to solder flipchip can also be achieved for material combinations with a low thermal mismatch. Ultrasonic flipchip bonding is even possible at room temperature. Better process stability and higher yields are achieved for fine pitch applications because the risk of shorts from epoxy or conductive paste is eliminated. This bonding method is an environmentally harmless process without the use of lead or epoxy. Ultrasonic flipchip is basically possible on any substrate with bondable metallization. Ultrasonic flipchip may prove more cost effective than other flipchip assembly methods because there is no additional equipment cost for printing, dispensing or curing.