Mechanical Analysis on Ball Bond Lifting of Wire Bonding in an LED Package for Backlight Unit

LED is replacing conventional CCFL as light source for BLU in displays of LCD TVs and laptop computers. It has the advantages of low power consumption, long durability, and mercury-free. In this paper, ball bond lifting failures in an LED package for BLU were investigated from the viewpoint of thermal stresses during curing and reflow processes after wire bonding. Interfacial tensile and shear stresses at ball bond interface were calculated by FEM simulation. From the simulation of LED packages with various wire loop shapes and pad layouts, failure mechanism of the ball bond lifting was described. In wire bonding for the LED packages, gold wires connecting pads on chips and lead frame have large loop height since the chips are mounted on a cavity designed for reflection of emitted light. Thermal expansion of thick layer of silicone encapsulant between the wire and lead frame induces large tension and bending of the wire, by which large interfacial tensile stress at the ball bond interface is developed during solder reflow. The mismatch of CTE between gold ball and sapphire chip induces interfacial shear stress at the interface during encapsulant curing. The interfacial tensile and shear stresses much depend on wire loop shape and pad layout, respectively, and optimal wire loop shape and pad layout were determined in order to minimize the stresses. When interfacial strength is weakened by defect, contamination, or process variation, the thermal stresses yield the ball bond lifting failure. The result agreed well with SEM and EDS of the ball bond interface and gathered data of failure rates.