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Mechanical Analysis on Ball Bond Lifting of Wire Bonding in an LED Package for Backlight Unit
Keywords: LED packaging, ball bond lifting, wire bonding
LED is replacing conventional cold cathode fluorescent lamp (CCFL) as light source for backlight unit (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. Interlaminar tensile and shear stresses at ball bond interface were calculated by finite element method (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 LED packages, gold wires connecting pads on chips and leadframe 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 leadframe induces large tension and bending of the wire, by which large interlaminar tensile stress at the ball bond interface is developed during solder reflow process. The mismatch of coefficient of thermal expansion (CTE) between gold ball and sapphire chip induces interlaminar shear stress at the interface during encapsulant curing. The interlaminar 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 scanning electron microscopy (SEM) of the ball bond interface and gathered data of failure rates.
Jong Woon Kim, Senior Engineer
Samsung Electro-Mechanics
Suwon, Gyunggi-Do 443-743,
Korea


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