Investigation on solder voids in flip-chip light-emitting diodes using thermal transient response

In order to reduce the cost of the LED packages and to minimize the thermal budget, the chip-on-board (COB) packages and the chip-scale package (CSP) LEDs, based on flip-chip die bonding process, have been developed. Although the flip-chip die bonding process for the COB and the CSP LEDs is useful to decrease the thermal resistance, a precise process control for minimizing voids in solder joint and an in-line inspection for bonding quality check are needed for reliability. We proposed a simple in-line void inspection method based on the thermal transient response of LED junction temperature. In order to carry out the feasibility test, we made three LED package groups showing different solder void qualities. By measuring voltages at two points in time domain, we could distinguish the LED packages showing a poor solder void quality.