Strength determination of light-emitting diodes and chip structure design

Light-emitting diodes (LEDs), representing a type of solid-state lighting, have been widely used as indicator lamps in the past few decades. It has attracted a great deal of attention in various illuminating applications in recent years due to its outstanding advantages, such as low power cost, long life time, and high efficiency. However, to make it possible to apply LED in daily life, a suitable package structure is necessary, which provides electrical interconnection and protection functions. Recently, the technology for a high power LED packaging that employs applied wire bonding process to achieve electrical interconnection has been widely adopted by LED packaging house. However, improper wire bonding parameters often result in LED die cracking or pad peeling. In this study, the strength of LED dies was investigated in order to improve the yielding of wire bonding. To determine its strength, point-load test associated with focusedion beam was utilized to measure the ultimate reactive force. Results of the experiment were further integrated with simulation technology based on the finite element method to evaluate its ultimate strength. In the PLT tests, direct contact pin-loading was applied to the epilayer surface of the LED dies and the ultimate force was measured. After the PLT tests, FIB was utilized to investigate fracture initiating location in the epilayer. The PLT results showed that the averaged ultimate force is about 75 g. According to the FIB results, the vertical load was validated as the driving force for pad peeling, epilayer crack, and LED die crack. Based on the experimental data, an FEM 3D contact model was utilized to analyze its detailed mechanical behaviours. Simulation results showed that stress concentration occurred near the edge of the pin and that the maximum stress took place in epilayer. In order to reduce the stress, three kinds of new LED structures that introduce the stress buffer layer between the Au pad and LED layers were evaluated, and the results showed good improvement of stress reduction in the epilayer. Nevertheless, the soft material applied for the stress buffer structure may cause another failure issue under thermal loading during the bonding process due to the mismatch of the coefficient of thermal expansion. Therefore, to achieve the optimal design and the best combination of design parameters, the simulation-based design methodology must be adopted to meet the design and production optimization goals, which would be impossible if done by conventional experiment-based trial-and-error design procedure.