A study on laser-assisted bonding (LAB) and its influence on luminescence characteristics of blue and YAG phosphor encapsulated InGaN LEDs

Abstract

This work is a sincere attempt to answer the question whether laser assisted bonding (LAB) is a feasible method for producing stable and reliable solder interconnections of LED devices without any negative impact on the LED chip performance. To meet these requirements, a bonding process was evaluated that induces marginal thermal and mechanical loads into the LED chip to prevent any changes in the quantum well (QW) structure. The bonding process uses a LAB "LaPlace" system with a 280W Q-CW solid-state, near infrared (NIR) laser source.By varying the laser parameters, the functionality of the bonded LED was characterized and a suitable process window, along with the corresponding energy threshold, was examined for the chosen LED configuration. A 700μm thick copper lead-frame test substrate was selected, and subjected to AgNi metallization. Furthermore, InGaN blue LED test chips (type EDI-FA445B) with 150μm pad-to-pad spacing, and 3μm SnAu 20/80 solder connections were used. Overall, this study compares a non-encapsulated LED version to an encapsulated (NYAG4454-L) one.To determine a suitable LAB process window, metallurgical properties of the soldered interface were examined microscopically by polishing a cross-section on the one hand, and conducting electrical qualification on the other. The mechanical load capacity was measured with a shear test unit, and the corresponding metallurgical fractures were optically inspected. The optical spectrum of the LED after bonding was studied with a "spectroradiometer" to identify shifts in the wavelength corresponding to electroluminescent emissions peaks. The impact of a "thermal-test" on the bonding quality was further evaluated. Additionally, the interface quality was correlated with the performance data in relation to LAB bonding parameters.Finally, this work introduces a laser assisted rework process i.g. laser assisted de-bonding (LAdB) for the de-bonding of faulty LEDs. The future prospects of intended reliability and stability for LED placement as well as a skillful approach for rapid, laser-assisted placement of mini- and micro-LEDs using the "LaPlace" systems are elucidated.