Fabrication and thermal analysis of flip-chip light-emitting diodes with different numbers of Au stub bumps Cheng-Chen Lin a, * , Liann-Be Chang a , Ming-Jer Jeng a , Chia-Yi Yen a , Atanu Das a , Chung-Yi Tang b , Ming-Yi Tsai b , Mu-Jen Lai a a Department of Electronic Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, ROC b Department of Mechanical Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, ROC article info Article history: Received 1 December 2009 Received in revised form 8 January 2010 Available online 12 February 2010 abstract The thermal performance of flip-chip (FC) light-emitting diodes (LEDs) with different numbers of Au stub bumps has been investigated by using thermosonic bonder. The LEDs were mounted on the alu- minium nitride (AlN) sub-mounts which have superior thermal conductivity (230 W/mK), and the high power Chip-on-Plate (COP) package was proposed to be used for our measurement. In order to under- stand the thermal performance of the high power FC-LEDs, the experimental measurement and finite- element model (FRM) numerical simulation have been used. It is found that the thermal performance of our 1 Â 1 mm 2 FC-LEDs can only be improved when using at least 6 Au stub bumps as interconnected metals. Moreover, the surface temperature of FC-LEDs is significantly reduced while using 20 Au stub bumps. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Recently, the high power gallium-nitride (GaN) based light- emitting diodes (LEDs) have achieved dramatic development due to their wide application in solid-state lighting illuminations [1– 3]. In order to fulfill the application in general lighting, high-power LEDs keep attracting many researchers’ interest [4]. However, since most GaN-based LED related epitaxial grown devices choose sap- phire as substrate which thermal conductivity are only 35 W/mK. With the increase of input power the thermal problem is becoming more and more important causing negative effect on the optical performance, reliability, and lifetime [5,6]. In order to solve the thermal problem, flip-chip technique has been developed in LED chip design [7–10]. Flip-chip (FC) LED means that the LED chip is flipped over and mounted on various heat-sink sub-mounts. Compared with the conventional LED, the light from the active layer emits through a backside of transparent sapphire substrate, therefore, no attenua- tion of light is made by the semi-transparent metal electrode. Moreover, adding a high reflectance contact layer can enhance up- ward light emission. At the same time, the heat generated in the LED will flow directly through the interconnecting metal bumps between the LED chip and the sub-mount improving the thermal conduction. Thus, FC-LED structure can achieve high power and high brightness performance. Conventional flip-chip bonding methods include thermocom- pression bonding using high force and heat, but problems such as reliability can be caused by the high force and temperature re- quired by this process. Adhesives are an attractive alternative, however it needs to apply adhesive on the chip or sub-mount sur- face reduced the effectiveness and will add cost of this process in high volume production. Thermosonic bonding used in this experiment is a thermocom- pression process which is aided by ultrasonic vibration of the chip during bonding. With ultrasonic vibration providing energy to interface, the need of force and temperature can be reduced [11]. Low force and temperature makes it particularly useful for fragile materials such as epi-layers of LEDs. This bonding process also pro- vides a short time that means high yield rate can be beneficial for mass production. In this study, aluminium nitride (AlN) is used as sub-mount which is acknowledged that it has very high thermal conductivity of 230 W/mK, and Au stub bump as the interconnecting metal by thermosonic bonding to achieve the FC process. Au stub bump has high thermal conductivity (300 W/mK), therefore, it can also provide a good spreading path to transmit the heat to the sub- mount. However, without enough spreading paths, the junction temperature of LED cannot be reduced, or even more, are higher than conventional LED (non-FC). In order to balance cost, yield, and spreading heat ability, the optimum numbers of Au stub bump had been used for FC-LED. The investigation was carried out by combining electrical and optical measurement, thermal simula- tion, and thermal measurement. 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.01.036 * Corresponding author. Tel.: +886 3 2118800x5357 E-mail address: breeze@hotmail.com (C.-C. Lin). Microelectronics Reliability 50 (2010) 683–687 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel