Effects of Bonding Materials on Optical-Thermal Performances and High-Temperature Reliability of High-Power LED

The die-bonding layer between chips and substrate determinates the heat conduction efficiency of high-power LED. Sn-based solder, AuSn20 eutectic, and nano-Ag paste were widely applied to LED interconnection. In this paper, the optical-thermal performances and high-temperature reliability of LED with these bonding materials have systematically compared and studied. The thermal conductivity, electrical resistivity, and mechanical property of these bonding materials were characterized. The LED module packaged with nano-Ag has a minimum working temperature of 21.5 °C. The total thermal resistance of LED packaged with nano-Ag, Au80Sn20, and SAC305 is 4.82, 7.84, and 8.75 K/W, respectively, which is 4.72, 6.14, and 7.84 K/W higher after aging for 500 h. Meanwhile, the junction temperature change of these LEDs increases from 2.33, 3.76, and 4.25 °C to 4.34, 4.81, and 6.41 °C after aging, respectively. The thermal resistance of the nano-Ag, Au80Sn20 and SAC305 layer after aging is 1.5%, 65.7%, and 151.5% higher than before aging, respectively. After aging, the LED bonded with nano-Ag has the better optical performances in spectral intensity and light output power, which indicates its excellent heat dissipation can improve the light efficiency. These results demonstrate the nano-Ag bonding material could enhance the optical-thermal performances and high-temperature reliability of high-power LED.

Compact Measurement of the Optical Power in High-Power LED Using a Light-Absorbent Thermal Sensor

LED (Light-Emitting Diode) presents advantages such as luminescence, reliability, durability compared with conventional lighting. It has been widely applied for life, healthcare, smart farm, industry, and lighting from indoor to the automotive headlamp. However, the LED is vulnerable to thermal damage originated from the high junction temperature, especially in high power applications. Hence, it requires precise qualification on the optical power and the junction temperature from the pilot line to secure reliability. In this study, the photo-thermal sensor is proposed by employing a sheet-type thermocouple composed of photo-absorbent metal film and thermocouple. This sensor aims low-cost qualification in pilot line for high-power luminous devices and optical monitoring of costly luminaire such as automobile LED headlamp. The sensor is designed to detect the increased temperature response of LED hot spots from the transferred thermal power and absorbed optical power. The temperature response of each sheet-type thermocouple is utilized as a signal output of the absorbed optical power and hot spot temperature based on the introduced sensor equation. The proposed thermal sensor is evaluated by comparing the experiment with the measured reference value from the integrating sphere and the attached thermocouple at a junction. The experiment result reveals 3% of the maximum error for the optical power of 645 mW.

Transparent Ceramics Enabling High Luminous Flux and Efficacy for the Next-Generation High-Power LED Light

By designing novel chemical compositions and controlling precursor powders, perfect transparent ceramics (TCs) of Gd₃Al₄GaO₁₂:2%Ce³⁺ garnets (GAGG:2%Ce³⁺) were achieved for the first time. Ce³⁺ distributions in ceramics were revealed by micromorphology and micro-cathodoluminescence. Benefiting from the components that are rich in red emission, warm light with a correlated color temperature of about 2800 K was generated when TC was used in high-power (hp) blue light emitting diodes (LEDs) (∼450 nm, ∼34 W driven at 1 A). The hp LED device shows high brightness with a luminous flux nearly 2100 lm. The luminous efficacy even reaches 388 lm/W, which is the highest value reported till now, indicating that the use of GAGG:2%Ce³⁺ TCs promises energy saving. We believe that this work will open a new perspective to develop TCs for next-generation hp LED lighting to substitute traditional hp xenon lamps and high-pressure sodium lamps.