Elsevier

Microelectronics Reliability

Volume 56, January 2016, Pages 53-60
Microelectronics Reliability

Thermal characteristics and fabrication of silicon sub-mount based LED package

https://doi.org/10.1016/j.microrel.2015.10.010Get rights and content

Highlights

  • The cost of an LED package is lowered by using a silicon substrate as the base attached to the chip.

  • A silicon sub-mount based LED package is proposed a different method.

  • The silicon sub-mount based COB package is applicable to high-power down-light type LED light sources.

Abstract

In this paper, the cost of a light emitting diode (LED) package is lowered by using a silicon substrate as the base attached to the chip, in contrast to the conventional chip-on-board (COB) package. In addition we proposed an LED package with a new structure to promote reliability and lifespan by maximizing heat dissipation from the chip. We designed an LED package combining the advantages of COB based on conventional metal printed circuit board (PCB) and the merits of a silicon sub-mount as a substrate. When an input current 500–1000 mA was applied, the fabricated LED exhibited the light output of approximately 112 lm/W at 29 W. We also measured and compared the thermal resistance of the sub-mount package and conventional COB package. The measured thermal resistance of the sub-mount package with a reflective film of Ag and the COB package were 0.625 K/W and 1.352 K/W, respectively.

Introduction

The demand for high-output light emitting diode (LED) packages has increased with growing demand for LED lighting and diversified fields of application. Increasing the package light output has the advantage of enhancing light conversion efficiency [1], [2], [3]. At the same time, energy other than light, which is mostly released as heat, now accounts for 75–85% of the total energy applied. As detailed in numerous, recent studies on LED light output, this means that the visible light conversion efficiency of incandescent light bulbs has been exceeded. By securing the highest visible light energy conversion efficiency, LED lighting is expected to be widely utilized in the lighting markets of the near future. However, the greatest disadvantage of the LED is that 75–85% of the total energy applied is still expelled as heat. Thus, for further advancement, the first obstacle to overcome is heat transfer [4], [5], [6].

The structure of conventional LED lighting has seven complex thermal nodes, including the LED package thermal node and the metal printed circuit board (PCB) thermal node. Heat transfer generated from the chip during LED operation is difficult for this structure. Due to the complex manufacturing process of conventional LED modules and the structurally complex thermal node, in order to improve the heat dissipation characteristics, chip-on-board (COB) packages are being developed [7], [8], [9], [10], [11]. Many research studies have been done on the thermal characteristics of the high-power multi-chip LED packages [8], [9]. The thermal mode analysis is used to optimize the thermal management with optimal locations and chip sizes for multi-chip package by Lai et al. [11]. Lee et al. have reported the low temperature co-fired ceramic chip-on-board (LTCC-COB) package with improved thermal characteristics without an insulation layer between the LED chip and metal base [10]. However, metal core PCBs generally used for COB packages have an isolation layer of low thermal conductivity between the chip mounting surface and aluminum metal PCB, which interferes with the heat transfer from the chip to the outside during LED operation. So, while the COB package has fewer thermal nodes, the low thermal conductivity of the isolation layer of the metal PCB hinders the heat transfer produced from the chip. Consequently, this impacts on LED reliability and lifespan. The ceramic substrate was introduced as part of the research to replace the dielectric layer having a low thermal conductivity [7]. However, ceramic substrate is often applied in small-scale applications due to the high cost. To overcome the low thermal conductivity of the isolation layer problems, silicon substrate was introduced in this paper which has the advantages of outstanding thermal conductivity, improving throughput with wafer-level packaging (WLP) technology, and reducing package material cost (lower cost module than AlN sub-mount). The silicon-based LED sub-mount technology has been introduced for reference [12]. This research shows that silicon LED sub-mounts lead to better thermal dissipation performance than do Al2O3 ceramic sub-mounts, and reveal acceptable insulation performance and high temperature reliability for silicon sub-mounts.

In this paper, a silicon sub-mount based LED package is proposed which employs a different method compared to the conventional COB package. This new package uses a silicon substrate on the base attached to the chip to reduce cost and to maximize heat transfer from the chip in order to improve reliability and lifespan. The LED package was designed with a structure that incorporates the advantages of the PCB based COB and silicon sub-mount, using the base as the substrate. Comparative analysis was conducted for the light output according to the input current, thermal resistance of the sub-mount package with Ag and Al reflective films, and thermal resistance of the conventional COB package. The silicon sub-mount package, however, still has very limited benefits. At present, silicon substrate does not bear the high-voltage power easily, and the manufacturing process of the silicon sub-mount package remains complicated. In addition, the market is also not mature, and the cost is more expensive than a conventional COB package. However, the cost of the silicon sub-mount package is similar to the cost of the package using chip-on-metal (COM) directly to the substrate to Al [13]. The cost of the Al substrate including the COM package is reasonable, although, the cost becomes more expensive, though, when adopting a high-gloss reflector to improve efficiency. The manufacturing process of the silicon sub-mount package is not over complex in that only one more step is added to the process. Furthermore, the silicon sub-mount package will be smaller because the technology can be integrated directly in the driving circuit itself.

Section snippets

Fundamentals of thermal properties

Thermal resistance in high-output LED packages is a critical factor affecting its reliability. It hinders the flow of heat between bonding interfaces from high temperature to low temperature. Moreover, thermal resistance is directly related to the chip junction temperature which can determine the reliability and lifespan of the LED device. Since temperature increase due to incomplete radiation deforms constituent materials or deteriorates the sealant and reliability of the chip itself, research

Design of LED package

The LED package was designed to combine the advantages of the COB based on a conventional metal PCB and silicon sub-mount using the base as the substrate. The metal PCB currently used structurally comprises an isolation layer which is composed of a material with very low thermal conductivity, of around 2–3 W/mk. This structure makes the release of heat generated from the package during LED operation to the outside difficult. Additionally, the junction between the LED package and metal PCB formed

Experimental results and discussion

The optical output properties of the silicon sub-mount based COB package with an Ag reflective film was measured using a 25 inch integrating sphere, installed spectrometer, and constant current source meter to apply an input current. Fig. 8(a) shows the change in flux according to the application of input current from 500–1000 mA. Fig. 8(a) reveals that at the forward current of 800 mA, the efficacy is approximately 112 lm/W. The increase in forward current results in the gradual increase in flux

Conclusions

In this paper, a silicon sub-mount based COB package was proposed which can rapidly transfer heat from the chip to a secondary cold-plate by removing the thermal resistance section of the metal PCB isolation layer and the thermal resistance section of the conventional PLCC package. This design maximized heat release from the chip, and lowered cost, by using a silicon substrate for the base upon which the chip is mounted. The silicon sub-mount based COB package with Ag reflective film had 367.1 

Acknowledgment

This work was supported by the Human Resources Development program (No. 20154030200940) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy. Also, this work was supported by the Regional Specialized Industry Development Project R&D (No. R0002504) funded by the Korea Government Ministry of Trade, Industry and Energy.

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