Moldless encapsulation for LED wafer level packaging using integrated DRIE trenches

doi:10.1016/j.microrel.2012.01.007

Microelectronics Reliability

Volume 52, Issue 5, May 2012, Pages 922-932

https://doi.org/10.1016/j.microrel.2012.01.007 Get rights and content

Abstract

This paper demonstrates a LED wafer level packaging process which employs the glob-top dispensing technique for encapsulation. The process utilizes the constraint effect introduced by the trenches to limit the spreading of encapsulant. This enables the geometry control of encapsulation. Several design and process parameters have been investigated. The study has considered the effect of the trench patterns. A 4-in. silicon wafer is fabricated with a pattern etched by the DRIE process. It serves as a substrate for an LED array employed in the present study. Using the wafer substrate and the glop-top dispensing technique, wafer level LED packaging incorporated with a moldless encapsulation process is realized.

Introduction

The light emitting diodes (LEDs) have been widely used since its invention in 1962. In its early age, LEDs could only emit red light. The power of the LED chips usually was below 0.1 W. Therefore, the conventional low-power LEDs normally were only used for decoration, signal indicators, or message display panels. They were not suitable for the general illumination applications. Afterwards, the AlInGaN blue and green semiconductors technologies were developed. It became feasible for LEDs to generate the white light [1], [2]. In the past few years, LEDs enjoyed a rapid development regarding its efficiency, power consumption, and light emission spectrum. These improvements further promoted the applications of LEDs. Nowadays, LEDs are essential elements in mobile phones, automobiles, display panels, traffic lights, and general illuminations. The High-brightness LEDs (HB-LEDs) have become a promising candidate for the solid-state lighting (SSL) [3], [4], [5].

There are two major factors leading to the prevalence of HB-LEDs for SSL applications. One is the optoelectronic efficacy. The other one is the cost. Both factors are closely related to the packaging technologies of LEDs. Currently most LEDs are packaged individually. Such a component level packaging process has low throughput, in addition to its relatively high cost. A more efficient packaging process, therefore, is in demand in the LED industry.

The demand has steered people to develop a wafer level packaging (WLP) process for LED fabrication. A silicone-based packaging platform for LED arrays was introduced by Sou and Huang [6]. A wafer level printing process for yellow phosphor coating for the white light LEDs was presented by Lee and Lee [7]. In order to realize WLP for LEDs, a new encapsulation process is needed. Currently the encapsulation processes are component based. This is not suitable for WLP. Chang-Hyun Lim, Won-Kyu Jeung and Seog-Moon Choi demonstrated a wafer level process to fabricate LED lens in batches, but it still required a molding process for encapsulation [8]. A moldless activated curing process was developed for the encapsulation of LEDs [9], [10]. However, the process was component-based. A comprehensive wafer level LED packaging process has not been achieved yet.

The objective of this study is to develop a wafer level LED packaging process which enables the whole packaging process of LED arrays to be directly completed on a wafer. Fig. 1 shows the conceptual process flow. A whole piece of wafer is first fabricated with the necessary patterns for a subsequent flip-chip mounting process. This wafer serves as a substrate for an LED array. The LED array is flip-chip mounted on the wafer substrate and then encapsulated, as shown in Fig. 1a. After packaging, the wafer is singulated into individual functional LED packages as shown in Fig. 1b and c. In this packaging process, a glob-top dispensing method is applied to perform the encapsulation. This eliminates the encapsulation mold required in a conventional process and hence, a moldless process can be accomplished.

In order to control the geometry of encapsulation in the moldless dispensing process, the constraint effect of the trenches is utilized to limit the spread of encapsulant on the wafer. An investigation is performed to evaluate several design and process parameters, which consider the constraint effect of trenches, the difference between a single-line trench pattern and a double-line trench pattern. A 4-in. wafer is fabricated with trenches etched by the deep-reaction-ion-etching (DRIE) process. It serves as a substrate for LED arrays. The objective of this prototyping exercise is to prove that WLP for LED arrays cane be achieved using the current moldless encapsulation technology.

Section snippets

Experimental setup

There were two sets of experiments studied. One set was about the control of the glob-top dispensing process. It was performed on some dummy wafers which bear no LED chip. The other set was about the application of the glob-top dispensing process for the fabrication of the wafer substrate which contained the LED arrays.

Glob-top dispensing process on dummy wafers

Due to high viscosity, the epoxy droplets spread very slowly on the wafer. It took more than 30 min to complete the spreading process and to attain a stable contact angle (i.e. the steady state). The result is shown in Fig. 7. It can be seen that, the final contact angle of the droplet is 15.0°. Since the ratio H/L is completely determined by the contact angle θ for the free-spread droplet, the geometry of the droplet is very flat.

Fig. 8 shows the final state of an epoxy droplet which has cured

Conclusions

The glop-top dispensing process on some dummy wafers has been studied. The constraint effects due to a flat surface, the single-line trench patterns, and the double-line trench patterns have been investigated. The results show that the contact angle of the droplets on a flat surface is too small to form ideal dome-shaped encapsulation. Some kinds of constraint are necessary to obtain encapsulation with a high H/L value.

The single-line trench and double-line trench patterns can bind the

Acknowledgments

This study was sponsored by the Hong Kong University of Science and Technology (HKUST) through an internal grant of RPC06-07.EG28. The authors wish to acknowledge this sponsorship. Also, the LED chips in this study were provided by the Advanced Packaging Technology (APT) Ltd. The authors would like to thank APT for their support.

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Introductory Invited PaperMoldless encapsulation for LED wafer level packaging using integrated DRIE trenches

Abstract

Introduction

Section snippets

Experimental setup

Glob-top dispensing process on dummy wafers

Conclusions

Acknowledgments

Illumination with solid state lighting technology

IEEE J Sel Top Quantum Electron

Red, green, and blue LEDs for white light illumination

IEEE J Sel Top Quantum Electron

LEDs challenge the incandescents

IEEE Circuits Devices Mag

Solid-state lighting: toward superior illumination

Proc IEEE

Life of LED-based white light sources

IEEE/OSA J Disp Technol

Silicon-based packaging platform for light emitting diode

IEEE Trans Adv Packag

Introductory Invited Paper
Moldless encapsulation for LED wafer level packaging using integrated DRIE trenches