Strength determination of high-power LED die using point-load and line-load tests

Abstract

The strength of high-power light emitting diode (LED) dies, cut from wafers with a laser, has to be determined for the need of design and quality control in order to assure the good reliability of packages in manufacturing and service. The objective of this study is to determine the strength of high-power LED die with a size of 1 × 1 × 0.1 mm³ by point-load test (PLT) and line-load test (LLT) associated with a plate-on-elastic-foundation configuration. ANSYS (one of commercial finite element codes) analysis is used to calculate the stress distributions of the die under both PLT and LLT. The ANSYS models of the PLT and LLT are validated by comparing with experimental force–displacement curves, and the results are further used to convert the die failure force from the tests into the die strength. The mechanism of tensile-stress dominated die strength has been discussed and validated in detail via these test results and analyses. The results of the PLT and LLT also indicate that for the die failure on chip surface, the average die strengths are about 1.44 GPa and 1.52 GPa from the PLTs with two different-radius pins, and about 1.2 GPa from the LLT. On the other hand, for failures on sapphire surface, the average die strengths are reasonably about 1.49 GPa and 1.26 GPa from the two PLTs, but the average one from the LLT is about 0.64 GPa (with less than 50% of the values from the PLT). The inconsistent data between two PLT and LLT for failure on sapphire surfaces were found to result from the edge chipping of the die specimen observed by scanning electron microscopy. It was also observed that the thin-layer GaN material has to be taken into account in the ANSYS analyses with a bi-material model of the LED die for precisely determining the die strength for failure on the chip surface. Otherwise, these strength data would be overestimated by a few tens of percent with a uni-material model of the LED die. All in all, this study has successfully demonstrated that the LED die strength can be determined by these feasible, easy-to-use and reliable test methods.

Introduction

High-power light emitting diodes (LEDs) have gradually gained popularity in many lighting applications today such as general illumination, automobile headlight and taillight, and backlighting in LCD displays and TVs. In these applications, the high-power LED dies could be subjected to mechanical, thermal, and environmental loads during packaging manufacturing processes (such as die bonding, wire bonding and encapsulation) and services. Therefore, the strength of LED dies, cut from the wafers after manufacturing processes (such as wafer grinding and laser dicing), has to be determined for the need to assure good design and reliability of the LED packages. In the literature, a ball-breaker test proposed by Hawkins et al. [1] was used to determine the strength of silicon die from the wafers after different grinding and polishing processes [2]. And further different test methods (including three-point bending, four-point bending and ball-breaker tests) [3] were evaluated for effectively determining the die strength. A ring-on-ring and four-point bending tests associated with Weibull probability analysis were employed for determining the die strength in an attempt to separate the surface grinding and edge dicing effects [4]. Recently, the geometry and sharpness of scratch tips for a few large and deep scratches created by wafer thinning process were found to significantly affect the strength data according to the three-point bending test results [5]. Two new test methods: the point-load test (PLT) and line-load test (LLT) have been proposed, and the silicon die strength has been determined [6], [7], [8]. It is found that there are four factors that influence the die strength: the surface conditions of the die (including grinding-mark direction and surface roughness), the edge crack of the die (so-called chipping created during the wafer sawing process), the weak plane of the crystal lattice of silicon, and, sometimes, the test methods with different loading types. Most recently, to precisely determine the die strength, the PLT has been reevaluated and improved by ANSYS analysis with contact-element models and the related formulation, which is associated with Hertzian contact theory [9]. However, from the literature, there is no report available on the determination of LED die strength. A typical high-power LED die with the size of 1 × 1 × 0.1 mm³ is so relatively tiny that the determination of the die strength by either convectional three-point bending, or four-point bending, or ring-on-ring tests would become very challenging and even impossible. Thus, this study aims to determine the strength of high-power LED dies by the PLTs with two different pin radii (0.063 mm and 0.25 mm), and LLT, associated with a plate-on-elastic-foundation configuration. The strength of the LED dies can be determined by these tests combined with ANSYS analyses, which are used to convert the die failure force into the die strength. Finally, an attempt is made to evaluate the factors that affect the variation of the die strength data after the tests.