Failure analysis on reflector blackening between lead frame electrodes in LEDs under WHTOL test

https://doi.org/10.1016/j.microrel.2015.01.004Get rights and content

Highlights

  • We report a new blackening failure mode of commercial LEDs.

  • Delamination and corrosive de-flash agent are the root causes of the failure.

  • Reflector cup corrosion and Ag migration are the features of the failure.

  • The effect of the blackening failure to LED performance is estimated.

Abstract

Blackening induced lumen decay in a QFN LED after WHTOL reliability test was reported and analyzed in this paper. A new LED blackening failure mechanism was proposed based on solid experimental results. We concluded that the failure process underwent delamination between lead frame and reflector polymer composites followed by chemical penetration, composite corrosion, silver migration, and finally caused blackening failure. Delamination and corrosive de-flash agent were the key factors for the failure mode. Besides, we also estimated the influence of the failure to the optical performance through simulation. Apart from other reported factors, this study highlighted that both composite corrosion and Ag migration could generate serious illumination decrease as well. The outcome of this study is valuable for LED manufacture and quality control in the future.

Introduction

WHTOL (wet and high temperature operating life) test is one of the standard tests applied in industry to evaluate product quality and reliability. Specializing in simulating failures induced by high temperature moisture, it is especially widely used in integrated circuit (IC) and light-emitting diode (LED) package industry [1], [2], [3]. During the test, the high temperature moisture can deteriorate adhesion between different materials at the interface [4]. Meanwhile, the thermal and hygro-mechanical stresses can significantly increase the possibility of delamination. The “First law of plastic packaging” concluded the importance of avoiding delamination [5]: “Perfect adhesion at any interface minimizes failure”. In another word, delamination is the dominant factor to packaging failure [6].

Up to now, various kinds of failure phenomena have been observed after WHTOL test [1], [2], [6], [7]. Among them, LED blackening is one of the most common failure phenomena. However, although the appearances of blackening failures are broadly similar [6], details like blackening position, morphology, composition and impact to device’s performance vary case to case. Such complexity indicates that the failure evolutions among all the cases can be quite different.

For general studies, LED blackening issues are reviewed and classified by failed components. The most typical one is the silver (Ag) plating on lead frame. Regularly, blackening on lead frame surface was caused by corrosion. Sulfur (S) or halogen elements from environment could react with Ag and severely damage the lead frame surface [8], [9]. The penetration paths of contaminants could be delamination at interfaces or the encapsulant. For certain lead frame structures, Cu diffusion was another reason that led to lead frame blackening [10]. This kind of blackening was emerged by the low reflectance of the diffused Cu on lead frame surface. Apart from this, blackening on encapsulant had been reported as well [11], [12]. With this failure mode, the delamination between encapsulant and chip could trap the light energy and cause encapsulant carbonization. The carbon residue directly worsened the optical performance [11]. Besides, die attach blackening had also been noticed [12]. For this mode, the bad heat diffusivity caused by delamination at chip and die attach interface not only resulted in die attach carbonization, but also generated encapsulant degradation and chip quenching. Eventually, blackening could be found around the chip. Other than these package level failure, failures at chip level had also been studied [13], [14]. In addition, due to optical importance, novel materials for reflector cup to avoid material degradation are continuously developing [15]. However, objectives are mainly on eliminating the effects of thermal aging and UV irradiation, other factors such as chemical corrosion have not been considered or systematically studied before.

On the other hand, electromigration has seldom been aware of relating to LED blackening. For WHTOL reliability test, the typical electromigration is known as electrolytic electromigration, or ion electromigration [16], [17]. It defines the phenomenon that Ag and certain amount of copper (Cu) are especially easy to migrate under biased DC voltage under WHTOL conditions [16], [18]. In fact, this phenomenon has been noticed and analyzed in IC industries for years. However, most LED researchers simply followed the steps of those IC engineers and focused on electrical properties [1], [2], [3]. The effect on optical performance was ignored. However, the lifetime of a LED device is defined by its lumen decrease percentage [19]. The optical output is also very sensitive to structure changes induced by electromigration besides electrical changes. Thus, evaluating the effect of such failure to LED performance is of great urgent.

This article stem from the suspicion on the abnormal blackening position of the failed QFN samples. Through nondestructive and destructive methods widely used in failure analysis, various phenomena such as delamination, penetration, matrix corrosion and silver migration were discovered. A new failure mode was proposed through integrating the processing technic and logically arranged clues. The corresponding failure evolution stages were also sketched. In addition, the impact of reflector matrix corrosion and Ag electrolytic electromigration to illumination output was evaluated through optical simulation as well. To prevent such delamination, enhancing the adhesion between reflector composite and lead frame, introducing interlocking structures and optimizing singulation parameters are proved as effective ways. Thus, this study is supposed to be beneficial for improving LED manufacturing quality.

Section snippets

Materials, test conditions and simulation

Commercial QFN (Quad Flat No-lead)-LEDs (2016 package) for general light applications were obtained from our suppliers. The samples were fabricated within the same batch in order to minimize systematic deviation. Methyl phenyl silicone with high refractive index (>1.5) was used as encapsulant. A transparent silicone was selected as the die attach. The emitting wavelength of the chip was 460 nm. The refractive indices of the sapphire, multi-quantum well (MQW) p-GaN, n-GaN, AlGaN and GaN were set

Experimental results analysis

Besides the cup wall, the reflection structure consists of three parts, which are (1) anode lead frame, (2) reflector composite between electrodes, (3) cathode lead frame, as shown in Fig. 1(a). Abnormal blackening after WHTOL tests was observed between two lead frame electrodes, as shown in Fig. 1(b1). Compared with the typical appearance of lead frame corrosion which was shown in Fig. 1(b2), such blackening position was quite strange. In order to clarify the failure mechanism, a series of

Conclusion

Starting from the suspension on abnormal blackening, a LED failure mode under WHTOL reliability test condition was discussed in this paper. Based on the characterization of LED morphology and components before and after test, a novel failure evolution of LED blackening was purposed. Delamination and de-flash chemical residue were the prime factors while reflector corrosion and Ag electrolytic electromigration were the following consequences. Meanwhile, as simulation results indicated, the

Acknowledgments

The authors would like to acknowledge the technical supports on SEM and EDS from Material Characterization and Preparation Facility (MCPF), The Hong Kong University of Science and Technology.

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