Fluid–solid coupling thermo-mechanical analysis of high power LED package during thermal shock testing

The virtual design by numerical simulation to model various accelerated reliability testing conditions is adopted to validate and improve the reliability of the high power LED package. In this study, the reliability of the high power LED package during thermal shock testing is investigated by fluid–solid coupling thermo-mechanical modeling by considering nonlinear time and temperature dependent material properties. Through fluid–solid coupling transient thermal transfer analysis, it is found that the maximum thermal gradient exceeds 75 K during the rapid cooling process and 91 K during the rapid heating process of the thermal shock testing which is ignored in the traditional isothermal assumption. The calculation results indicate that the equivalent plastic strain range of the bonding wire within the LED package with consideration of the temperature gradient is much higher than that with the isothermal assumption. The assumption of the isothermal condition is not appropriate which will lead to overestimation of the predicted lifetime. The viscoelastic behaviors of the silicone have significant influences on the lifetime prediction of the bonding wire and silicone with low elastic modulus and coefficient of thermal expansion (CTE) can significantly enhance the reliability of the bonding wire under the thermal shock loading. The results in this study could provide a guideline on design for reliability in the high power LED packaging.     

Early degradation of high power packaged LEDs under humid conditions and its recovery — Myth of reliability rejuvenation

A sharp rise in lumen degradation was observed for packaged high power LEDs during the initial period of operation under high humidity and temperature conditions, and the degradation reaches a peak value, followed by a “recovery” in lumen output, a sign of reliability rejuvenation. The time to reach the peak degradation is shorter with higher relative humidity. Scanning acoustic microscopy (SAM) tomography is employed to study the effect of moisture at different time intervals. With the help of moisture diffusion modeling using ANSYS simulation, the phenomenon is found to be due to the increasing moisture absorption of silicone resulting in subsequent light scattering as light is emitting from the dice. The “recovery” is the result of moisture absorption by die attach material that sucks the moisture from the silicone. Thus the “recovery” of lumen degradation is actually associated with the degradation in the internal structure of the LED package which is not reversible. C-SAM results are in accordance with the simulation and experimental results. The implication of this finding on temperature–humidity test of high power LEDs is described, and the material parameters of silicone to reduce this initial degradation are also presented.     

Research on lumen depreciation related to LED packages by in-situ measurement method

In this paper, lumen depreciation of LED in reliability experiment was monitored by in-situ measurement method. The partial LED flux on the receiving surface of fiber cable was captured, and it was proportional to the total luminous flux of LED light source when we provided an exact distance. The high temperature operating life test was used to find the weakness elements of LED packages with a limiting maximum temperature stress of 125 °C. Four kinds of packaged samples were constituted with difference components, and the lumen depreciations were presented. Combined with the lumen depreciation data and sampling inspection, the results could be summed up as follows: (i) the luminous flux of LED chip had a steady and slow depreciation, however, that of the samples coated with the phosphor–silicone composites had an initial sharp decline and then reach the stable state. (ii) The samples of only chip encapsulated by silicone and those of commercial white LEDs were carbonized on the center surface between chip and materials of encapsulant. As a conclusion, the silicone as an LED encapsulant could induce flaws, the material properties in larger coefficient of thermal expansion (CTE) and stronger adhesion should be considered in the package design stage, and the degradation of phosphor–silicone composites led to a fast light energy loss during the initial high temperature aging test, and then reached up to steady.     

Analysis of electrical parameters of InGaN-based LED packages with aging

As the light-emitting diode (LED) becomes a mature technology in the general illumination space, there is a tendency to operate LEDs at high current densities and temperatures in order to gain higher light output at lower cost. Further, there is interest among intelligent-lighting platform developers to offer predictive maintenance capabilities to users. The existing useful life prediction model defines LED lifetime based on parametric failure; however, there is a need for a useful life prediction model based on catastrophic failure, which can occur with the degradation of components in an LED package. Electrical parameters, especially package series resistance, are good indicators of LED package health (i.e., remaining useful life) and could potentially be sensed real-time in an application. In this study, the series resistance variation pattern until catastrophic failure was measured at different current and temperature stress conditions. The degradation mechanisms at each phase of variation were explained and, using available models, activation energies and exponents were extracted. The experimental data suggest electromigration-induced metal migration from the contact metallization layer to the semiconductor is the cause of short circuit catastrophic failure of LED packages. The variation patterns of ideality factor and reverse leakage current support this hypothesis. The information presented can be used to develop a catastrophic life estimation model for LED packages under current and temperature stress.     

Prediction of the epoxy moulding compound aging effect on package reliability

Most semi-conductor devices are encapsulated by epoxy moulding compound (EMC) material. Even after curing at the prescribed temperature and time in accordance with the supplier’s curing specifications often the product is not yet 100% fully cured. As a consequence, the curing process of a product continues much longer, leading to curing effects of the EMC during the lifetime of the package. In this paper, the effect of EMC curing during lifetime on package reliability is investigated. The visco-elastic mechanical properties of two commercial EMC materials are measured as a function of aging time. The resulting data is used to construct material models that are used in FE calculations. Aging effects on critical semi-conductor failure modes die cracking, compound cracking, wedge break, and delamination are addressed. Die and compound crack risks are predicted by common stress analysis. The risk of wedge break occurrence is investigated by detailed 3D modeling of the actual wires in the package using a global-local approach. Conclusions on delamination risks are made based on a parameter sensitivity analysis using a 3D cohesive zones approach to predict transient delamination. The package reliability study shows that the effect of EMC aging affects relevant failure modes in different ways.     

大功率白光LED封装设计与研究进展

封装设计、材料和结构的不断创新使发光二极管(LED)性能不断提高.从光学、热学、电学、机械、可靠性等方面,详细评述了大功率白光LED封装的设计和研究进展,并对封装材料和工艺进行了具体介绍.提出LED的封装设计应与芯片设计同时进行,并且需要对光、热、电、结构等性能统一考虑.在封装过程中,虽然材料(散热基板、荧光粉、灌封胶)选择很重要,但封装工艺(界面热阻、封装应力)对LED光效和可靠性影响也很大.     

Fabrication process simulation and reliability improvement of high-brightness LEDs

To enhance the light extraction efficiency and thermal performance of AlGaInP light-emitting diodes (LEDs), the wafer bonding technique which can replace the GaAs substrate with other high thermal conductivity substrates was applied. However, this technique may make the film crack during either the removal etching process of the GaAs substrate or the annealing process after the GaAs removal. Therefore, this crack problem is an important issue in the reliability/yield of high-brightness LEDs. In this research, a detailed finite element model of the high-brightness AlGaInP LED, which is replaced by the GaAs substrate with high thermal conductivity substrate through the Au–In metal bonding technique, was developed and fabricated. In addition, the mechanical behavior of wafer-level metal bonding was also simulated by finite element analysis (FEA) and validated by experimental measurements. Hence, the above validated simulation technique combined with process modeling is used to understand the stress variation of the multilayer structure of AlGaInP LED during the fabrication process and to find the principal cause of the film crack.     

大功率白光LED封装技术可靠性研究

首先介绍了大功率白光LED封装的前景和其主要功能,然后对大功率白光LED封装的关键技术,包括荧光胶封装工艺、外封胶选取、大尺寸晶片封装、可靠性测试与评估方面做了阐述。并对光斑改善和光通量提高做了一些具体的研究。     

Design methodologies for reliability of SSL LED boards

This work presents a comparison of various LED board technologies from thermal, mechanical and reliability point of view provided by an accurate 3-D modelling. LED boards are proposed as a possible technology replacement of FR4 LED boards used in 400 lumen retrofit SSL lamps. Presented design methodology can be used for other high power SSL lamp designs. The performance of new LED board designs were evaluated by numerical modeling. Modeling methodology was proven by measurement on reference FR4 LED board. Thermal performance was compared by extracting of LED boards thermal resistances and thermal stress has been inspected considering the widest temperature operating range according to standards (−40 to +125 °C). Thermo-mechanical and reliability analysis have been performed to study parameters of each LED board technology, using thermal boundary conditions extracted from the thermal simulation of a whole LED lamp. Elastic–plastic analysis with temperature dependent stress–strain material properties has been performed. The objective of the work is to optimize not only the thermal management by thermal simulation of LED boards, but also to find potential problems from mechanical failure point of view and to present a methodology to design SSL LED boards for reliability.     

Crack-guided effect on dynamic mechanical stress for foldable low temperature polycrystalline silicon thin film transistors

In the thin film transistors (TFTs) device research for foldable display, the degradation effect by the mechanical stress is crucial. Here, the crack position is critical for TFT reliability. However, it is difficult to characterize the crack position due to the random generation of the crack by mechanical stress. In this paper, the crack-guided low temperature polycrystalline silicon (LTPS) TFT test structures are fabricated and the crack-guided effects on mechanical stress of the tested TFT structure are analyzed. To strain on the foldable LTPS TFTs, 50,000 cycles of tensile and parallel direction dynamic mechanical stresses were applied with 2.5-mm bending radius. Based on the results, the generating crack position can be guided and controlled and also TFT reliability for foldable display can be enhanced.     

层压封装平面LED光源的耐候实验

研究了层压封装的平面LED光源在高温高湿与水下环境的可靠性。平面LED光源采用标准层压工艺封装,对封装后的LED模组进行高温高湿耐候试验与水下环境试验,并与未封装的LED模组进行对比实验。实验结果表明,在环境温度为80℃、相对湿度为80%,模组工作电流为300 mA,连续33天高温高湿条件下,层压封装的平面LED模组的照度变化和温度均高于未封装的LED模组。在40℃水下环境下连续工作400 h,层压封装的平面LED模组的照度略有变化,且光衰小于1%。因此,层压封装能有效阻断外界高温高湿环境对LED模组可靠性的影响,更适合在常温水下照明应用。     

A Review on the Physical Mechanisms That Limit the Reliability of GaN-Based LEDs

We review the failure modes and mechanisms of gallium nitride (GaN)-based light-emitting diodes (LEDs). A number of reliability tests are presented, and specific degradation mechanisms of state-of-the-art LED structures are analyzed. In particular, we report recent results concerning the following issues: 1) the degradation of the active layer induced by direct current stress due to the increase in nonradiative recombination; 2) the degradation of LEDs submitted to reverse-bias stress tests; 3) the catastrophic failure of advanced LED structures related to electrostatic discharge events; 4) the degradation of the ohmic contacts of GaN-based LEDs; and 5) the degradation of the optical properties of the package/phosphors system of white LEDs. The presented results provide important information on the weaknesses of LED technology and on the design of procedures for reliability evaluation. Results are compared with literature data throughout the text.     

Mean-time-to-failure evaluations of encapsulation materials for LED package in accelerated thermal tests

The mean-time-to-failure (MTTF) evaluation of encapsulation materials of LED package in accelerated thermal tests is presented. The Weibull and Arrhenius theory was used to characterize the thermal degradations of encapsulation materials for LED package in a quantitative investigation. The shape parameter, scale parameter, MTTF and activation energy (E_a) could be obtained to potentially provide clear values for comparison and understanding. The results showed that the glass as encapsulation material of LED modules exhibited better thermal stability than the silicone encapsulation by about 7 times in lumen loss, 2 times in chromaticity shift, and 3 times in correlated color temperature at 150 °C. Furthermore, an order magnitude MTTF improvement at room temperature was achieved in the glass encapsulation case.     

Correlation of flip chip underfill process parameters and materialproperties with in-process stress generation

Electronic packaging designs are moving toward fewer levels of packaging to enable miniaturization and to increase performance of electronic products. One such package design is flip chip on board (FCOB). In this method, the chip is attached face down directly to a printed wiring board (PWB). Since the package is comprised of dissimilar materials, the mechanical integrity of the flip chip during assembly and operation becomes an issue due to the coefficient of thermal expansion (CTE) mismatch between the chip, PWB, and interconnect materials. To overcome this problem, a rigid encapsulant (underfill) is introduced between the chip and the substrate. This reduces the effective CTE mismatch and reduces the effective stresses experienced by the solder interconnects. The presence of the underfill significantly improves long term reliability. The underfill material, however, does introduce a high level of mechanical stress in the silicon die. The stress in the assembly is a function of the assembly process, the underfill material, and the underfill cure process. Therefore, selection and processing of underfill material is critical to achieving the desired performance and reliability. The effect of underfill material on the mechanical stress induced in a flip chip assembly during cure was presented in previous publications. This paper studies the effect of the cure parameters on a selected commercial underfill and correlates these properties with the stress induced in flip chip assemblies during processing     

A statistical study to identify the effects of packaging structures on lumen reliability of LEDs

The development of high-power light-emitting diode (LED) devices has been bedeviled by the reliability problems. And most reliability issues are caused by the packaging materials rather than the chips. However, which packaging material is the most influential remains unrevealed. To answer this question, a statistical method was introduced in this paper. Optical simulations were conducted to calculate the optical output power of LED package according to the orthogonal experimental design. Range and variance analyses were carried out to determine the significance of the relevant factors on the LED's light output. The results showed that the dome lens among the non-luminescent packaging materials had the most significance in affecting the light output. It is concluded that this method is useful in detecting the most significant part of LED packaging materials during the development of new packaging structures and is beneficial for enhancing the whole reliability of LED package effectively.     

Integrated vapor pressure, hygroswelling, and thermo-mechanical stress modeling of QFN package during reflow with interfacial fracture mechanics analysis

In this paper, a comprehensive and integrated package stress model is established for quad flat non-lead package with detailed considerations of effects of moisture diffusion, heat transfer, thermo-mechanical stress, hygro-mechanical stress and vapor pressure induced during reflow. The critical plastic materials, i.e., moldcompound and die attach are characterized for hygroswelling and moisture properties, which are not easily available from material suppliers. The moisture absorption during preconditioning at JEDEC Level 1, and moisture desorption at various high temperatures are characterized. The moisture diffusivity is a few orders higher at reflow temperature than moisture preconditioning temperature. Due to coefficient of moisture expansion mismatch among various materials, hygro-mechanical stress is induced. The concept is analogous to coefficient of thermal expansion mismatch which results in thermo-mechanical stress. Thermal diffusivity is much faster than the moisture diffusivity. During reflow, the internal package reaches uniform temperature within a few seconds. The vapor pressure can be calculated based on the local moisture concentration after preconditioning. Results show that the vapor pressure saturates much faster than the moisture diffusion, and a near uniform vapor pressure is reached in the package. The vapor pressure introduces additional strain of the same order as the thermal strain and hygrostrain to the package. Subsequently, the interfacial fracture mechanics model is applied to study the effect of crack length on die/mold compound and die/die attach delamination.     

Color shift acceleration on mid-power LED packages

Mid-power LED packages are now widely used in many indoor illumination applications due to several advantages. Temperature stress, humidity stress and current stress were experimentally designed and performed to accelerate the color shift of mid-power LED packages and color shift mechanisms have been discussed based on the color shift results obtained from measurements. Conclusions could be drawn:

• -Linear function fitting demonstrates a good linear relationship between color shift (Δu′ Δv′) and aging time almost for all the aging conditions. We can extrapolate the color shift Δu′ and Δv′ based on the fitted regression equations and then make the prediction for the total color shift Δu′v′.
• -Current stress can induce a different failure mode. Peak intensity reduction analysis reveals that the current stress accelerates the degradation of LED die.
• -Humidity test induced a substantial color shift both in u′ and v′. The u′ has an increased degradation rate after aging of 3000 h at 85%RH & 85 °C, there should be different degradation mechanisms during the whole humidity test. The molecular structure decomposition of silicone plates and then follows the silicone carbonization due to the long-term (3000 h) accumulated high localized temperature aging.

     

Failure and degradation mechanisms of high-power white light emitting diodes

The investigation explores the factors that influence the long-term performance of high-power 1 W white light emitting diodes (LEDs). LEDs underwent an aging test in which they were exposed to various temperatures and electrical currents, to identify both their degradation mechanisms and the limitations of the LED chip and package materials. The degradation rates of luminous flux increased with electrical and thermal stresses. High electric stress induced surface and bulk defects in the LED chip during short-term aging, which rapidly increased the leakage current. Yellowing and cracking of the encapsulating lens were also important in package degradation at 0.7 A/85 °C and 0.7 A/55 °C. This degradation reduced the light extraction efficiency to an extent that is strongly related to junction temperature and the period of aging. Junction temperatures were measured at various stresses to determine the thermal contribution and the degradation mechanisms. The results provided a complete understanding of the degradation mechanisms of both chip and package, which is useful in designing highly reliable and long-lifetime LEDs.     

InGaA1P超高亮LED性能及可靠性

InGaAlP超高亮LED是近年来发展的新型可见光LED,具有发光效率高,电流承受能力强及耐温性能好等特点,应用于各种户外显示与照明装置,本文汇集各厂家InGaAlP超高亮LED芯片,并制成器件,对其多种性能进行对比分析,测试超高亮LED的发光强度与电流的关系以研究饱和电流的大小,并进行电耐久性试验以考核超高亮LED芯片的可靠性,简要介绍了封装工艺设计对超高亮LED性能参数的影响,并提出了超高亮LED性能及可靠性的其它要求,为客户选用超高亮LED提供相对的依据。     

Lifetime assessment of Bisphenol-A Polycarbonate (BPA-PC) plastic lens,used in LED-based products

In this investigation, the accelerated optical degradation of two different commercial Bisphenol-A Polycarbonate (BPA-PC) grades under elevated temperature stress is studied. The BPA-PC plates are used both in light conversion carriers in LED modules and encapsulants in LED packages. BPA-PC plates are exposed to temperatures in the range of 100–140 °C. Optical properties of the thermally-aged plates were studied using an integrated sphere. The results show that increasing the exposure time leads to degradation of BPA-PC optical properties, i.e. decrease of light transmission and increase in the yellowing index (YI). An exponential luminous decay model and Arrhenius equation are used to predict the lumen depreciation over different time and temperatures. Accelerated thermal stress tests together with the applied reliability model are used to predict the lifetime of plastic lens in LED lamps in real life conditions.