D.G. Yang

Highly accelerated life testing of LED luminaries

With rapid development of lighting emitting diode (LED) market, more people are focusing on reliability testing method of LED luminaries system. Based on the previous exploration, to assess the whole system reliability with a fast way, authors propose to divide the system into several subsystems, and then carry out step stress accelerated testing (SSAT) to get failure mechanism and reliability distribution of each subsystem, finally calculate the life distribution of the whole system by combining with system statistics analysis methodologies. The first step, the most important step, is to know the stress limit of the subsystem before conducting SSAT testing. The intention of normal highly accelerated life test (HALT) process is to subject the item under test to stimuli well beyond the expected field environments to determine its operating and destruct limits, but without enough attention on those degradation parameters of products, especially like LED products which present their life characteristics on many output parameters, such as lumen maintenance, color rendering index (CRI), chromaticity coordinate, junction temperature and optical efficiency, which included important life information. Therefore, Focused on the LED light source subsystem, it is not practical to accelerate it with HALT by only focused on failure or destructive data. In this paper, different HALT procedure with different objects is introduced, and two types of LED system from separated vendors are conducted proposed HALT before SSAT testing. Test results suggested that it is feasible to observe output parameters of LED products to decide the stress limitation during HALT testing, pseudo junction temperature and Lumen maintenance can be regarded well as key parameters. For other parameters of system, different parameter has changed with different rule, which indicates that key concerned factor should be fixed. Meanwhile, it is found that the lumen maintenance decreases very fast when temp stress over 110 °C. When focused on some failure mechanisms without change even if overloading the stress, a good solution for a fast qualification method will be worked out. Deep study on this found should be carried out in future.

Reliability study on high power LED with chip on board

In this paper the reliability of high power LED street lamp subjected to temperature cycle is analyzed. Two kinds of finite element models of LED street lamp structure with 60W input power are established by using ANSYS software. The temperature cycling condition from −5°Cto 50°C is loaded. The increasing and decreasing rate of temperature is 5°C / min and the holding time is 30 minutes. This process is consisted of 8 cycle periods and returns to room temperature 25°C for initial temperature. The results demonstrate the deformation of integrated LED street lamp structure can reach 2.17μm while that of LED lamp array is only 5nm. The results provide the basic for further study on LED street lamp failure mechanism and reliability design for LED street lamp.

Study on thermo-mechanical reliability of embedded chip during thermal cycle loading

With the development trend of microelectronic system with small size, high speed, high frequency and high density, passive and active components are directly embedded into a core or high-density-interconnect layers. This System-in-Package (SiP) technology could shorten interconnection between the die and substrate and reduce the inductance and noise interference. However, there are many electrical and mechanical reliability issues including the reliability issue for embedded structure. An embedded structure was chosen in this study. The embedded chip was surrounded by epoxy. An epoxy was selected as the adhesive to embed the chip. The active surface of the chip was face up, to form a planar surface with the substrate. Benzocylobutene (BCB) was chosen as the dielectric polymer for embedding technology because of its low curing temperature. One quarter 3D model of embedded structure was loaded on six thermal cycles according to the temperature cycling standards JESD22-A104C. The thermo-mechanical reliability was investigated and the modified Coffin-Manson equation was employed to predict the fatigue life of copper films. FEA simulation results revealed that the fatigue life of copper film is 35.7 cycles. Stresses in the die always lead to various failures in manufacturing and using process, so equivalent von Mises stress and peel stress were also analyzed in this study.

The effect of diaphragm on performance of MEMS pressure sensor packaging

MEMS (Micro Electro-Mechanical System) is a technology that offers significant advantages over various microscopic elecromechanical devices. In the field of MEMS products, pressure sensor represents a considerably mature technology and has been extensively used in a variety of applications. Nevertheless, packaging is a key issue among the processes of MEMS manufacture due to the specialty and complexity of MEMS devices. For isolated packaged MEMS pressure sensor, design and fabrication of insulated diaphragm structure is a key factor as it directly affects the accuracy and reliability of packaged pressure sensor. Thermal expansion will emerge among the parts of pressure sensor when working temperature is extremely high. In this paper, finite element method is applied to study the issue on thermal expansion of silicon oil under using flat diaphragm and corrugated diaphragm, respectively. Simultaneously, the volume expansion of silicon oil under using the two diaphragms is calculated. Through comparison, it can be found that the volume expansion of silicon oil under using corrugated diaphragm is less than that when using flat diaphragm along different path. Therefore, conclusion can be achieved that optimization of insulated diaphragm structure will be superior in realization of transmission of pressure almost without loss and relief of additional pressure to the sensor chip induced by thermal expansion of silicon oil.

A reliability analysis method for LED luminaires based on step stress accelerated degradation test

To consider the complex system integration of LED luminaires, a novel analysis method combined with reliability test is presented. Initially, the whole system is divided into three subsystems (Drive, fixture and LED light source) so that reliability test can be conducted on the objective subsystem with a stress level as high as possible. And further step stress accelerated degradation test (SSADT) is implemented on the light source subsystem, with a goal to achieve its reliability distribution function. Only the light source subsystem is accelerated inside the thermal chamber, and connected with other subsystems outside the chamber. And then the reliability of LED light source subsystem is extrapolated with the methodology model of SSADT. Finally, the whole LED system reliability is integrated reasonably with the help of Fault Tree and Monte Carlos Algorithm. The proposed method considers the stresslimit difference of subsystems, and solves the difficulty to assess the system reliability of LED luminaire with complex system integration. The result of an example case shows that the SSADT method is very effective based on the subsystem-isolation acceleration way, and that the whole LED system lifetime meets closely the lifetime remarked by vendor. The proposed method is expected to be very useful in future LED luminaire fast qualification test.

Effect of temperature and voltage on LED luminaries reliability

In this paper, the reliability of LED luminaries system after thermal-ageing test was investigated with experimental test and numerical method. Samples were aged at 45oC, 85oC and 95oC using step-stress accelerated test. The thermal resistance was measured and the failure modes were verified. After 1,692 hours, four categories of failure modes were found and main failure mechanism was analysed. The total thermal resistance of sample C and D increased by 3.24 K/W and 5.09 K/W. The thermal stress is about 687 MPa, 1,106 MPa and 1,116 MPa at different ageing temperature.

Spreading Resistance Analysis of LED by Structure Function

In this paper, an investigation on structure function of LED for thermal resistance testing is presented. LED was placed on aluminum plate in different locations. Four group experiments were applied to determine thermal resistance of LED. A simple and effective solution is proposed and discussed on the spreading resistance. The experimental results show the relationship between junction temperature and different locations.

Prediction of die failure in copper-low-k flip chip package with consideration of packaging process-induced stresses

In Flip Chip package, the curing process of the underfill polymer will induce extra residual stress and strain fields. For simplicity reasons, in thermo-mechanical analyses, the curing induced stress state was usually neglected by assuming a so-called “stress-free” temperature. However, such simplification is not verified, in particular for advanced IC chips such as copper-low-k interconnects, which is very sensitive to the stress level it undergoes. An investigation on the die failure issues in copper-low-k Flip Chip Package with consideration of packaging process-induced stresses was presented in this paper. Firstly, a cure-dependent viscoelastic model was applied to describe the properties of the underfill resin during the curing process and subsequent thermal cycling. Secondly, prediction of die fracture failure probability was conducted. Weibull statistics model was used to describe the probability distribution for the die strength test. Model parameters were obtained by fitting to the test results. Fracture failure probability of the die backside was calculated based on the Weibull statistics model and the stress states induced in the curing processes and test condition. Thirdly, the stress state on the copper-low-k layer was investigated. The results show that maximum stress occurs at top interface of the low-k layer structure. The cure-induced stresses play a significant role on the total stress level. The effect of the packaging process-induced stress cannot be simply neglected.

Heat transfer analysis of vapor chamber heat pipe for high power LED package

In this paper, the high power LED system dissipation equipment is analyzed, in this LED system,consisting of nine chips and each chip is 3W. The novel dissipation equipment is design and analyzed, which is composed of vapor chamber, heat pipe array and fins. Vapor chamber as the thermal diffusion layer, it is effectively reduces the spreading resistance from LED chips to heat pipe. The other two models were built as the comparison. The designs were then being calculated on its transfer behavior using simulation with professional thermal analysis software Flotherm, simulation results show that the novel design has better heat dissipation ability than the other two comparison models, and the junction temperature is 71.6° C at the natural convection condition, after optimization of the fins, it is drop to 65.6° C.

Reliability study on high density LED packaging with Chip On Board structures

In this paper an improved LED (Light emitting diode) packaging structure was proposed, which is based on COB (Chip On Board) technology. This research mainly analyzed the thermal reliability of a 4×4 square LED die array with COB technology during temperature load. Simple intuitive for observation, two specific red glue curing profiles were compared with different curing temperature, curing time, temperature rate, and room temperature holding time, and the influences of the temperature load to LED packaging were compared with and without consideration red glue curing before the subsequent thermal cycles loaded. Last the prediction of the Silicon substrate failure was carried on referring to the Weibull distribution. The light system consists of sixteen chips that each chip is 1.2W. Every die in the LED array can be regarded as working under the same condition, the heat distribution in each die is uniform. 3D finite element model of the LED packaging was modeled.