Research On Thermal Reliability And Lifetime Prediction Of High Power LED Packaging

High power light-emitting diode(LED)has been widely used in people's daily life due to its advantages of high luminous efficiency,long lifetime,low power consumption and environmental friendliness.It is believed that LED will be the next generation light source.In general,the life of a LED product can be as long as tens of thousands of hours.Conducting life test under normal working conditions can cost a lot and there exist many uncontrollable factors during the process.Thus,it is nearly impossible to estimate LED's life effectively by normal life testing method.In addition,in practical use,the LED devices would encounter harsh environmental conditions such as high temperature,high humidity,vibration,thermal shock and so on.These conditions would enlarge the defects,accelerate the degradation of the materials,worsen the reliability and shorten the useful life of the LED devices.Therefore,it is of great importance to study the reliability of high power LED and evaluate its life effectively.In this dissertation,the effects of the packaging technologies and materials on LED's reliability and failure analysis and the accelerated life prediction of high power LED module were investigated.The main work and achievements are as follows:1)An online testing method for accelerated thermal reliability test of high power LED was proposed.By parameter optimization,it was found that the optimal distance between the optical fiber's entrance surface and the LED light source was 12 mm and that between the fiber's exit surface and the sensing probe was 5 mm.The feasibility of the online testing system was also verified.The light output was measured under various driving currents by both the online testing system and the integrating sphere system.The results showed that they had a good consistency with each other.A 6000 h benchmark according to LM-80-08 standard also validated the feasibility of the online testing system.Besides,the error analysis showed that the system error and random error of the online testing system in high temperature aging experiments were 0.2% and 0.02% respectively.2)The effect of LED chip bonding process on the optical properties and thermal reliability of LED was studied.Firstly,the impact of chip offset on LED's optical properties was investigated by optical simulations.It was found that the chip offset would result in the decrease of luminous efficiency and yellow-blue-ratio(YBR)and deterioration of angular color uniformity.And this impact increased with the chip offset magnitude.Then,the simulation results were validated by experimental results.Finally,the effect of chip bonding process on LED's thermal reliability was explored by accelerated aging experiments.The results showed that chip offset would worsen the reliability and the impact also increased with the chip offset magnitude.3)The failure modes and mechanisms in high power LED during operation in hightemperature environments were studied in terms of packaging materials.The results showed that the silicone gel would lead to defects like carbonization,voids,etc.and phosphor thermal quenching would cause fast light energy loss initially.The effect of silicone amount on LED's reliability was also investigated and it was found that the transmittance decrease of silicone gel was responsible for the influence of silicone gel on LED's light output.From the angle of improving LED's reliability,a new packaging method of combining the blue LED chip with phosphor plate was proposed and explored.The phosphor temperature was measured by thermocouple plug-in measurement.The results showed that the phosphor temperature is 58.5 ? while the current is 350 mA.After analysis,the measured results were overestimated and the reason was due to the light energy absorption and transformation by the thermocouple's bead.4)A statistical method was proposed to identify the effects of packaging structures on LED's reliability.A common LED packaging structure was taken as an example.Range and variation analyses showed that the influence of the optical lens among four packaging materials and structures was the most significant.Then a structural design method for LED package was proposed base on reliability estimation.And the structural design suggestions were given after analyzing two typical LED packaging structures.5)Accelerated life tests of high power LED module were carried out.The function mechanisms of acceleration factors in high temperature and current stressed accelerated life test were analyzed and the experimental design was conducted based on the current effect.The results showed that the light output degradation and color shift increased with temperature under high temperature accelerated life test.Weibull distribution was used to fit the accelerated lifetime and a lifetime prediction model was built by using the Arrhenius model.Comparison experiments under 150? showed that the relative error of the lifetimes derived from the lifetime prediction model and experimental data was 7.67%,3.29% and 2.69% respectively under the failure probabilities of 5%,10% and 63.2%.Besides,the current effect in high-temperature-current accelerated life test was analyzed.The current effect was not zero and should not be ignored.When building the lifetime prediction model,a model considering both temperature and current effects should be used instead of the Arrhenius model which only considers temperature effect.In this dissertation,the thermal reliability and lifetime prediction of high power LED packaging are studied and progress has been made.The conclusions of this research may play a guiding role in the accelerated aging and reliability analysis of LED packaging.