LED light source is composed of III-V compounds, well-known as epitaxial grain (chip) which has longer product lifetime, robust and has nice chemical resistance. However, to activate LED chips required input current by external device, therefore small size chip require to be mounted on lead frame with electrode and cap with polymer, which is known as packaging process. Number of LED package will be assembled into module/luminaire according to applications.
Due to multi process involved in LEDs product manufacture, the failure mechanism is not easy to locate the root cause, normally the chip is robust, the damage may happen on package material(weak mechanical property, thermal stress, aging…etc.). To understand the failure mode required optical, chemical, material, electrical and physical knowledge to have better improvement plan.
DEKRA-iST has over 5 year experience in LED failure analysis and development in analysis method. Following example sharing based on previous case study.
No light emitted after power applied, Open/Short circuit is commonly observed. When open circuit happened, Lab could simply check by multimeter and locate open position (wire broken) by X-ray, and inspection by SEM to confirm the defect of structure. Short circuit happened when current route changed and not go through the chip, therefore no light emit on LEDs, which may cause by (a.) electrical migration (b.) wire loop distortion (c.) die bond package glue spread on wall…etc. Failure mechanism above could be analysis through IV curve, X-Ray, chemical decap with optical microscope inspection.
Discoloration on LEDs could be observed by bare eye(LF blacken, phosphor abnormal) and optical measurement result(chromacity coordinate shift), the abnormal happen on lead frame may cause by chemical reaction(oxidation/Sulphur corrosion), composition analysis is recommended(EDS, XPS and AES). The packaging material discoloration is due to degradation of polymer (eg. Epoxy might degrade under uv radiation, that’s why silicone is common used in LED package.
LED leaf frame discolor
This failure means that the light intensity emitted by the LED is lower after test. As mentioned in the previous section, the degree of light failure has become an important indicator of the life of LED lighting products, so this type of failure analysis is very important. Overall, the analysis of this type of failure is very complex, because the factors that affect the light intensity contain large scope, such as chip degradation, reflection cup deterioration, glue and chip between the delamination, plastic transparency decreased, wire connection resistance rise, high thermal resistance…etc. When it comes to white LED, should consider the problem of deterioration of the phosphor, white LED usually use one or more of the fluorescent powder, they will be affected by heat or moisture, Resulting in a change in the color and light output. Analysis of the techniques include: non-destructive testing, such as visual inspection, LED electrical properties, optical measurement, to approach possible causes, followed by destructive analysis, FTIR, TEM (Transmission Electron Microscope) ,SEM (Scanning Acoustic and so on) to locate root cause.
White Light LEDs Lumen decay analysis
Electrostatic discharge may have influence on chip destruction, in LED scope, due to GaAs chip is conductive, so this failure is not significant, however, The white LED which made of a non-conductive sapphire substrate, and the mismatch between the substrate and GaN and other materials due to lattice will form an internal defect shows more sensitive to ESD damage. Electrostatic discharge may result in immediate loss of semiconductor junctions or permanent drift of potentials and potential damage resulting in an increase in the rate of attenuation. There are several phenomena that can be used to help determine whether the chip is subject to electrostatic damage, such as reverse bias leakage current greatly increased, chip with partial light, chip surface melted and so on. Sometimes, the influence of static damage is not obvious on LED electrical characteristics, luminous characteristics, chip surface integrity but this damage will be gradually increased due to accumulation, and sometimes it may also survive the entire product Life cycle, once the production line is not doing electrostatic protection measures, the production of the product's passenger rate will be fluctuated, in the face of this phenomenon, failure analysis may not be able to find the root cause, so ESD protection on production should be emphasized.
Sudden failure is often due to thermal stress, when the epoxy resin package reaches the glass transition temperature (Tg), the resin will be very rapid expansion of the semiconductor and solder joints in contact with the location of mechanical stress to weaken or break It, and at very low temperatures will cause the package to crack. In addition, high-power LED are current density sensitive, uneven current density distribution in the junction (junction) may produce hot spots, there is the risk of burn out, if the substrate heat transfer inhomogeneous will cause serious problem, which is commonly happen in the welding material of the hole or the electronic migration which effect and Kirkendall empty.
LED light grain and chip structure and the common IC is relatively simple, but in the application but also involves light, electricity, heat and so on, so the reason for the failure is more complex, in the failure analysis of the implementation of the steps but more diverse, At the same time LED chip and packaging structure due to the patent, the products are not the same, the analysis of the sample preparation required for the complexity is much larger than the conventional IC, so robust sample production technology, complete analytical equipment with a wealth of experience Comprehensive judgment, in order to obtain the correct failure analysis results.
LED failure analysis services
DEKRA-iST provide different analytical methods and processes for LED products from EPI, packaging process to finished product assembly. In the epitaxial part, we focus on the bonding, leakage and electrostatic protection of PAD and Wire Bonding. , The photoelectric characteristics, etc .; in the packaging process, providing a surface material micro-pollution analysis technology, the Leadframe, Housing and other material quality can be effectively controlled; the other part of the sealant, due to the heat generated by the sealant Serious, troubled by many packaging plants, the company also gathered in the multi-party information, and through material analysis to find out the problem with the color of the functional basis, can be used as a comparison, help clarify the problem; in the Light Bar or module products , The LED first focus on the strength of the solder joint, because most of the LED backlight using aluminum plate (MCPCB), in the surface mount assembly (SMT), the need to consider the part of the relationship between temperature and solder joint strength to avoid the problem of insufficient solder joint strength As well as the long-term use of heat dissipation.
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