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 many years of experience in LED failure analysis and development in analysis method and has collected the examples from previous case studies as below
 
  • LED Dosen't Light Up
<Caused by the broken wire>
 
No light up emitted after power applied, Open/Short circuit is commonly observed. When open circuit happened, Lab could simply check by multi-meter 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 
<LED leaf frame discolored>

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/Sulfur 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.
   
  • Lumen Decay
<White Light LEDs Lumen decay>
 
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. 
   
  • ESD failure

<GaN LED ESD failed from I-V curve>
 
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.
 
<LED damaged by ESD>
   
  • Others
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.

 General failure modes collected by DEKRA iST 
 
 
The structure of LED package and chips is relatively simple compared with common ICs, but they also involve light, electricity, heat and other phenomena during application. Therefore, the failure modes are more complicated, and the methodology of failure analysis is more diverse. At the same time, the LED chip and package structure are different from each other manufacture due to patents, and the complexity of sample preparation required is much greater than conventional ICs. Therefore, the correct failure analysis result is recommended and supported by the laboratory with excellent sample preparation technology, comprehensively analytical instrument and rich experience for neutral judgment.
 
<LED Failure Analysis Flow>

 

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