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>
<LED damaged by ESD>
- Others
General failure modes collected by DEKRA iST
<LED Failure Analysis Flow>
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