LED Walls
Maintenance and Diagnostics
Methods for Analysis
LEDs present a broad range of failure modes, which can be approached by using different methods of analysis. The objective of the failure analysis is to be able to allocate the observed failure pattern to a possible root cause and then show a way to avoid it. Because of the complexity, a failure analysis puts high requirements on technological knowledge and the availability of methods for analysis13.
Approach to find errors:
- Questions: In which environment was the LED/Module operated? For how long? Which driving mode was used?
- Measurements: Current/Voltage curves, intensity of light, wavelength/colour, radiation characteristics
- Non-destructive analysis: x-ray, light optical microscopy, scanning acoustic microscopy
- Destructive analysis: cross-sectioning, opening the package
- Physical analysis: FIB/REM, OBIRCH / Emission microscopy, EBIC, cathodoluminescence
In order to limit the extent of the examinations, it makes sense to develop an efficient strategy that will cover the most comprehensive amount of historical information at the beginning of the failure analysis13.
Visual light microscopy inspection
The visual inspection provides information about the external construction, the external integrity of the LED package and accessible parts of the internal construction13.
Opto-electric measurements
Essential opto-electric parameters are the optical power, the spectrum (colour), and the current-voltage characteristics. The radiation characteristics in the far- and the near field discover the inhomogeneities of the light emitting area. Measurements of thermal resistance show problems in the thermal coupling. Increased voltage in the forward direction points to a disturbance in the current path. Increased leakage current in reverse bias is a sign of a severe disorder in the epitaxy layers. This could because, for example, by ESD. The comparison of fine structures in the current/voltage curve between failed and working LEDs can help to differentiate between the various causes of errors13.
Other non-destructive procedures
Because LEDs are usually packed in plastic housings, the detachment of the bond wire of the LED chip can be detected by using an x-ray microscope. The scanning acoustic microscopy is also suitable for identifying delamination on the interface13.
Destructive physical methods
To localise failures inside the LED, direct access to internal parts like the LED-chip or bond interfaces is required. Cross-sectioning or removing encapsulate materials by using chemical solvents are adequate treatment methods.
The inside structures are then accessible for high definition optical or SEM microscopy, and failures at the interfaces can be detected. Material analysis with EDX can determine impurities in the case of corrosion effects and give an indication to possible causes13.
Failure analysis on chip level
To localise failures on chip level, methods are available that reveal defective areas on the chip, like dark spots or dark lines. Such methods are based on localized stimulation of light emission (cathodoluminescence) or current flow by pointing an external electro beam (EBIC) or laser beam (OBIC) onto the chip. Emission microscopy uses very sensitive detection of light created by leakage current. Special methods like TEM (Transmission Electron Microscopy) and sample preparation using FIB (Fused Ion Beam) are required for the deeper analysis of failures in the epitaxy system layer. Evaluation of the results requires the possibility for comparisons to the known error patterns and good models because the defective structures within the semiconductor are very dependent on the technology used. These methods are very expensive and only used in special cases13.