Depth analysis of LED failure

Depth analysis of LED failure

LED light source is the hair of the so-called III-V compounds that form, that is known as the epitaxial grain (chip), the solid compounds itself nature is very quiet, regulated in product under the conditions of the use and not easy to damage, but in general application environment doesn't chemical reaction, so have long product lifetime. However, in order to make this LED chip to shine, and must by outside access, so the current average size is small parts of the chip in particular in Taiwan (or says wire frame) and metal line or soldering materials such as the positive and negative poles chip connected, then use the polymer materials in Taiwan, the whole claddings so-called packaging process, through this period of process on the market after each of a common LED lamp grains. In practical application, also will see the number needed a LED lamp grain assembly ChengMo group, and finally to and other function module combination become the end products. From that, a LED on product failure, causes the product may come from any one part, so the cobwebs can find the true to its failure reason. From LED lights for failure, for complete for grain, grain of LED lights LED chip itself is very strong, but the encapsulation of the chip claddings material is vulnerable to damage, so, LED lamp grain of failure many attributable to the packaging materials damage to or degradation. If want to fully analytic this kind of failure, involving optical, chemistry, material science, electronic physics and other areas of professional knowledge, and the instrument of tie-in and rich practical experience, to be sure is true because failure point, and then put forward the improving measures.

1. Do not light: this failure is to point to LED after electrify completely not emit light. Generally speaking, the conductive path breaker "(open)" is the major cause of this kind of failure, one of the method is also confirm open circuit is simple, to common with three meters can test and verify. However, to find open circuit point, must be done further analysis, for example: usable X-ray to confirm whether lineup break from, or by SEM (scan type electronic microscope) observation profile structure can check the sticky part of the defects crystal, etc. This kind of failure of the second reason is "short circuit (short)", this is because the current did not really LED chip, but by "box", it flows through LED lamp grain of natural will not shine, such as: for electronic migration happened to electrode of metal atoms don't normal diffusion, such as indium tin oxide (ITO), silver or GaN/InGaN diode linings of metal, probably due to mechanical stress, are high current density or corrosive environment in such different shape happened. Other possible reasons for the lineup is offset, sticky rubber adhesives etc. Crystal climb This failure must use i-v curve (i-v chart) to judge, as for failure point because from appearance can't check to the above defects, so must first to X-ray to confirm; Or chemical solution removes LED packaging materials, and then use the optical (OM) or electronic microscope (SEM) close examination.

2. Angry: this kind of failure is to point to light up the situations not LED, appearance or plastic material transparency and new product is different, with the naked eye can see, usually in the product USES period of time or in finish reliability test happened after. Generally speaking, color change area can be divided roughly for what happened in the wire rack or encapsulation glue material, if has two kinds in wire frame, it's usually because the surface and the chemicals in the environment of the reaction, such as oxidation or to distinguish belongs to which kinds of sulfide, depends on component analysis, can use instrument contains EDS (Energy Dispersive X-ray Spectroscope), XPS (X-ray Photoelectron Spectrometer), AES (Augur Electron Spectroscope), and so on. If the rubber material happen with packaging color, belong to degradation of polymer materials phenomenon, such as epoxy resin vulnerable to high temperature or uv light yellow, so affected and white LED many to replace silicon glue of mining. Analysis such special attention, because lost effect rubber material itself has the certain transparency, sometimes the discoloration of wire frame by glue material for cover and mistaken for glue material change color, leading to the improvement of the wrong direction.

3. Light failure: in this failure means the light intensity LED below and new products. As previously mentioned, chapter has become a light failure degree judge LED lighting products of life is an important index, so this kind of failure analysis is quite important. Overall, this kind of failure analysis is very complicated, and the factors that affects the light intensity is very much, such as chip degradation, reflex, cup, the rubber material and degradation between chip off layer, glue material reduced transparency, lineup jointing surface resistance, thermal resistance is too high, and so on up, if produce in a white LED, it must also take into account the fluorescent powder, because the deteriorating white LED usually use one or the DuoZhong fluorescent powder, they will be affected by heat or the effects of moisture and attenuation and reduce the efficiency, leading to the output of the light color change. Analysis technique including: not to destroy testing, such as inspection, LED electrical curve, integral ball optical properties and so on, the use of such data synthesis judgement, one by one to rule out possible reasons for that, when, then performs destructive analysis, the use of sample preparation technology to produce a suitable for analysis of samples, again complementary with all kinds of instruments such as SAT (Scanning Acoustic Tomography) ultrasound scan, FTIR (Fourier Transform InfraRed) Fourier transformation InfraRed spectrometer, TEM (Transmission Electron Microscope) through electronic Microscope, SEM (Scanning Electron Microscope) and so on to verify it, can to really for. 4. ESD failure: this is a kind of electrostatic discharge caused by the destruction in MOS IC chip, the product attach great importance to this phenomenon, in the past because of LED, gallium arsenide (GaAs) chip itself can conduct electricity, so this kind of problem is not common, but due to the use of white LED not conductive sapphire substrates, and substrate and GaN etc material for lattice mismatch between will form the internal defects (such as dislocations), the more sensitive to ESD damages. Electrostatic discharge may produce semiconductor juncture (junction) of failure or characteristics of the immediate permanent drift and potential damage will lead to increased rate of decay. There are several phenomenon can be used to help judge whether chip from static damage, such as a reverse bias leakage current, chip only local glow, chip surface local melting point, etc. In Sometimes, the beginning of the static electricity the extent of the damage is not high, LED the electrical characteristics of, the luminescence properties, chip surface integrity, but this is all of destruction would by accumulated gradually obvious, but sometimes may also went through the whole product life cycle, once the production line not doing static electricity protective measures, and the production of products of the guest will be back, and high rate low, in the face of this kind of phenomenon, failure analysis also may not be found, so do so because of static electricity protective measures is the best production line in solving the problem.

5. Other: suddenly often because of the failure of the thermal stress caused by epoxy resin, when the encapsulation of the glass to transfer temperature (Tg), resin will the expansion of the very fast in the semiconductor and solder joints, the position of the mechanical stress produced contact to weaken or break it, and in very low temperature will let packaging produce rift. In addition, high power LED to the current crowded, the uneven sensitive current density distribution in the juncture (junction), may produce local hot spots, there are heat burned risk, if appear again of the substrate is uneven heat transfer, will make the problem worse, this common in the welding materials of the holes or electronic transfer effect and Kirkendall empty, so hot burned is also LED common failure modes.

Conclusion: LED lamp grain and the structure of the chip and common IC to relatively simple, but in than application but also involves light, electricity, heat wait for a phenomenon, so happened is relatively complex, the failure reasons of the failure analysis to carry out the steps in a DuoYuan instead, and LED chip and packaging structure for the reason, all the patent product is not the same, for analysis of the complexity of the sample preparation ambitious also, so the conventional IC good sample production technology, full analysis of the instrument and rich experience comprehensive judgments, can obtain the correct failure analysis results.

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