Common Invalidation Problems and Precautions of LED Device

  • 2022-06-13

The failure mode of LED lighting product can be classified into: chip failure, packaging failure, electrical stress failure, thermal stress failure, and assembly failure. Through the analysis and improvement of these phenomenon, it will provide help for manufacturers to design and produce LED lighting products with high reliability.

 

Chip Failure

Chip failure refers to the failure of chip itself or other factors. There are a variety reasons: chip crack is caused due to inappropriate bonding process conditions, which brings about great stress, with the accumulation of heat, thermal mechanical stress has also been strengthened, producing micro cracks on the chip. Micro cracks will be further expanded by constantly increasing injected current during working, until the device is completely failure. Second, if the chip has source damage, gradual degradation will happen in the charging process and final failure, which can also cause light decline or failure in the process of using. Moreover, if the chip bonding process is not good, in use process it will lead to the separation of chip bonding layer from bonding surface and make the sample fail at open circuit, and also cause "dead lights" death occur in the process. The reasons for bad chip bonding process are that using overdue silver paste, long exposure time, too little silver paste usage, long curing time, solid crystal base surface contamination, etc.

 

Packaging Failure

Packaging failure refers to the device failure caused by inappropriate packaging design or production technology. Epoxy resin material used in the packaging will have degradation problems, resulting in the reducing service life of the LED. The degradation problems include: light transmittance, refractive index, expansion coefficient, hardness, water permeability, air permeability, packing performance and so on, among which light transmittance is the most important one. In addition, there is another important factor affecting the service life of the LED - corrosion. In LED application, generally the main cause of corrosion is the water vapor infiltration in the encapsulation material, which causes metamorphic lead, PCB copper corrosion; sometimes, with the introduction of water vapor, movable conductive ion will reside in the chip's surface, causing leakage. In addition, the device encapsulating with bad quality has a lot of bubbles inside the packaging, and those bubbles will also lead to the corrosion of the device.

 

Thermal Stress Failure

Temperature has always been the important factors that affect the LED optical properties, and in the research of LED failure modes, the scholars at home and abroad consider the working environment temperature as the accelerated stress to make life-accelerated test for LED. This is because under the premise of unchanged thermal resistance, the temperature of welding point of packaging will increase, leading to the LED failure in advance.

 

Electrical Stress Failure

If LEDs use EOS in the case of overcurrent or static shock damage the chip, the chip will open circuit and cause the electrical stress failure. For example, GaN is WBG materials, which has high resistivity. If you use this kind of chip, it will be difficult for induced charge in the process of production because of electrostatic generation to disappear. When its accumulated to a certain point, it can produce high electrostatic voltage, and if it exceeds the endurance of material, breakdown phenomenon occurs and discharges, making the device fail.

 

Improvement Measure

 

Heat Dissipation Technology

Heat dissipation technology is always an important part having the impact on LED application. If the LED device is not able to timely dissipate heat, the temperature of chips will greatly increase, then the luminous efficiency will drop sharply, and its reliability (such as service life, color change, etc.) will be worse; at the same time, the high temperature will produce mechanical stress in the LED encapsulation structure, which may further cause a series of reliability problems. Therefore, in the manufacturing process, you can choose a base with good thermal conductivity and make the LED heat dissipation area as big as possible, so as to increase the cooling performance of the device.

 

Anti-electrostatic Technology

A big problem of LED device with GaN as chip is the electrostatic effect, and failing to deal with this problem will seriously affect the life of the device. In LED design, therefore, you have to fully consider the anti-static design, in order to avoid the device failure phenomenon because of high electrostatic voltage breakdown.

 

Packaging Technology

Epoxy resin material used in the packaging will cause the degradation of light transmittance because of the light and temperature rise. It is showed that the transparent material becomes brown in use and affects the original spectral power distribution of the device. Therefore, in the LED packaging process, we should strictly control the temperature of the curing and avoid advanced aging during the process. On the other hand, in order to prevent the corrosion phenomena of the device, we should choose packaging material with good transparency and at the same time, pay attention to clean the bubble inside the material in the injection molding process to reduce the residual amount of moisture and the risk of corrosion of the device.

 

Improving Manufacturing Process

LED manufacturing process needs suitable bonding conditions, and too big bonding will crush the chip, which will cause inadequate bonding strength and makes it easier to be loose. Therefore, at the same time of guarantee the device bonding strength, we need to try to reduce damage to the chip bonding process to achieve the purpose of optimizing. The bonding of the chip should be done in appropriate range of temperature and time, so as to reach the requirements of densification, no empty, and small residual stress.

 

Reasonable Selection

Before the delivery of LED, we can add a screening process to do reasonable aging and screening test to some samples, eliminating some possible failure of the device in advance, in order to reduce the LED early failure phenomena in actual use.

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