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What are the causes of LED display dead lights

Time:2018-10-12 Views:213
At present, LED technology is becoming more and more mature, and its long life advantages have been a focus of public attention.However, in recent years, we still encounter a lot of "dead lights" in the production and application of LED.The so-called dead light, also known as the extinguishing light, is the LED light source is not on.The dead light generated in both production and application is a headache for manufacturers, which not only faces the loss caused by product defects, but also affects consumers‘ confidence in LED products.
Therefore, the study and analysis of some common causes of LED dead lights can help us reduce and prevent the repeated occurrence of LED product failures, guarantee product quality and improve product competitiveness, and provide references for technological improvement and improvement of enterprises, so as to create greater economic benefits for enterprises.
The common causes of LED dead lights are as follows:
For the "dead light", we should first determine whether the LED is short or open. If it is open, we will generally consider whether the welding line inside the LED lamp is disconnected.The welding line inside the LED lamp is disconnected, resulting in the LED has no power supply voltage, which is one of the common reasons for the LED dead light.The common disconnection position of welding line has 5 places, as shown in figure 1: point A, B, C, D, E:
A point: where the chip electrode meets the gold ball;
Point B: where the gold ball meets the gold line, that is, the ball neck;
C point: the range of welding line arc;
D point: the joint of the two welding points of the support and the gold wire;
E point: the joint of the bracket welding spot and the bracket plating layer.
The fracture location of the weld line can be checked after the sample is dissected by optical microscope and electron scanning microscope (SEM), which is helpful for further analysis of the cause.The following cases are presented, where the weld line breaks and the cause of the break are different.
    
Failure lamp bead no. 5730.The lamp bead appears dead after 100 cycle cold and hot shock test.After the cross-section analysis of the failed sample, it was found that the silicone around the first welding spot and the second welding spot had burst, and the second welding spot D had broken, as shown in fig.2-4.
 
Failure sample section surface
Because of the silica gel and gold wire thermal expansion coefficient difference is bigger, after 100 cycles after hot and cold impact test, silica gel with gold thread in constant expansion and contraction, and gold wire solder joints bend is the concentration, therefore, the most likely to cause around the solder joint blowout of silica gel, silica gel crack in welding line second solder joint is the weakest point D disconnect, eventually appear dead lamp sample.
    
The failure lamp ball model is the imitation flow lamp ball batch lamp ball appears dead lamp after using on the lamp for a period of time, when lighting, the current distribution of each lamp bead on the lamp is about 500mA.First of all, after sol-gel examination on some of the failed samples, we found that all the failed lamp beads were 4 first solder joints broken, while 4 second solder joints remained intact, as shown in figure 5 ~ 8.
After that, we conducted a cross-section analysis of the failed samples, and found that the silica gel directly above the chip had burst, as shown in figs. 9 and 10, the silica gel in other areas was intact.
 
Since the 4 first welds that broke were concentrated on top of the chip, the 4 second welds that remained intact were on the holder.It is very likely that the silicone cracking on top of the chip caused the disconnection of 4 first solder joints, and the location of the silicone bursting was mainly concentrated on the chip, namely, directly above the heat source.
In addition, when the lamp beads were lit, the current was relatively high (500mA), which could be speculated that the overheating of the chip caused the silicone on the chip to burst.The heat dissipation path of the lamp bead is carefully checked, and it is found that the overheating of the lamp bead chip is probably related to the heat sink at the bottom of the lamp bead, which adopts thermal silicon-conducting resin to fit the PCB board, and the heat dissipation effect of the high-power lamp bead is not good enough.
    
For some LED lamp beads with vertical chip, the underside of solid crystal layer and the peeling of bracket plating layer are the common causes of dead lamp.
    
The failure sample is a straight plug-in LED lamp bead, and the dead lamp appears in the process of use, with an error rate of 1.5%.After cross-sectional examination of the failed samples, we found that all the gold solder joints remained intact, as shown in figs. 11 ~ 13.However, it was found that the solid crystal layer and the scaffold coating were completely separated, and the encapsulation glue and the holder cup wall were also separated, as shown in figure 14.
 
It can be judged from the above observed phenomenon that the cause of the lamp bead lamp is the peeling phenomenon between the encapsulation glue and the bracket interface. The extent and area of the stripping will expand with the intensification of the use process, which further causes the solid-crystal glue and bracket peeling, and finally leads to the dead lamp in the sample.It is also possible that poor adhesion of encapsulating glue caused the stratification between the encapsulating glue and the scaffold interface.
    
In some cases, the lamp is not necessarily the lamp itself problem, but may be caused by the power supply used.
    
The failed sample is a imitated LED lamp bead, which appears dead light after a period of use.Inspection on multiple failure after the lamp bead sol, failure light bead chip found near 2 P gold wire solder electrode and electrode graphic lines have been burned, 2 N gold wire solder electrode and electrode graph line and four second spot on the stent are intact, burned or fracture has not been found, the following figure 15 and figure 16.
 
Obviously, the burning of chip P electrode is the direct cause of the lamp.So what caused the P electrode to burn?Next, we made the following analysis.
We randomly selected several lamp bead samples that could be normally lit to simulate the high-voltage impact experiment, and applied 20V instantaneous high voltage to each lamp bead separately.The experimental results showed that the high-voltage impact rear lamp bead appeared dead light in an instant, and the sol-post-inspection found that it was also the open circuit dead light caused by the burning of p-electrode circuit on the chip, as shown in figs. 17 and 18.
 
Through the above inspection and verification test, it can be inferred that the root cause of this batch of lamp bead dead lamps of the customer is the excessive surge current during the use of the lamp bead. Because the resistance value of chip P area is higher than N area, when the current is concentrated through the P electrode, the P electrode will burn down first and lead to the open-circuit dead lamp.
Lamp bead in the process of using the surge current (or voltage) is too large, probably with the driving power of lamps and lanterns in the surge current on or off, it is possible that the chip P electrode, the defective cause P electrode welds a momentary contact undesirable situation, cumulative when have more LED series high pressure on the poor contact contact cause instantaneous high current lamp bead welding wire burned and sealant was blackened.

The previous cases of dead lights are all open road phenomenon, the following for you to give a short circuit phenomenon of the case.
    
The failure sample is the imitation flow lamp bead, the lamp bead ageing process discovered these lamp bead appear dead lamp, dark light and so on undesirable situation.After soling the defective products, it was found that the chip electrode was corroded and the electrode was peeled in many areas, as shown in figs. 21 and 22.
 
The corrosion area of the chip was analyzed with the X-ray energy spectrometer (EDS), and it was found that the corroded area of the chip electrode contained more Na, Cl and K elements, as shown in figs. 23 and 24.
 
Based on the chemical composition of the elements, it is speculated that the chip may be contaminated by NaCl and KCl.When the heat and water vapour coexist, the chip electrode will be corroded, resulting in the metal corrosion of the chip electrode and the adhesion of the electrode line will drop down, and even cause the local area to fall off.The migration of the electrode solute shorted the P and N electrodes and led to the die lamp.
    
There are many reasons for LED dead lights, from packaging, application, and use of every link is likely to appear dead lights phenomenon, the case mentioned above is just a brick to attract jade.How to reduce and eliminate dead lights and improve product quality and reliability is the key problem every LED enterprise needs to face.
The analysis of the cause of LED dead lights is one of the important ways to reduce and eliminate LED dead lights. The failure analysis of LED products, in addition to powerful equipment hardware, needs to be supported by the production experience of chip, packaging and application, so as to give play to the equipment‘s ability and solve problems for customers.Deronser goes online
 
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