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How to analyze the failure of lighting LED?

Do you still remember Op in the double eleven success? 200 million yuan, we see the LED has slowly become popular. However, though popularized, we still have a lot of LED failures during production and use. Here we will talk about the LED failure mode.
There are mainly two kinds of LED as general illumination at present: One is SMD LED that can carry on SMT work; The other kind is integrated and array type COB and copper base module. Of course, there are still DOB, but today we do not discuss.
First, SMD LED failure mode
First, let's look at the failure mode of the SMD LED. SMD LED types too much, 2835,3528,5050,5630,3014,4014 and so on. Plastic stent from PPA, PCT, until now the EMC and so on. Regardless of package size, the failure modes are not very different, and I summarize a few points.
1.1 The most serious failure is dead lamp
Death lights should be divided into real death or suspended animation.
The more common case of suspended animation is the LED backflow time, the virtual connection between the LED and the PCB. The Weld is better to find and solve. Through spot measurement and aging can be found Weld position, and through the heat station or hot air gun LED correction and replacement can be. Another type of suspended animation is the virtual connection inside the LED or broken gold wire but also next to each other. As shown in Figure 1 below, the typical LED gold wire inside the virtual connection (already disconnected, but still next to each other, the LED can still be lit, but after a while because of the colloidal stress caused disconnection head separate death lights) .
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▲ Figure a broken gold wire failure mode
If death is true, the situation is more, but the common is the golden line fracture (Figure 1, such as a broken head is not a real death next to a). Another is the over-current led to chip and gold burn, this visual is very obvious, as shown in Figure II below. Summarized to death in several situations: one is the line fault problem may result in poor bonding failure line, it may internal stress (thermal expansion and contraction of the force) lead to line failure; one is more than the necessary current and voltage Problem; there is a static breakdown LED problem (the failure mode visual effect is not obvious, the chip PN junction is breakdown, need to aging for some time before gradually die).
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▲ Figure 2 overcurrent causes chip failure mode
So the question has come, how to avoid or reduce the dead lamp so serious failure yet.
The most important thing is to ensure the reliability of line welding, the reliability of these adverse consequences are fatal. First of all, to ensure the stability of the machine, as well as wire mode, and some need to be tail lag, or tail plus a tail. Second, is to ensure the wire and chip key and compatibility. We used to use 9999 pure gold, now using K gold (more than 60% gold), or alloy (more than 8% gold) and CHIP PAD key and compatibility and welding strength need to be taken into account.
In addition, the glue has a relationship with the hardness, if the glue is too hard, then the stress (thermal expansion and contraction of the force) will pull off the chip. This failure mode is also more common. Why? Because silicone is too expensive, the sealing is not good, more people tend to use hard silicone and epoxy resin as potting. Therefore, when we choose packaging materials, we must take into account the stress and package matching, not too hard, suitable for use in the environment.
For example, now 9V1W 2835 as a bulb light source, pin temperature TS often 105-110 ℃ between. So for such a high temperature, our packaging glue hardness must be smaller, otherwise over time will be too much stress pull off the gold line and die lamp. Of course, if it is 0.06W LED, epoxy resin should be cheap and practical packaging glue, the impact of stress is not too large. As for static electricity protection, almost all electronic devices in the electronics industry are required to be protected by static electricity, such as establishing an air shower, leading the body to charge, humidifying air, grounding the equipment, increasing a plasma fan, and the like. Electrostatic protection of universal measures, I will go into details here.
1.2 Another LED failure mode is: short life, color drift and other major indicators of abnormal parameters
We define 70% of the initial luminous flux as the end of LED life (this is not the same definition of dead lights). Life expectancy can also be understood as lumens maintenance. Now almost all the major companies have SMD LED LM-80 test, test 6000H, or 9000H, simulated life expectancy of 36000H or 54000H. Of course, the design is wonderful, the reality is cruel. LED in its application conditions and our LM-80 test conditions are so different, so life expectancy is not as expected.
For example: give guests samples 100Ma @ 9V 2835LED. The customer installed a bulb aged 100H and found that the lumen maintenance rate was only 90% and the difference from the LM-80 curve was too large (at least a 98.8% maintenance rate), which is the failure mode with a typical life expectancy less than expected. Of course, there are often accompanied by internal color black LED, CCT blue shift ah (visual LED white light turns blue). Here I only analyze this common failure mode.
If only the LED luminous flux attenuation is too powerful, no other minor serious complications, then we speculate that the LED itself material deterioration. Macroscopic LED light out of low efficiency, micro-words or LED internal photons can not get out, or LED photons less. The light may not come out, may be the reflector (stent) inefficient, such as PPA or PCT yellowing, glue refractive index change, metal reflective layer reflectivity and so on.
So LED photons will not be reflected, into heat, there may be complications (other core parameters such as CCT changes). If the photon is less, it should be specifically LED CHIP photon less, it should be that the chip is degraded. The number of electrons and holes that CHIP moves freely decreases. Material deterioration is a normal phenomenon. However, if the current is too large, LED overheating will lead to deterioration of packaging materials, the speed is far more than the normal rate of deterioration, which is what we call abnormal flux maintenance. (Abnormal life)
Another common failure color temperature, color rendering index, VF and other anomalies. CCT blue shift, R9 change, VF change and so on. These are generally LED abnormalities, of course, there are exceptions. First of all, to say the following exceptions, that is, everyone said the temperature rise (hot and cold data contrast), this is a normal phenomenon, cold and hot state above the parameters due to the heat will appear within the allowable range Variety.
To say that common abnormalities, that is, the data gap after the thermal stability is too large, such as CCT, R9 or LM equivalent fluctuations. The reason is relatively simple, in addition to the impact of over-current overheating, as well as the thermal stability of the material itself is not very good. Such as red phosphor nitride, its thermal stability will not be too good, will affect the CCT, R9 and other factors. Another common anomaly is the effect of chemicals on LEDs such as the chemical reaction of S (volatile sulfide), Br (volatile halide), o (oxygen), and silver plating, or the effects of volatile organic compounds .
Below we cite a typical case, look at this halogenated (vulcanization failure mode) common pollution failure mode. Through the spectrum of black parts of the elemental composition analysis. Case as shown below:
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Second, COB failure mode
Above we have said SMD LED common failure mode. Now let's talk about COB failure mode. In terms of the general failure mode, whether it is SMD LED or COB LED, are more or less the same. However, the failure mode of COB may be due to its different package, the failure of the pattern will be a little more.
2.1 In the case of a dead lamp, the fundamental failure mode is the same as the failure mode in a SMD LED. However, because there are more COB strings, the definition of a dead lamp is not all the dead lights, but one or a few strings of dead lights. The other strings are still lit normally, as shown in Figure III. COB dead lamp is nothing more than the following situations: one is the line fault problem may result in failure of the line lead to failure; may also be stress (thermal expansion and contraction of the force), COB is more obvious, so COB are Strict control of the pin temperature does not exceed 85 ℃ (SMD LED 1W 2835 105 ℃ can be used normally) lead to line failure; one is more than the allowable current and voltage problems, as shown in Figure 4 below; the other is the electrostatic breakdown problem .
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Figure III COB broken dead lamp mode. Only a bunch of light, a few other strings of dead lights
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▲ Figure four COB burning dead lamp mode. Chip surface black, glue carbonization.
So how can we reduce the possibility of these failures? COB LENS (yellow light-emitting area) is relatively fragile, we want to protect the LENS area (including white dam area), blister box packaging need to add protective layer and so on. In addition, LED drive current is very large, so the circuit requires an over-current protection design. When used, strict control of a reasonable temperature is also very important protection measures. Specifically, the temperature of geometry?
In fact, the most important is the COB colloidal glass temperature (some 180 ℃, some 200 ℃ or other temperature) will change the hardness of the glue, resulting in internal stress (thermal expansion and contraction of the force) increases, pulling off the gold line. Some people may want to ask, the maximum chip temperature is 125 ℃, you 180,200 high temperature come from? We want to say is that the highest point of the LED temperature is actually colloidal surface temperature, why? Because photons will be extinguished in the colloidal surface into heat.
2.2 Another failure mode LED is: short life, color drift and other major indicators of abnormal parameters. Chip, glue, phosphor, silver coating and SMD LED are close, so the failure mode and SMD LED failure mode similar. I will not waste everyone time to read again.
2.3 Here are some other typical COB failure modes.
A, plastic cracking: changes in colloidal hardness resulting in plastic cracking, severely led to broken gold wire (dead lamp). As shown in Figure 5 below.
B, flux pollution. Flux contaminated area after aging black, and accompanied by local plastic cracking, serious will lead to dead lights. As shown in Figure 6 below.
C, high temperature lead to chip degradation, glue carbonization and COB solder mask color change. As shown in Figure VII below.
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▲ Figure five COB plastic crack
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▲ Figure VI COB colloidal surface black
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Figure VII COB high temperature failure picture
From the above mode of failure of SMD and COB point of view, are more or less the same. May be slightly different because of the package or application area. Therefore, we can summarize the following: Reasonable temperature conditions allow the LED to run well; stable current can make the LED lamps rise in quality; away from volatile S, halogen and volatile organic compounds; proper operation and good operation (Such as adding a blue film COB, after welding is completed and then tear off), can reduce the loss caused by the operation; good electrostatic protection, to prevent the potential risk of failure.
 
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