Packaging technique:

In the process of package, every step may cause an effect on the performance of LED street light chips. In which bonders and bonding wires (empty solders) are most likely to raise resistances and thermal resistances. If bonders are too tick, or with air bubble, impurities, virtual sticky in them, the resistance and heat resistance also will be increased. However, since the degeneration of fluorescent powder is too obvious. The influence from this kind of packaging defects can be ignored.

 

 

Light attenuation caused by misuses:

1. Improper power suppliers. This situation may happen when users are not familiar with electric parameters of white LED lamps. Improper power suppliers will cause LED chips working outside the ASO, and consequently light attenuation. Surrounding temperature is measured when LED chips are working in stable conditions. For example, a piece of LED high bay lamp has been installed in the middle of a room, wait until it is in the stable condition, measure the internal temperature of the lamp (near LED chips) rather than the room temperature. If the surrounding temperature exceeds a threshold, lamp will be degenerated or even damaged.
2. Insufficient cooling methods: the radiation of heat requires enough area for contact surfaces. Some people prefer to add copper blocks on PCBs or aluminum plates, which is meaningless. It cannot solve the heat problem but raise the cost and heavy of products. Most heat generated by PCBs or aluminum plates will be gathered in these copper blocks. When they reach the peak of their thermal capacity, no more heat will be absorbed. Using thermal capacities of materials to help radiate heat cannot solve this problem thoroughly. It can only cool the device for a short time. Designers should not choose auxiliary cooling materials according to their thermal capacities but their specific heat capacities. Water has powerful specific heat capacities, which is widely used cooling materials. The specific heat capacity is only half of aluminum. However, neither copper blocks nor aluminum blocks are unsuitable for white LED lamps. The best way is natural cooling. Natural cooling relies on the area of radiators’ surface not the volume. If these heat generated by white LED lamps can be radiated in time, the light attenuation phenomenon will be slowed down effectively.

There are three types of bases of SMD LED Street Light chips: glass fiber, plastic, ceramic material. When in packaging, each LED chip will be fixed directly in a copper sheet, for that copper has a very high thermal conductivity. However, the volume of copper sheets is so small that heat will be radiated into external PCBs though a very short path. Experiments prove that SMD LED Street Lights chips with good cooling designs have a much longer life than plug-in LED chips. Similar low power LED chips but different packaging (low power supports/high power bases) will have different working lives. For example, 9 10 x 12 mil² LED chips fixed in parallel on base surface, under a total working current of 350 mA. After 7600 hours’ constant work, the light attenuation is about 23%. In accordance with this trend, the estimated working life will be 18000 hours. High power packaging chips have surface current density (0.324 mA/mil) twice as much as low power packaging chips (0.167 mA/mil), but has a much longer life than them (almost 9 times), as a result of using materials with higher thermal conductivities and bases with larger area. By doing so, heat can be transmitted not only in radial but also in lateral.

 

 

Other materials used in package:

Heat is the main cause of the light attenuation of white LED lamps. Thus every material related to heat radiation should be taken into account. In previous section, we discussed the LED base problem. Now let us talk about other materials used in packaging process: solid crystal glues, mix powder glues, sealants. Tests results indicate that the life of white LED lamps using silver glues is longer than using epoxy resins, but initial luminous flux is lower by 1/3. Using epoxy resins as mixed glue has a shorter working life than using silica glues, but initial luminous flux is increase by 25%. The most commonly used sealants are epoxy resins and silica glues. As a matter of fact, these two types of materials have very weak heat radiating capabilities. The thermal conductivity of iron is 230 times as much as that of epoxy resins and 150 times as much as that of silica glues. In addition to their thick, they are somehow with no capability of heat radiation.

Currents higher than the normal working current will speed up the light attenuating process, dramatically reducing the working life of white LED Lights. There are several other facts can prove that fluorescent powder’s degeneration is the main factor fir the short life of white LED lamps: 1. Normally, we will find that after aging tests, most white-ray LEDs send out a kind of blue-white rays instead of white rays, indicating the degeneration of fluorescent powder; 2. Some people tested the performance of blue-ray LED chips and found that although the capability of the entire LED lamp has dropped more than 30%, the degeneration of LED chips is only a few percent. This shows that most degeneration is from fluorescent powder. 3. Same chips, high power package has a much longer life than low power package. (will be discussed more in details in next section)

 

 

Overall, under the influence of temperatures, the degeneration of fluorescent powder is inevitable. The higher the temperature, the fast the degeneration.

 

The degeneration of blue-ray LEDs:

Among LED chips, blue-ray LEDs have the shortest working life. The average life of low power plug-in blue-ray LEDs under normal working current (20mA) is around 7000-10000 hours. Low power plug-in red-ray LEDs working under 50mA current, they will suffer no light attenuation even after 8000 hours. Besides, the average life of yellow-ray LEDs and green-rays LED Street Light are longer than 10000 hours. Thus this congenital defect of blue-ray LED chips is another main factor for the short lives of white LED lamps. The internal materials of LED chips cannot be changed, but scientists still can apply themselves on finding better materials for external parts of LED chips.

 

Materials made up LED bases have bad thermal conductivities:

Usually, low power plug-in LEDs will be fixed on based made up by iron. From the view of heat radiation, iron has a very low thermal conductivity. Meanwhile, cross areas of supporters are usually small, which cannot radiate heat in time. Materials and constructions of LED bases will cause the weak heat radiation behavior of low power plug-in LED lamps. Copper has much better heat radiating capability than iron. In consideration of its high price, LED manufacturers have to reduce the cost at the cost of performances and lives of their products.

At present, white LED lamps have very serious light attenuation problems, especially for low power white LED Street lights. Light attenuation phenomenon dramatically reduces the working life for them. Generally, white LED lamps are very expensive in LED lighting market. If manufacturers want to stay in business, they have to increase the working life of their products. But they cannot. There are several causes for the problem: the degeneration of fluorescent powder in high temperatures; the degeneration of blue-ray LEDs; the degeneration of LED bases; other packaging materials; packaging technique; misuse (improper power supply, bad cooling). In below, we will discuss them separately in more details.

 

 

The degeneration of fluorescent powder in high temperatures:

The stimulated light efficiency of fluorescent powder will drop with the increase of temperature. A testing report from a fluorescent powder factory in Guangzhou shows that the stimulated light efficiency of fluorescent powder drops by 2% under 80 ^oC. However, this is just a short-term experiment. When temperatures drop back to the normal level, fluorescent powder regains the original performance. If we intend to find whether this degeneration is restorable or not after long-term work, aging tests are necessary.

 

In some cases, we will find that after a period of aging tests (1000 hours for lower power white LEDs, 2000 hours for high power white LED Street Light), white LED lamps become brighter. This situation may cause by following reasons:

1. Fluorescent powder used together with epoxy glue lead to the degeneration of fluorescent powder. In the initial effect of temperatures, the stimulated lighting emitting ability of fluorescent powder will be recovered to some degrees.
2. In some cases, the stimulated lighting emitting ability of fluorescent powder will be raised rather than dropped for being combined with epoxy glue.
3. The performance of blue-ray LED chips will be enhanced after a period of work.

 

Among the aging tests of low power LED white lamps, scientists found that the luminous flux of them will increase within 200 hours’ continuous work. But after 200 hours, situations are completely different. They will inevitably meet with light attenuation problems. Conditions are a little bit of better for high power white LED lights. The luminous flux of these lamps will become unstable within the working range from 100 hours to 6000 hours. However, after 6000 hours work, they will also meet with attenuation problems.