At present, LED technology is more and more mature. Since its introduction, the advantage of long service life of municipal lighting has become one of the focuses of public attention. However, in recent years, we still encounter many "dead lights" in the production and application of LED energy-saving factory lights. The so-called dead light, also known as off light, means that the LED energy-saving projection light source does not emit light. It is a very difficult problem for manufacturers to produce dead lights either in production or in application. It not only faces the loss caused by inferior products, but also affects consumers' confidence in LED products. Therefore, the research and analysis of some common causes of LED dead lights will help us to reduce and prevent the recurrence of LED product failures, ensure product quality and improve product competitiveness, and provide reference for enterprise technology improvement and promotion, thus creating greater economic benefits for enterprises.

Shandong Lanyue Photoelectric Technology Co., Ltd., as a professional LED light manufacturer, has accumulated a large number of dead light cases since its establishment in 2008. To sum up, the common reasons for LED energy-saving floodlights dying are as follows:

1、 Disconnection

For the "dead light", we should first determine whether the LED is short circuited or open circuited. If it is an open circuit, we usually consider whether the bonding wire inside the LED light is disconnected. The welding wire inside the LED lamp is disconnected, resulting in no power supply voltage for the LED, which is one of the common reasons for LED extinguishment. There are 5 common disconnection positions of welding wire

Case 1.1

Invalid lamp bead model is 5730. The lamp bead will die after 100 thermal shock tests. After cross section analysis of the failure sample, it was found that the silica gel around the first and second solder joints of the failure sample was broken, and the second solder joints had been disconnected.

Due to the large difference in thermal expansion coefficient between silicone and gold wire, after 100 thermal shock tests, silicone and gold wire continue to expand and contract, while the bending of gold wire solder joints is a stress concentration point, which may lead to the rupture of silica gel around the solder joints, and the cracking of silica gel leads to the disconnection of the weaker part of the two solder joints of the wire, and then the sample shows dead light.

Case 1.2

The invalid lamp bead model is a fake lumen lamp bead. After the bulb bead has been used for a period of time, the bulb will go out. When the lamp is on, the current distributed to each lamp bead is about 500 mA. First, we carried out sol inspection on some unqualified samples, and found that all unqualified lamp beads were broken by four one solder joints, while four two solder joints were still intact. Then, we carried out cross section analysis on the failed samples, and found that the silica gel above the chip was broken, while the silica gel in other areas was intact.

The four disconnected first solder joints are concentrated on the chip, while the four intact second solder joints are on the bracket. This means that the rupture of the silica gel above the chip leads to the disconnection of four solder joints. The rupture of the silica gel is mainly concentrated on the chip, that is, directly above the heat source. In addition, when the lamp bead is lit, the current is very high (500mA), and it can be considered that the chip is overheated, causing the silica gel above the chip to break. Carefully check the heat dissipation path of the lamp bead. It is found that the overheating of the lamp bead chip may be related to the bottom of the lamp bead and the heat sink of the PCB board. The heat dissipation effect of this high-power lamp bead is not good enough.

2、 Bonding layer peeling

For some LED energy-saving project lights with vertical chips, the common reasons for dead lights are the peeling off of the bottom of the chip bonding layer and the peeling off of the bracket coating.

Case 2.1

The failed sample is a direct plug-in LED lamp bead, which has been turned off during use, and the defective rate is 1.5%. After cross section inspection of unqualified samples, we found that gold wire solder joints remained intact. However, it was found that the adhesive layer and bracket coating were completely stripped, and the packaging adhesive and bracket cup wall were also stripped.

From the above observation, it can be determined that the reason for the dead lamp bead is the peeling phenomenon between the sealant and the bracket. With the intensification of the use process, the degree and area of peeling expand, which further leads to the peeling of adhesive and bracket. The sample has a dead light. It may also be that the poor adhesion of the packaging glue causes delamination between the packaging glue and the bracket interface.

3、 Burned solder joints

In some cases, the dead light of the bulb may not be caused by the bulb itself, but also by the power supply used.

Case 3.1

The failed sample is an LED lamp bead that imitates lumen. After a period of use, the LED lamp bead will disappear. After checking several failed lamp bead sols, it was found that two gold wire solder joints of P electrode and the electrode pattern circuit near the failed lamp bead chip had been burned, and two gold wire solder joints of N electrode, namely, the electrode pattern circuit and the solder joints of the support for 4 seconds, remained intact, and no burns or cracks were found.

Obviously, the burning of the P electrode on the chip is the direct reason why the lamp does not light up. So, what causes the chip P electrode to burn out? Next, we conduct the following analysis.

We randomly selected several lamp bead samples that can be normally lit to simulate the high-voltage impact test, and then applied 20 V instantaneous high voltage to each lamp bead. The experimental results show that the lamp bead will die immediately after being impacted by high voltage. The post sol inspection found that the P electrode circuit on the chip was also burned and the lamp was turned on.

Through the above inspection and verification test, it can be inferred that the root cause of the dead lamps of the customer's batch of lamp beads is the excessive surge current when using the lamp beads. Because the resistance value in the P area of the chip is higher than the resistance value in the N area, when the current is concentrated through the P electrode, the P electrode burns first and causes an open circuit dead light. When the lamp driving power supply is turned on or off, the excessive surge current (or voltage) during the use of lamp beads may be related to the surge current. The P electrode wire of the chip may also have defects, which will cause the P electrode solder joints. In the case of poor instantaneous contact, when multiple LEDs are connected in series, the high voltage will accumulate and cause the instantaneous large current to pass. Poor contact will cause the lamp bead line to burn and the sealant to burn black.

4、 Chip corrosion

The previous energy-saving floodlights and energy-saving floodlights are open circuit. The following is the condition of short circuit dead light.

Case 4.1

The failed sample is a simulated lumen lamp bead. During the aging process of lamp beads, it is found that these lamp beads have bad conditions, such as dead light and dim light. After the inferior product is sol, it is found that many areas of the chip electrode are corroded and the electrode is stripped.

The X-ray energy spectrometer (EDS) was used to analyze the elements in the corrosion area of the chip, and it was found that the corrosion area of the chip electrode contained more Na, Cl and K elements.

According to the chemical composition of elements, it is speculated that the chip may be contaminated by NaCl and KCl. When heat and water vapor coexist, it will corrode the chip electrode, causing metal corrosion of the chip electrode, reducing the binding force of the electrode wire, and even part of the area falls off. The migration of dissolved substances in the electrode will short-circuit the P and N electrodes of the chip and cause the chip to die.

To sum up

There are many reasons why LED energy-saving floodlights are dead. Dead light may appear in every link from packaging, application to use. The above case is just an introduction. How to reduce and eliminate dead lights and improve product quality and reliability are the key issues that every LED company needs to face. Analyzing the cause of LED dead light is one of the important methods to reduce and eliminate LED dead light. In addition to powerful equipment hardware, failure analysis of LED products also requires the production of chips, packages and applications. Only experienced support can give full play to the functions of the equipment and solve problems for customers.