One article: understand the causes of LED chip failure and package failure

Date：2022-08-08 16:39:00Views：1167

The core framework of LED chip is a semiconductor chip, which is composed of p-type semiconductor and n-type semiconductor. P-type semiconductor is dominant in the hole of the chip. When p-type semiconductor and n-type semiconductor are connected, a p-n junction will be formed. When the current acts on the semiconductor wafer through the wire, the electrons will be pushed to the P-region. In the P-region, the electrons and holes coincide and emit energy in the form of photons. This is also the working principle of the LED chip. The wavelength of light is also the color of light, which will be determined by the material of the p-n junction. The main material of LED chip is monocrystalline silicon. As the core component of LED light source, its quality determines the performance and reliability of products. Any improper use may damage the chip and cause the chip to fail during use. For application engineers, chip failure analysis is one of the most difficult problems. The following is a brief analysis of LED chip failure for your reference.

LED chip features:

1. It is siyuanxi, prepared by MOVPE process

2. Good reliability

3. Widely used

4. High safety

5. Long service life

The main categories are:

1. MB chip: metal adhesive chip, a patented product of UEC

2. GB chip: adhesive bonding chip, patented product of data UEC

3. TS chip: transparent substrate chip, a patented product of HP

4. As chip: absorption substrate chip

LED chip structure is flip chip structure and vertical structure

Important parameters of LED chip are:

LF: forward working current

VF: forward working voltage

V-I characteristics: voltage current relationship of light emitting diodes

4: Luminous intensity

-90 ° - + 90 °: luminous angle of LED chip

Δλ： Spectral half width

IFM: Maximum Forward DC current

VRM: maximum reverse voltage

PM: allowable power consumption

LED light source has appeared in traditional lighting and other fields, but there are still many unsolved problems in LED light source. These include poor consistency, high cost and poor reliability. The most important problem is stability and reliability. Although the life of the LED light source is currently predicted to exceed 50000 hours. But this life refers to the theoretical life, the service life of the light source at 25 ℃. In the actual use process, it will encounter high temperature, high humidity and other harsh environments, amplify the defects of LED light source, accelerate the aging of materials, and make the LED light source rapidly fail.

Physical mechanism of failure mode

LED lamp beads are a system composed of multiple modules. The failure of each component will lead to the failure of LED beads. From the light-emitting chip to the LED lamp beads, there are nearly thirty failure modes, as shown in Table 1, the failure modes of LED lamp beads. Here, the LED is divided into two parts: chip and external package. Then, the mode and physical mechanism of LED failure are also divided into chip failure and package failure.

Table 1 failure modes of LED lamp beads

1、 The causes of LED chip failure mainly include static electricity, current and temperature.

Electrostatic discharge can release instantaneous ultra-high voltage, which brings great harm to LED chip. LED chip failure caused by ESD can be divided into two modes: soft failure and hard failure. The high voltage / current caused by static electricity causes the LED chip to short circuit and become a hard failure mode.

The reason for the short circuit of LED chip is that the electrolyte is broken by the high voltage, or the current path is generated in the chip by the high current density.

The slightly lower voltage / current released by the static electricity will cause the soft failure of the LED chip. The soft failure is usually accompanied by the decrease of the chip reverse leakage current, which may be caused by the disappearance of a part of the leakage current path due to the high reverse current.

Compared with vertical LED chips, static electricity is more harmful to horizontal LED chips. Because the electrodes of the horizontal LED chip are on the same side of the chip, the instantaneous high voltage generated by static electricity is more likely to short circuit the electrodes on the chip, thereby causing the LED chip to fail.

High current will also bring LED chip failure: on the one hand, high current will bring relatively high junction temperature; On the other hand, electrons with high kinetic energy enter the PN junction and break the MG-H bond and the ga-n bond.

The breaking of MG-H bond will further activate the carriers of the p layer, so that the LED chip has a light power rising stage at the beginning of aging, and the breaking of ga-n bond will form nitrogen vacancies.

Nitrogen vacancies increase the possibility of non radiative recombination, which explains the attenuation of optical power of the device. It is a long process for the formation of nitrogen vacancies to reach equilibrium, which is the main reason for the slow aging of LED chips.

At the same time, large current will bring current crowding in the LED chip. The larger the defect density in the LED chip, the more serious the current crowding phenomenon.

Excessive current density will cause electromigration of metal and make LED chips fail. In addition, under the dual effects of current and temperature, the highly unstable mg-h2 complex will appear in the effectively doped p layer of InGaN light emitting diode.

The effect of temperature on LED chip is mainly to reduce the internal quantum efficiency and shorten the life of LED chip.

This is because the internal quantum efficiency is a function of temperature. The higher the temperature, the lower the internal quantum efficiency. At the same time, the aging effect of temperature on the material will make the ohmic contact and the internal material of the LED chip worse.

In addition, the high junction temperature makes the temperature distribution in the chip uneven and generates strain, which reduces the internal quantum efficiency and the reliability of the chip. When the thermal stress is large to a certain extent, the LED chip may be broken.

2、 The factors causing LED package failure mainly include temperature, humidity and voltage.

At present, the most in-depth and extensive research is the influence of temperature on the reliability of LED packaging. The causes of LED module and system failure due to temperature are as follows:

(1) High temperature will accelerate the degradation of packaging materials and reduce the performance;

(2) Junction temperature has a great influence on the performance of LED. Too high junction temperature will cause the phosphor layer to burn black and carbonize, resulting in a sharp reduction in LED light efficiency or catastrophic failure.

In addition, due to the mismatch of refractive index and thermal expansion coefficient between silica gel and phosphor particles, the conversion efficiency of phosphor will be decreased at too high temperature, and the higher the proportion of phosphor doped, the worse the light efficiency will be;

(3) Due to the mismatch of thermal conductivity between packaging materials and the uneven temperature gradient and temperature distribution, cracks may occur inside the materials or delamination may occur at the interface between materials.

These cracks and delaminations will cause the light efficiency to decrease. The delamination between the chip and the phosphor layer can reduce the light extraction efficiency. The delamination between the phosphor layer and the potted silica gel can reduce the light extraction efficiency by more than 20%.

The delamination between the silica gel and the substrate may even cause the gold wire to break and cause catastrophic failure.

Through the experimental research on high humidity environment, it is found that the invasion of moisture not only makes the LED light efficiency decline, but also may lead to the catastrophic failure of LED.

Through the high temperature and high humidity reliability acceleration experiment at 85 ℃ / 85% RH, it is found that moisture plays an important role in the formation of delamination defects. The delamination phenomenon causes the light efficiency of led to decrease, and different chip surface roughness leads to different failure modes.

The above is the related content of LED chip failure analysis compiled by Chuangxin testing small. I hope it can help you. Our company has professional engineers and industry elite teams, and has three standardized laboratories with an area of more than 1800 square meters. It can undertake various test projects such as electronic component testing and verification, IC authenticity identification, product design and material selection, failure analysis, functional testing, factory incoming material inspection and tape weaving.