Six problems of LED display chip

LED chip is the heart of LED products. Its main function is to convert electric energy into light energy. In the process of use, LED display chip will also have a variety of problems. Now let's take a look at the six common problems of LED chip!

Low forward voltage, dark light

(1) One is the ohmic contact between the electrode and the luminescent material, but the large contact resistance is mainly caused by the low concentration of the material substrate or the electrode defect.

(2) One is the non ohmic contact between the electrode and the material, which mainly occurs in the extrusion or pinch of the first layer of electrode during the preparation of chip electrode.

In addition, the decrease of forward voltage may be caused in the process of packaging. The main reasons are that the silver glue is not cured sufficiently, and the electrode of the bracket or chip is contaminated, resulting in large contact resistance or unstable contact resistance.

When the forward voltage decreases, the current passing through the chip is small, which leads to dark spots. Another dark phenomenon is that the chip itself has low luminous efficiency and normal forward voltage drop.

Pressure resistant welding

(1) Non stick: mainly because the electrode surface oxidation or glue

(2) There are some problems, such as loose contact with luminescent materials and thickened wire layer, in which the thickened layer falls off.

(3) Pierce electrode: it is usually related to chip materials. Brittle and low strength materials are easy to pierce the electrode. GaAlAs materials (such as high red and infrared chips) are easier to pierce the electrode than gap materials.

(4) Pressure welding debugging should be adjusted from welding temperature, ultrasonic power, ultrasonic time, pressure, golden ball size, bracket positioning, etc.

Luminous color difference

(1) The color difference of the same chip is mainly due to the problem of epitaxial materials. The quantum structure of AlGaInP four element material is very thin, so it is difficult to ensure that the composition of each region is consistent（ The composition determines the band gap, and the band gap determines the wavelength).

(2) Gap yellow and green chip, the luminous wavelength will not have a big deviation, but because the human eye is sensitive to the color of this band, it is easy to detect yellow, green. As the wavelength is determined by the epitaxial material, the smaller the area is, the smaller the color deviation concept appears. Therefore, there is a neighbor selection method in M / T operation.

(3) Some gap red chips are orange yellow, which is due to the indirect leap mechanism. Due to the influence of impurity concentration, when the current density increases, the impurity energy level shift and luminescence saturation are easy to occur, and the luminescence begins to turn orange.

 

Gate fluid effect

(1) It is that led can not be turned on under normal voltage. When the voltage is increased to a certain extent, the current will change.

(2) The reason for the phenomenon of gate fluid is that the reverse interlayer appears when the epitaxial sheet of luminescent material grows. The positive voltage drop of LED tested at if = 20mA is hidden. In the process of use, the two pole voltage is not large enough, which is not bright. The test information instrument can be used to test the curve from the transistor grapher, or it can be found by the forward voltage drop under the condition of small current if = 10uA, The positive voltage drop under small current is obviously larger, which may be caused by this problem.

Reverse leakage current IR

Under limited conditions, the reverse leakage current is the basic characteristic of diode. According to the previous conventional regulations of LED, it refers to the reverse leakage current when the reverse voltage is 5V. With the improvement of LED performance, the reverse leakage current will be smaller and smaller. The smaller the IR, the better. The reason is the irregular movement of electrons.

(1) The quality problem of the chip itself may be caused by abnormal cutting of the chip itself.

(2) If there are too many silver glue spots, it will lead to short circuit. The reverse leakage caused by epitaxy is mainly caused by the internal structure defects of PN junction. In the process of chip fabrication, the side corrosion is not enough or silver glue is adhered to the measuring surface. It is strictly forbidden to use organic solution to prepare silver glue. In order to prevent the silver glue from climbing to the junction area through the capillary phenomenon.

(3) Electrostatic injury. Epitaxial materials, chip manufacturing, device packaging, testing, generally 5V reverse leakage current is 10uA, can also be fixed reverse current test reverse voltage. The reverse characteristics of different types of LED vary greatly: the reverse breakdown of common green and common yellow chips can reach more than 100 V, while the reverse breakdown of common red chips is between 10 and 20 v.

(4) Improper control of wire pressure leads to chip collapse and IR rise.

Solution

(1) The amount of silver glue should be controlled at 1 / 3 ~ 1 / 2 of the wafer height;

(2) The static electricity of human body and machine should be controlled below 50V;

(3) The pressure of the first point of the welding line should be controlled between 30 and 45g.

Dead light phenomenon

(1) The leakage current of LED is too large to cause PN junction failure, which makes the LED light not on. Generally, this situation will not affect the work of other LED lights.

(2) The internal connection lead of the LED lamp is disconnected, causing the LED no current to pass through and causing the dead lamp. This situation will affect the normal operation of other LED lamps. The reason is that the working voltage of the LED lamp is low (red, yellow, orange, led 1.8v-2.2v, blue, green and white LED 2.8-3.2v). Generally, series and parallel connection are used to adapt to different working voltages, The more LED lights in series, the greater the impact. As long as the internal wiring of one of the LED lights is open, the whole series of LED lights in the series circuit will not be on. It can be seen that this situation is more serious than the first one.