What problems LED chips usually encounter

Time:2018-10-09 Views:373

LED chips are the heart of LED displays and other products. The main function of LED chips is to convert electrical energy into light energy. In the process of use, LED chips also have various problems.

Forward voltage reduced, dark light

(1) one is that the electrode is in contact with the luminescent material as ohm, but the contact resistance is large, which is mainly caused by the low concentration of the material substrate or the defect of the electrode.

(2) one is the non-ohm contact between the electrode and the material, which mainly occurs in the extrusion printing or pinching printing when the first layer electrode is evaporated in the preparation process of the chip electrode.

In addition, positive pressure may be reduced during the packaging process, mainly due to insufficient solidification of silver glue and large or unstable contact resistance caused by the contamination of scaffold or chip electrode.

When the chip with forward voltage reduction is tested at a fixed voltage, the current through the chip is low, thus showing dark spots. Another dark phenomenon is that the chip itself has low luminous efficiency and the forward voltage drop is normal.

Difficult to bonding

(1) non-stick: mainly due to oxidation of the electrode surface or adhesive

(2) there is poor contact with the luminescent material and the thickened welding line layer, in which the thickened layer falls off mainly.

(3) piercing electrode: usually related to chip material, it is easier to Pierce the electrode if the material is brittle and not strong, and GAALAS material (such as high-red, infrared chip) is easier to Pierce the electrode than GAP material.

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

Color difference

(1) the obvious difference in luminous color of the same chip is mainly due to the problem of epitaxial wafer materials. ALGAINP four-element materials adopt a very thin quantum structure, and it is difficult to ensure the same components in all regions when growing.(composition determines band width and band width determines wavelength).

(2) GAP yellow and green chip, the luminous wavelength will not be very different, but because human eyes are sensitive to the color of this band, it is easy to detect partial yellow and partial green.As the wavelength is determined by the epitaxial material, the smaller the region, the smaller the concept of color deviation will appear, so there is an adjacent selection method in M/T operation.

(3) some of the luminescence color of GAP red chip is orange yellow, which is because its luminescence mechanism is indirect leap forward.Under the influence of impurity concentration, when current density increases, impurity level deviation and luminescence saturation are easily produced.

Gating effect

(1) the light-emitting diode cannot conduct under normal voltage. When the voltage is increased to a certain level, the current will be mutated.

(2) brake fluid phenomenon causes is luminescent material in growth of epitaxial wafer reverse intercalation, there is the phenomenon of the LED in the IF = 20 ma test positive pressure drop when the hidden, in use process is out of the poles voltage is not big enough, the show is not bright, available test instrument information from transistor curve tracer test, but can be by small current IF = 10 ua under positive pressure drop to found that small electrical flow of large, positive pressure drop may be due to the problem.

Reverse leakage current IR

The reverse leakage current is the basic characteristic of the diode under the limited condition. According to the previous conventional regulation of LED, it refers to the reverse leakage current when the reverse voltage is at 5V.As the performance of the led increases, the reverse leakage current will become smaller and smaller.The smaller the IR, the better, because the electrons move irregularly.

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

(2) too many silver glue points will lead to short circuit in serious cases.The reverse leakage caused by epitaxy is mainly caused by defects in the inner structure of PN junction. In the process of chip making, the side corrosion is insufficient or the silver glue wires are attached to the test surface. It is strictly forbidden to use organic solution to prepare silver glue.To prevent the silver glue from climbing to the junction through the capillary phenomenon.

(3) electrostatic injury.Epitaxial materials, chip fabrication, device packaging, testing generally the reverse leakage current at 5V is 10UA, and the reverse voltage can also be fixed.Different types of LED reverse characteristics vary greatly: pu green, pu yellow chip reverse breakdown can reach more than 100 volts, while pu red chip is between a dozen or twenty volts.

(4) improper pressure control of welding line, resulting in inboard collapse and IR rise.

The solution

(1) the amount of silver glue should be controlled from 1/3 to 1/2 of the chip height.

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

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

Die modulation phenomenon

(1) the leakage current of LED can cause the PN junction to fail, so the LED light cannot be lit, which generally will not affect the work of other LED lights.

(2) when the internal connection lead of the LED lamp is disconnected, the LED has no current passing and produces a dead light, which will affect the normal operation of other LED lights. The reason is that the working voltage of the LED lamp is low (red, yellow and orange LED working voltage 1).8 v - 2.2v, blue, green and white LED working voltage 2.8-3.2v), usually connected by series or parallel connection to adapt to different operating voltages. The more LED lights in series, the greater the impact will be.