6 major problems often encountered in LED chips

(1) One is that the electrode is in ohmic contact with the luminescent material, but the contact resistance is large, mainly due to the low concentration of the material substrate or the defect of the electrode.

(2) One is that the electrode and the material are non-ohmic contacts, mainly occurring in the extrusion printing or the printing position when the first layer electrode is evaporated during the preparation of the chip electrode.

In addition, the forward pressure may also be reduced during the packaging process. The main reason is that the silver gel is not sufficiently cured, and the contact or chip electrode is contaminated, resulting in large contact resistance or unstable contact resistance.

When the chip with reduced forward voltage is tested at a fixed voltage, the current passing through the chip is small, thereby expressing a dark spot. Another dark light phenomenon is that the chip itself has low luminous efficiency and a normal forward voltage drop.

(1) Non-stick: mainly because the surface of the electrode is oxidized or has glue

(2) There is a weak contact with the luminescent material and the thickened bonding wire layer is not strong, and the thick layer is mainly detached.

(3) Puncture electrode: usually related to the chip material, the material is brittle and the strength is not high, the material is easy to penetrate the electrode. Generally, the GAALAS material (such as high red, infrared chip) is easier to penetrate the electrode than the GAP material.

(4) Pressure welding adjustment should be adjusted from welding temperature, ultrasonic power, ultrasonic time, pressure, gold ball size, bracket positioning and so on.

(1) The difference in the color of the same chip is mainly due to the problem of the epitaxial wafer material. The ALGAINP four-element material is very thin in quantum structure, and it is difficult to ensure that the composition of each region is uniform. (The component determines the forbidden band width and the forbidden band width determines the wavelength).

(2) GAP yellow-green chip, the emission wavelength will not be greatly deviated, but because the human eye is sensitive to the color of this band, it is easy to detect yellowish and greenish. Since the wavelength is determined by the material of the epitaxial wafer, the smaller the region, the smaller the concept of color deviation, so there is a proximity selection method in the M/T operation.

(3) Some of the GAP red chips have a yellowish color, which is due to the indirect leap forward. Affected by the impurity concentration, when the current density is increased, impurity level shift and luminescence saturation are likely to occur, and the luminescence starts to turn orange.

(1) The LED cannot be turned on under normal voltage. When the voltage is raised to a certain level, the current is abrupt.

(2) The phenomenon of generating thyristor is caused by the reverse interlayer when the epitaxial wafer of luminescent material grows. The forward voltage drop of the LED with this phenomenon is hidden when IF=20MA, and the process is due to insufficient voltage of the two poles. Large, the performance is not bright, the test information instrument can be used to test the curve from the transistor plotter, or it can be found by the forward voltage drop under the small current IF=10UA. The forward voltage drop under the small current is obviously too large, it is possible It is caused by this problem.

The reverse leakage current under the limited conditions is the basic characteristic of the diode. According to the previous conventional regulations of the LED, it refers to the reverse leakage current when the reverse voltage is 5V. As the performance of LEDs increases, the reverse leakage current will become smaller and smaller. The smaller the IR, the better, due to the irregular movement of electrons.

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

(2) Too many silver glue points can cause a short circuit when it is serious. The reverse leakage caused by epitaxy is mainly caused by the internal structural defects of the PN junction. The side corrosion is not enough or the silver rubber wire is attached to the measuring surface during the chip making process. It is strictly forbidden to use the organic solution to prepare the silver paste. In order to prevent the silver glue from climbing to the knot area through capillary phenomenon.

(3) Electrostatic injury. Epitaxial material, chip fabrication, device package, test, the general leakage current is 5UA under 5V, and the reverse voltage can be tested under fixed reverse current. The reverse characteristics of different types of LEDs are quite different: the green color, the reverse breakdown of the yellow chip can reach more than one hundred volts, and the red chip is between ten and twenty volts.

(4) Improper control of the wire pressure causes the wafer to collapse and cause an increase in IR.

solution:

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

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

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

(1) The leakage current of the LED is too large, causing the PN junction to fail, so that the LED light is not bright. This situation generally does not affect the operation of other LED lights.

(2) The internal connection lead of the LED lamp is disconnected, causing no current to pass through the LED and generating a dead light. This situation will affect the normal operation of other LED lamps, because the operating voltage of the LED lamp is low (red yellow orange LED working voltage) 1.8v-2.2v, blue green white LED working voltage 2.8-3.2v), generally have to use string, parallel to connect, to adapt to different working voltage, the more LED lights in series, the greater the impact, As long as one of the LED lights is internally connected, the entire LED of the series circuit will not light up, which is more serious than the first case.