LED chips often encountered six major problems

The forward voltage is reduced, dim light

(1) One is that the electrode is in ohmic contact with the luminescent material, but the contact resistance is large, which 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 printing or the printing and the distribution at the time of evaporating the first electrode in the preparation of the chip electrode.

In addition the packaging process may also cause the forward voltage drop is low, mainly due to insufficient silver plastic curing, stent or chip electrode contamination caused by contact resistance or contact resistance instability.

The forward voltage drop of the chip in the fixed voltage test, the current through the chip is small, which shows the dark spots, there is a dark light phenomenon is the chip itself, low luminous efficiency, normal pressure drop normal.

Difficult to pressure welding

(1) play non-stick: mainly because the electrode surface oxidation or glue

(2) there is not strong contact with the luminescent material and thicker wire layer is not strong, in which the thick layer off mainly.

(3) piercing the electrode: usually associated with the chip material, brittle and low strength materials easy to break through the electrode, the general GAALAS materials (such as high red, infrared chip) than the GAP material easy to break through the electrode.

(4) pressure welding debugging should be from the welding temperature, ultrasonic power, ultrasonic time, pressure, the size of the ball, bracket positioning and other adjustments.

Luminous color difference

(1) There is a significant difference in the luminescent color of the same chip mainly because of the material of the epitaxial wafer. The quantum structure of the four-element ALGAINP material is very thin, and it is difficult to ensure the consistent growth of the components in all the regions. (Component determines the forbidden band width, the forbidden band width determines the wavelength).

(2) GAP yellow-green chips, the wavelength of light will not have a great deviation, but because of the human eye is sensitive to the color of this band, it is easy to detect the yellow, green. Since the wavelength is determined by the material of the epitaxial wafer, the smaller the area is, the smaller the concept of color deviation occurs is. Therefore, there is a proximity selection method in the M / T operation.

(3) GAP red chip some glow color is orange, which is due to its light-emitting mechanism is an indirect jump. Affected by the impurity concentration, current density increases, easy to produce impurity level shift and luminous saturation, the light is beginning to change to orange.

Throttle fluid effect

(1) is the light-emitting diode can not be turned on at normal voltage, when the voltage increases to a certain extent, the current mutation.

(2) The cause of the thyristor phenomenon is that the epitaxial layer of the light-emitting material grows reversely when it grows. The LED with this phenomenon has a hidden voltage drop when tested at IF = 20MA. In the process of using, the bipolar voltage is not enough Large, the performance is not bright, available test information Instrument from the transistor graphical test curve, but also through the small current IF = 10UA forward voltage drop to find that the small current forward voltage drop significantly larger, it may be This problem is caused.

Reverse leakage current IR

Under limited conditions, the reverse leakage current is the basic characteristics of the diode, according to the previous conventional LED rules, refers to the reverse voltage reverse leakage current of 5V. As the LED performance increases, the reverse leakage current will be smaller and smaller. The smaller the IR the better, resulting from the irregular movement of electrons.

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

(2) too much silver paste point, in severe cases can cause short circuit. The reverse leakage caused by the epitaxy is mainly caused by the internal structural defects of the PN junction. The side surface corrosion during the chip manufacturing is not enough or there is silver glue attached to the measuring surface. The silver paste is prohibited to be dispensed with the organic solution. In order to prevent the silver glue through the capillary phenomenon climbed knot area.

(3) Electrostatic injury. Epitaxial materials, chip production, device packaging, testing the general 5V reverse leakage current of 10UA, you can also test the reverse voltage fixed reverse voltage. Different types of LED reverse characteristics vary greatly: Pu green, yellow reverse breakdown of the chip can reach more than 100 volts, while the general red chip in a dozen twenty volts.

(4) wire pressure control properly, resulting in intracranial collapse led to IR.

solution:

(1) The amount of silver glue to be controlled in the height of the chip 1/3 ~ 1/2;

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

(3) the pressure of the first wire should be controlled between 30 ~ 45g better.

Dead lights phenomenon

(1) LED leakage current is too large cause PN junction failure, the LED lights do not light, this situation generally does not affect the work of other LED lights.

(2) The internal connection leads of the LED lamp are disconnected, resulting in the dead lamp caused by no current passing through the LED. In this case, the normal operation of other LED lamps will be affected because the working voltage of the LED lamp is low (red, yellow and orange LED operating voltage 1.8v-2.2v, blue-green white LED working voltage 2.8-3.2v), generally have to use the string, parallel connection, to adapt to different operating voltage, the greater the impact of series LED lights, As long as one of the LED's internal wiring is open, the entire LED string of the series circuit will not be lit. This shows that the situation is more serious than the first case.

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