LED chips are the heart of LED products. The main function is to convert electrical energy into light energy. In the process of use, LED display chips will also have various problems. Now let's take a look at the common occurrences of LED chips. Six questions!
01. Forward voltage reduction, dark light
(1) One is that the electrode and the luminescent material are in ohmic contact, 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 that the electrode and the material are in non-ohmic contact, which mainly occurs during the process of evaporating the first layer of electrode in the chip electrode preparation process.
In addition, the forward voltage may also be reduced during the packaging process. The main reasons are that the silver glue is not fully cured, and the bracket or chip electrodes are stained, resulting in high 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, thus showing dark spots. There is also a dark light phenomenon that the chip itself has low luminous efficiency and normal forward voltage drop.
02. Difficult pressure welding
(1) Non-stick: mainly because the electrode surface is oxidized or glued
(2) The contact with the luminescent material is not strong and the thickened wire layer is not strong, among which the thickened layer falls off.
(3) Piercing electrodes: usually related to chip materials, materials with brittle materials and low strength are easy to penetrate electrodes. Generally, GAALAS materials (such as high red, infrared chips) are easier to penetrate electrodes than GAP materials.
(4) Pressure welding debugging should be adjusted from welding temperature, ultrasonic power, ultrasonic time, pressure, gold ball size, bracket positioning, etc.
03. Differences in luminous color
(1) The obvious difference in the luminous color of the same chip is mainly due to the problem of the epitaxial wafer material. The ALGAINP four-element material adopts a very thin quantum structure, and it is difficult to ensure that the components of each region are consistent during growth. (The composition determines the band gap, and the band gap determines the wavelength).
(2) For GAP yellow-green chips, the light-emitting wavelength will not vary greatly, 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 epitaxial wafer material, the smaller the area, the smaller the concept of color deviation, so there is a neighboring selection method in the M/T operation.
(3) Some of the light-emitting color of the GAP red chip is orange-yellow, which is due to the indirect leap forward in its light-emitting mechanism. Affected by the 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-yellow.
04. Thyristor effect
(1) The light-emitting diode cannot be turned on under normal voltage. When the voltage is increased to a certain extent, the current will change abruptly.
(2) The reason for the thyristor phenomenon is that there is a reverse interlayer when the epitaxial wafer of the luminescent material is grown. The LED with this phenomenon has hidden forward voltage drop when tested at IF=20MA. During the use process, the voltage between the two poles is not enough. If it is large, it does not appear to be bright. You can use the test information instrument to test the curve from the transistor chart, or you can find out by the forward voltage drop under the small current IF=10UA. The forward voltage drop under the small current is obviously too large. is caused by this problem.
05. Reverse leakage current IR
Under limited conditions, the reverse leakage current is the basic characteristic of the 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 the performance of light-emitting diodes, the reverse leakage current will become smaller and smaller. The smaller the IR, the better, and the cause is the irregular movement of electrons.
(1) The reason for the quality problem of the chip itself may be caused by abnormal cutting of the chip itself.
(2) Too much silver glue dots will lead to short circuit in severe cases. The reverse leakage caused by epitaxy is mainly caused by the internal structural defects of the PN junction. During the chip manufacturing process, the side corrosion is not enough or there is silver glue attached to the measurement surface. It is strictly forbidden to use organic solutions to prepare silver glue. In order to prevent the silver glue from climbing to the junction area through capillary phenomenon.
(3) Static damage. For epitaxial materials, chip fabrication, device packaging, and testing, the reverse leakage current is generally 10UA under 5V, and the reverse voltage can also be tested under a fixed reverse current. The reverse characteristics of different types of LEDs are quite different: the reverse breakdown of common green and common yellow chips can reach more than one hundred volts, while the common red chips are between ten and twenty volts.
(4) Improper control of bonding wire pressure, resulting in chip collapse and increased 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 static electricity of the human body and the machine should be controlled below 50V;
(3) The pressure of the first point of the welding wire should be controlled between 30 and 45g.
06. Dead light phenomenon
(1) The leakage current of the LED is too large, causing the PN junction to fail, so that the LED lamp does not light up. This situation generally does not affect the work of other LED lamps.
(2) The internal connection lead of the LED light is disconnected, causing the LED to have no current passing through and producing a dead light. This situation will affect the normal operation of other LED lights. 1.8v-2.2v, blue-green-white LED working voltage 2.8-3.2v), generally connected in series and parallel to adapt to different working voltages, the more LED lights in series, the greater the impact. As long as one of the LED lights has an open circuit, the entire string of LED lights in the series circuit will not light up. It can be seen that this situation is much more serious than the first situation.
