Six Major Problems That Often Occur In Led Display Chips
May 25, 2021
1. Decrease in forward voltage, dark 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 defect of the electrode.
(2) One is that the electrode and the material are in non-ohmic contact, which mainly occurs when the first layer of electrode is evaporated during the preparation of the chip electrode, and the position of the extrusion or clamping.
In addition, the forward pressure may also be reduced during the packaging process, mainly due to insufficient curing of the silver glue, contamination of the bracket or chip electrode, etc., which cause high contact resistance or unstable contact resistance.
When a chip with a reduced forward voltage is tested at a fixed voltage, the current passing through the chip is small, thus showing a dark spot. There is also a dark light phenomenon that the chip itself has low luminous efficiency and the forward voltage drop is normal.
2. Difficult to pressure welding
(1) No stickiness: Mainly due to oxidation or glue on the electrode surface
(2) There are weak contact with the luminescent material and the thickened wire bonding layer, among which the thickened layer falls off.
(3) Piercing electrode: Usually related to the chip material, materials with brittle and low strength are easy to pierce the electrode. Generally, GAALAS materials (such as high red, infrared chips) are easier to pierce the electrode than GAP materials.
(4) The pressure welding debugging should be adjusted from the welding temperature, ultrasonic power, ultrasonic time, pressure, gold ball size, bracket positioning, etc.
3. Luminous color difference
(1) The obvious difference in the luminous color of the same chip is mainly due to the material problem of the epitaxial wafer. The four-element ALGAINP material adopts a very thin quantum structure, and it is difficult to ensure the consistency of the composition in each region. (The composition determines the band gap, and the band gap determines the wavelength).
(2) The GAP yellow-green chip does not have a large deviation in the emission wavelength, but because the human eye is sensitive to the color of this band, it is easy to detect the yellowish and greenish color. Since the wavelength is determined by the material of the epitaxial wafer, the smaller the area, the smaller the concept of color deviation, so there is a neighbor selection method in the M/T operation.
(3) Some of the light-emitting color of the GAP red chip is orange-yellow, which is because the light-emitting mechanism is an indirect leap forward. Affected by the impurity concentration, when the current density increases, the impurity level shift and luminescence saturation are likely to occur, and the luminescence starts to turn orange-yellow.
4. Brake fluid effect
(1) The light-emitting diode cannot be turned on under normal voltage. When the voltage is increased to a certain level, the current has a sudden change.
(2) The thyristor phenomenon is caused by the reverse interlayer during the growth of the luminescent material epitaxial wafer. The forward voltage drop of the LED with this phenomenon is hidden when the IF=20MA. The use process is due to insufficient voltage between the two electrodes. If it is large, it is not bright. You can use the test information instrument to check the test curve of the transistor graph instrument. It can also be found by the forward voltage drop under small current IF=10UA. If the forward voltage drop under small current is obviously too large, it may Is caused by the problem.
5. Reverse leakage current IR
Under limited conditions, the reverse leakage current is the basic characteristic of the diode. According to the conventional regulations before the 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. 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) There are too many silver glue dots, which will cause a short circuit in severe cases. The reverse leakage caused by the 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 the silver glue wire sticks to the measuring surface. It is strictly forbidden to mix the silver glue with organic solution. In order to prevent the silver glue from climbing to the junction area through the capillary phenomenon.
(3) Static damage. Epitaxial materials, chip production, device packaging, testing generally reverse leakage current under 5V is 10UA, it is also possible to test the reverse voltage under a fixed reverse current. The reverse characteristics of different types of LEDs are quite different: the reverse breakdown of normal green and normal yellow chips can reach more than 100 volts, while the normal red chips are between ten and twenty volts.
(4) Improper control of the bonding wire pressure causes the chip to collapse and the IR rises.
solution
(1) The amount of silver glue should be controlled at 1/3～1/2 of the height of the chip;
(2) The amount of static electricity of the human body and the machine must be controlled below 50V;
(3) The pressure at the first point of the welding wire should be controlled between 30 and 45g.
6. Dead light phenomenon
(1) The excessive leakage current of the LED causes the failure of the PN junction and the LED lamp does not light up. This situation generally does not affect the work of other LED lamps.
(2) The internal connecting lead of the LED lamp is disconnected, causing no current to pass through the LED and causing a 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, and orange LED working voltage). 1. 8v-2. 2v, blue, green and white LED working voltage 2. 8-3. 2v), generally must be 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.
