[Questions and Answers] Five major technical issues in the use of LED chips

  I saw a senior engineer summarize several common problems encountered in the use of LED chips and how to solve them on this website, and shared them with everyone. Please see below.

　　1. Forward voltage decreases, dim light:

　　A: One is that the electrode and the light-emitting material are in ohmic contact, but the contact resistance is large, which is mainly caused by low concentration of the material substrate or electrode defects.

　　B: One is that the electrode and the material are in non-ohmic contact, which mainly occurs in the extrusion or clamping print when the first layer of electrode is evaporated during the chip electrode preparation process, and the distribution position.

　　In addition, the packaging process may also cause a decrease in forward voltage drop. The main reasons are insufficient curing of the silver glue, contamination of the bracket or chip electrodes, etc., resulting in large or unstable contact resistance.

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

　　2. Difficult pressure welding: (mainly non-stick, electrode falling off, electrode penetration)

　　A: Non-stick: Mainly because the electrode surface is oxidized or has glue

　　B: There is a loose contact with the luminous material and a loose thickened welding wire layer, with the thickened layer falling off being the main problem.

　　C: Punch-through electrode: usually related to chip material. Materials that are brittle and low in strength are easy to punch through electrodes. Generally, GAALAS materials (such as high red and infrared chips) are easier to punch through electrodes than GAP materials.

　　D: The pressure welding debugging should be adjusted from the welding temperature, ultrasonic power, ultrasonic time, pressure, gold ball size, bracket positioning, etc.

3. Difference in luminous color:

　　A: The main reason why the same chip has obvious differences in luminous color is the material of the epitaxial wafer. The quantum structure of the ALGAINP four-element material is very thin, and it is difficult to ensure the consistency of components in each region during growth. (The components determine the bandgap width, and the bandgap width determines the wavelength).

　　B: For GAP yellow-green chips, the emission wavelength will not have a large deviation, but because the human eye is sensitive to the color of this band, it is easy to detect yellowish or greenish. Since the wavelength is determined by the epitaxial wafer material, the smaller the area, the smaller the color deviation, so there is a neighbor selection method in M/T operations.

　　C: Some GAP red chips have an orange-colored emission color, which is due to the indirect transition mechanism. Affected by the impurity concentration, when the current density increases, it is easy to produce impurity energy level shift and luminescence saturation, and the luminescence begins to turn orange.

　　4. Scything fluid effect:

　　A: The light-emitting diode cannot conduct under normal voltage. When the voltage is increased to a certain level, the current changes suddenly.

　　B: The reason for the thyristor phenomenon is the reverse interlayer when the epitaxial wafer of the light-emitting material grows. The forward voltage drop of the LED with this phenomenon is hidden when tested at IF=20MA. During use, it is because the voltage between the two poles is not large enough, so it does not light up. You can use the test information instrument to test the curve from the transistor diagram instrument, or you can find it through the forward voltage drop under a small current IF=10UA. If the forward voltage drop under a small current is obviously too large, it may be caused by this problem.

　　5. Reverse leakage:

　　A: Reason: Epitaxial materials, chip production, device packaging, the test generally shows that the reverse leakage current is 10UA at 5V, and the reverse voltage can also be tested at a fixed reverse current.

　　B: The reverse characteristics of different types of LEDs vary greatly: the reverse breakdown of ordinary green and yellow chips can reach more than 100 volts, while that of ordinary chips is between 10 and 20 volts.

　　C: Reverse leakage caused by epitaxy is mainly caused by internal structural defects of PN junction. During chip manufacturing, the side corrosion is insufficient or silver glue is attached to the test surface. It is strictly forbidden to use organic solvent to mix silver glue to prevent silver glue from crawling into the junction area through capillary phenomenon.