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Common Quality Problem Analysis and Strategies of LED Chips

1. High direction pressure drop, dark light

A: Electrode and luminescent materials are ohmic contacts, but the contact resistance is large, which is mainly caused by the substrate materials’ low concentration or electrode defect.

B: Electrode and materials are non-ohmic contacts, which mainly happens to be the extrusion seal or the chip seal and the distribution position of the first layer evaporation electrode in the chip electrode fabrication process.

The encapsulation process can also cause the forward voltage drop, the main reason is that the silver glue is not fully solidified; the stent or the chip electrode contamination has caused the big contact resistance or unstable contact resistance.

The chip of the forward voltage drop will show the dark spots when testing the fixed voltage, the current through the chip is small, the dark light phenomenon indicates that the chip’s luminous efficiency is low, the forward voltage drop is normal.

2. Difficult bonding: (mainly the bonding is not sticky; the electrode falls off, punching the electrode)

A: Difficult bonding: mainly because the electrode surface has oxidation or glue

B:The contact with luminescence materials is not firm and the thickening welding line is rickety, especially the thick layer falls off.

C:Punching the electrode: it is usually relevant to the chip material, brittle materials with low intensity are easy to punch the electrode, general GAALAS materials (such as the infrared chip) is easier to punch the electrode compared with the GAP materials.

D:Bonding debugging should begin with adjusting the welding temperature debugging welding, ultrasonic power, ultrasonic time, pressure, golden ball size, stents location.

3.Illuminant color differences:

A: The illuminant color of the same chip has obvious difference which is mainly caused by the extension materials. ALGAINP four elements materials adopts the thin quantum structure, it was difficult to ensure that the regional growth is the consistent components. (Components determine the forbidden bandwidth; the forbidden bandwidth determines the wavelength).

B: For GAP yellow and green chip, the shine wavelength does not have a large deviation, but because the band color is sensitive to the human eye, it is easy to find out the slant yellow and the slant green. Since the wavelength is determined by the extension materials, smaller the area is, the color deviation concept is smaller, so there is an adjacent selection method in the M/T assignments.

C: The illuminant colors of GAP red chip is slant orange, this is because the emitting mechanism is the indirect leaps. Effected by the concentration of the impurities, when the electric current density increases, it is easy to generate impurities level migration and glowing saturation, illuminance starts to change into orange-yellow.

4. Brake fluid effect:

A: The light emitting diode under the normal voltage cannot conduct, when the voltage rises to a certain degree, the current have produced mutations.

B: The reason of producing the brake fluid phenomenon is that the epitaxial wafer of the luminescence materials appears the reverse interlayer when it grows. The testing forward voltage of the LED with this phenomenon has hidden value when IF = 20 MA, the use process will show that it is not bright enough because the polar voltage is not big enough. The information testing instrument can be used to test the curve from the transistor curve tracer, it can be discovered through the forward voltage drop under the small current IF = 10 UA. The forward voltage under the small current is obvious large, it may be caused by the problem.

5. Reverse leakage:

A: Reason: extension materials, chip making, device packaging, testing the reverse leakage current under 5V, it is usually 10 UA. We can also fix the reverse current to test the reverse voltage.

B: The reverse characteristics of different types of LED differs: the reverse breakdown of the general green and general yellow chip can achieve more than one hundred volt, but the general chip is between ten and twenty volt.

C: The reverse leakage caused by the extension is mainly resulted from the PN junction internal structure defect, the side corrosion is not enough or the silver glue silk attaches on the side, it is strictly prohibited to use organic solution to allocate silver glue in order to prevent the silver glue from climbing to the punction area through the capillarity.

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