Analysis of several common quality problems of LED chips

Now LED headlamp is used more and more. LED chip is the core of LED headlamp. In daily life, there are several common quality problems of LED chip. Let's make a brief summary here.

1. Color difference may exist in the same batch of chips

Most of the time, we can see the same batch of chips purchased by naked eyes, and there are differences in the color of the light outside the chips. The main reasons are as follows

Reason 1: the difference in the color of the same batch of chips is mainly due to the different external materials, but the structure of the external weak materials is very weak, the size and thickness of the external materials will seriously affect the wavelength of the chip and other unpredictable differences in all aspects.
 

Reason 2: gap yellow green chip, there will be no big deviation in the light-emitting wavelength, but because the human eye is very sure about the color of this band, it is very easy to see that it is yellow and green. Because the wavelength is determined by the external material of the chip, and the size and thickness of the external material determine the wavelength, the smaller the deviation of the external material is, the smaller the probability of color deviation will be, so there is proximity selection method in M / T operation.
 

Reason 3: gap red chips have orange color, which is due to different color coordinates. What will affect the color coordinates of the light-emitting chip? We have to think of the raw materials for making the chip, and there will be some impurities in everything. These impurities will directly affect the color coordinates of these chips. The first thing we think about is the proportion of impurities, the proportion is different, the current density will be different, and the light-emitting will be obviously different, which will cause the light-emitting to turn orange.
 

2. The positive and negative working differences of the same batch of chips affect the current:

Reason 1: in a finished lamp, for example, the voltage of a disk of chips is some 2.8-3.0v. The voltage of this disk of chips is a positive and negative voltage, and there will always be a deviation when the finished product is made, and then there will be a small current deviation, and there will be a light and dark theory.
 

Reason 2: one is the non ohmic contact between the electrode and the material, which mainly occurs in the chip manufacturing process. The surface materials will have different proportions, and the slightly small time and location will affect the voltage of the finished product.
 

In addition, the voltage may also be different in the packaging process. The main reason is that there will be some weak physical changes in the surface layer, which is called forward voltage drop change, resulting in large contact resistance such as chip electrode contamination or unstable contact resistance.
 

3. Difficult to weld: (mainly including not sticking, electrode falling off, electrode penetration)

Cause 1: sticking: mainly due to impurities on the electrode surface, the presence of contact surface does not affect the welding
 

Reason 2: incomplete contact with solder, or too little solder, in which there may be other foreign matters in the midway, which will increase or decrease the temperature, and the upper welding layer will fall off in the midway
 

Reason 3: puncture electrode: it is usually related to chip material. The weak material with low hardness and strength is easy to puncture electrode. Generally, GaAlAs material (such as high red, infrared chip) is easier to puncture electrode than gap material.
 

Reason 4: the welding temperature, welding time, pressure force, welding area and solder size should be adjusted in the process of pressure welding.
 

4. Gate fluid effect

Reason 1: LED can't light up normally under a certain normal voltage. When the voltage rises to a certain extent, the current will reverse suddenly.
 

Reason 2: the reason for the gate fluid phenomenon is that there are other changes in the growth of the outer layer of the light-emitting material called reverse interlayer. When if = 60mA, the positive voltage drop measured is hidden. In the use process, the voltage at the two poles is not large enough, which means there is no response directly. The measurement information instrument can test the curve from the transistor graph or the positive voltage at low current if = 10uA It is found that the positive voltage drop is obviously larger under low current, which may be caused by this problem.
 

5. Reverse leakage:

Reason 1: cause: surface material, chip manufacturing process, device packaging, test reverse voltage under fixed current, or reverse leakage current under 5V is 10uA
 

Reason 2: the reverse characteristics of different kinds of LED are quite different: common green, common yellow chip reverse breakdown can reach more than one hundred volts, and common chip is between ten and twenty volts.
 

Reason 3: the reverse leakage caused by epitaxy is mainly caused by the different process ratio of internal structure. In the chip manufacturing process, the ratio of various materials in the surface layer varies, and the manufacturing time will also affect the structure of the surface layer. It is strictly prohibited to use organic solution to mix silver glue. In order to prevent the silver glue from climbing to the junction area through capillary phenomenon.