I. LED Chips
An LED chip is a solid-state semiconductor device. The heart of an LED is a semiconductor chip. One end of the chip is attached to a support, which is the negative electrode, and the other end is connected to the positive electrode of the power supply. The entire chip is encapsulated in epoxy resin.
LED chips are generally very small: low-power chips are typically 8mil, 9mil, 12mil, 14mil, etc., as thin as a human hair. Previously, counting them manually was extremely laborious and inaccurate. In 2012, Xiamen Haojing Technology developed software specifically for counting LED chips. The instrument integrates high-definition digital technology to solve this most difficult counting problem. The dedicated LED chip counter consists of hardware with a megapixel industrial-grade CCD and a megapixel lens, combined with software integrating high-definition image digital technology. It is mainly used to calculate the number of LED chips. This dedicated LED chip counter accurately and quickly counts LED chips through high-speed image acquisition and visual recognition processing. It is simple to operate and easy to use.
II. Main Causes of Dark Cracks in LED Chips
The main causes of dark cracks in LED chips are three improper operations:
(i) Improper parameter adjustment
1. Other parameters are set incorrectly.
2. Improper setting of ejector pin height
3. Improper setting of die-bonding height
4. Improper crystal suction height setting
(ii) Improper institutional adjustments
1. The three points are not aligned correctly.
2. Improper welding head pressure
(iii) Malfunctioning tools
1. Insufficient vacuum pressure
2. Wear and tear on the suction nozzle and ejector pin.
III. Common Causes of Leakage in LED Chips
1. Contamination (a significant and frequently occurring problem)
LED chips are very small, and dust and other contaminants can easily contaminate them. Importantly, dust, water vapor, and various impurity ions adhere to the chip surface, and not just the surface itself.
There's an effect within the chip that can diffuse into the chip and cause further problems. For example, copper and sodium ions easily diffuse into semiconductor materials, and even very small amounts can severely degrade the performance of semiconductor devices. If the packaging plant's cleanliness doesn't meet requirements, LED quality issues will arise.
Leaks caused by pollution will exhibit various characteristics when their current-voltage (volt-ampere) properties are observed. These include: different current-voltage characteristics in the forward and reverse directions; changes in reverse breakdown voltage creep; and creep in the forward current-voltage curve. In severe cases, it will show both forward and reverse states. Pollution leaks also exhibit instability; in some cases, the leak will temporarily return to normal.
2. Stress
In LEDs, different materials have different coefficients of thermal expansion. During repeated temperature changes, each material cannot return to its initial state upon contact, and there will be a certain amount of stress between them, but this is not necessarily harmful.
Large stresses can only remain when the coefficients of thermal expansion differ too much and the process conditions are unsuitable.
Such severe stress can crush the chip, cause it to break, lead to leaks, crack and fail to emit light in certain areas, and even completely disconnect the circuit and stop emitting light. Even when the stress is not too high, it can sometimes have serious consequences.
Initially, there are dangling bonds on the side of the LED. Stress causes micro-displacement of surface atoms. The electric field of these dangling bonds is even in an unbalanced state, leading to changes in the energy level state at the end face and PN junction, resulting in leakage.
3. Subsequent processes pose potential risks.
After the front-end electrode fabrication is completed, the chip exists as a single epitaxial wafer. In the back-end processes, the wafer is ground to thin the substrate, preparing for the subsequent laser cutting process. After grinding, laser cutting is performed, and the entire wafer is divided into individual chips. Next, the chips are inspected, sorted, and packaged, completing the entire LED manufacturing process. Two potential causes of chip leakage current exist in the back-end processes: First, during laser cutting, the laser beam's focal point is close to the epitaxial layer, causing cracks to extend into the epitaxial layer during the cutting process. This damages the epitaxial layer on the chip's sides, increasing the probability of external impurity ions entering the chip, forming deep impurity energy levels, and increasing non-radiative recombination of electrons and impurity ions, thus increasing leakage current.
Secondly, during the packaging process after the chip completes testing, improper handling can cause the chip to become contaminated. The contaminated chip surface contains impurity elements such as Na and Cl metal elements, which can easily penetrate the protective layer and enter the semiconductor material, making the chip more prone to leakage.
