In the process of production and use, LEDs often encounter product vulcanization, resulting in product failures, causing significant losses to customers and manufacturers. In particular, SMD LEDs and high-power LEDs use a large amount of polymer materials such as silica gel and silica gel modified materials in the manufacturing process, which has become the hardest hit area for product vulcanization. How to prevent LED vulcanization during manufacturing and use has become a common problem faced by various LED manufacturers.

LED vulcanization refers to the process in which -2 valent sulfur and +1 valent silver react chemically due to sulfur (S), or sulfur-containing substances under certain temperature and humidity conditions, to produce black silver sulfide. Since LED products encapsulated by organic silicon materials have high moisture and oxygen permeability, product failure caused by LED vulcanization is particularly serious in such products.

The vulcanization of LED products mainly occurs in the following links. First, the LED production process. The vulcanization of raw materials and finished products is preventable, and the probability of occurrence is also low; second, during the SMT process. The finished product is vulcanized, which can be prevented and has a high probability of occurrence; thirdly, during use. The finished product is vulcanized, which can be prevented and has a high probability of occurrence.

1. Prevention of vulcanization during production

The first link to prevent LED vulcanization is in the manufacturing process. The following uses SMD packaged with silicon material to illustrate the materials that may be vulcanized during the production process:

①The silver-plated stent is vulcanized, the stent turns yellow, black and loses its luster, and the reliability of the solid crystal process is reduced;

②The silver glue is vulcanized, the silver glue turns black, the color is dim, and the solid crystal process reduces the reliability or fails;

③Silicone bonding glue is vulcanized, causing curing hindrance, unable to fully cure, bonding process, decline or failure;

④Silicone dispensing powder is vulcanized, causing curing hindrance, unable to fully cure, dispensing, dispensing, packaging process, product failure;

⑤ Phosphor contains sulfur, which causes curing hindrance, powder dot process, and product failure.

The vulcanization phenomenon in the production process mainly occurs in the die bonding and dispensing and packaging processes. The vulcanization is mainly caused by silver-containing materials and silicon-based film materials. In the production process, we must pay great attention to the vulcanization prevention of these two links.

The main possible causes of vulcanization pollution are:

Sulfur-containing gloves, finger cots, sulfur-containing masks, sulfur-containing PCBs, sulfur-containing cleaning agents, vulcanized ovens, vulcanized boxes, clamps, etc.

The prevention method is to use:

Desulfurization gloves, finger cots, sulfur-free masks, use after cleaning and desulfurization, sulfur-free cleaning agent, independent baking, set up a special sulfur-free oven, use separately, distinguish sulfur-free materials, box clamps and fixtures.

It can be seen from the above analysis that in the LED manufacturing process, prevention is the main focus. Raw materials (silver, silicon) that are easily vulcanized must be prevented from being sulfur-containing tools, clamps, gloves, Contamination of sulfur-containing materials such as masks leads to vulcanization.

The best preventive measure is to avoid using sulfur-containing materials and fixtures and other sulfur-containing auxiliary tools and materials during the production process. The production environment of LEDs is basically a dust-free workshop, and the isolation of sulfur in the environment is very high. As long as the isolation measures for sulfur-containing materials, fixtures, and auxiliary materials in production are done, they will generally not be easily vulcanized in production.

2. Prevention of vulcanization during SMT process

Vulcanization in the SMT process is the most serious part of the vulcanization of LED products, mainly because a large number of sulfur-containing materials will be encountered in the SMT process. PCB boards, solder paste, washing water, and other irrigation materials are found during the inspection process. There is a record of sulfur element in the filling rubber. During the SMT operation, these materials will enter the reflow oven together with the LED. After reflowing for a period of time, the silver-plated part of the bracket will be blackened, mainly because the sulfur-containing substances are in the reflow process. It penetrates into the LED, causing the silver to be vulcanized and the product to fail.

The PCB products on the market all contain sulfur to varying degrees. Although PCB manufacturers clean the plates for desulfurization during the manufacturing process, it is difficult to completely remove them. Therefore, during SMT operations, quality control of the sulfur content of PCB boards is required. Generally, the maximum sulfur content of PCB copper foil should not exceed five thousandths as the upper limit.

In view of the fact that sulfur is relatively active in a high temperature environment, during SMT operations, the PCB board can be pre-processed through a high temperature reflow oven and then subjected to a surface cleaning treatment to reduce the sulfur content on the PCB surface. If the process does not allow, the PCB surface can be cleaned directly before SMT. After the reflow is completed, the surface of the product must be cleaned once to reduce the probability of LED vulcanization by cleaning the board surface and solder joints. When cleaning, you need to choose a cleaning agent that does not contain sulfur and does not contain other acids.

In the process of SMT operation, it is necessary to prevent the use of sulfur-containing materials to cause vulcanization of the LED, and to prevent the use of vulcanized fixtures and machines. It is best to use them separately, and it is best to establish a separate sulfur-free SMT production line or workshop to ensure that the products are not polluted by sulfur during the process of making LED products SMT.

Let's study the composition of glue. Generally, it is used as LED potting glue. There are two main types. One is low refractive index (usually around 1.43), methyl glue, the price is relatively low, but the water permeability is strong, so this The probability of vulcanization of similar glue is very high. The second type is phenyl glue, which has a high refractive index (usually above 1.5) and is very expensive, because its molecular structure is more complicated, and it is difficult for water and gas to enter it. Yes, the ability to prevent vulcanization is very high.

If the LED is sealed with methyl glue, it is equivalent to carrying the root of the disease when it leaves the factory. When the customer meets any conditions that can cause the LED to vulcanize, it will cause its disease to be excited, causing the LED to vulcanize. However, LEDs encapsulated with phenyl glue will be strong when they leave the factory, without any cause of disease, so there will be no vulcanization during application.

3. Prevention of vulcanization during use

Sulfur is a very common odorless non-metal. Pure sulfur is a yellow crystal, also known as sulfur. Sulfur has many different valences, common ones are -2, 0, +4, +6, etc. In nature, it often appears in the form of sulfide or sulfate. It is a component of a variety of amino acids, and thus is a component of most proteins.

It is mainly used in fertilizers, but is also widely used in gunpowder, lubricants, insecticides and antifungal agents. It can be used to make sulfuric acid, sulfite, plastic, enamel, synthetic dyes, vulcanized rubber, bleach, medicine, paint, etc.

In life and production, we are ubiquitous in contact with sulfur and sulfur-containing items, so we need to pay special attention when using LED products in our daily lives to prevent LED lamps or other products from being vulcanized. LED originals and LED application products should not be stored in a unified environment with sulfur-containing substances. If LED application semi-finished products and finished products are not sealed, they should be avoided from being exposed to the natural environment, especially to avoid exposure to acidic environments.