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LEDs often face the issue of vulcanization during both production and use, which can lead to product failure and cause significant financial losses for both customers and manufacturers. Especially in the case of SMD LEDs and high-power LEDs, a large amount of polymer materials—such as silicone gel and modified silicone materials—are used in the manufacturing process, making these products particularly vulnerable to vulcanization. As a result, preventing LED vulcanization has become a common challenge across the industry.
Vulcanization occurs when sulfur (S) or sulfur-containing substances react chemically under specific temperature and humidity conditions. This reaction typically involves -2 valence sulfur reacting with +1 valence silver. Since LED packages made with silicone are highly hygroscopic and oxygen-permeable, they are especially prone to severe failures caused by vulcanization.
The main stages where vulcanization commonly occurs include:
1. **During the manufacturing process** – Although the risk is relatively low, improper handling of silver-based materials or silicone components can lead to vulcanization.
2. **During the SMT (Surface Mount Technology) process** – This is the most critical stage, as various sulfur-containing materials such as PCBs, solder pastes, and cleaning agents may come into contact with the LED components.
3. **During product use** – Once the LED is in the field, exposure to environmental sulfur compounds can still trigger vulcanization, especially if the product was not properly sealed or stored.
### Prevention During Production
Preventing vulcanization starts at the very beginning of the manufacturing process. Common issues that occur during this phase include:
- **Silver-plated substrates** turning yellow or black due to sulfur exposure.
- **Silver glue** darkening and losing its effectiveness.
- **Silicone-based encapsulation materials** failing to cure properly.
- **Phosphor powders** containing sulfur, leading to instability during processing.
The primary sources of sulfur contamination during production include gloves, masks, PCBs, cleaning agents, and fixtures. To prevent this, it's essential to use sulfur-free alternatives, such as desulfurized gloves, non-sulfur masks, and sulfur-free cleaning agents. Additionally, maintaining a clean, controlled environment with separate sulfur-free ovens and fixtures is crucial.
While it’s nearly impossible to eliminate all sulfur from the environment, minimizing its presence through strict material control and process isolation can significantly reduce the risk of vulcanization during production.
### Prevention During SMT
The SMT process poses the highest risk of vulcanization due to the frequent exposure of LEDs to sulfur-containing materials like PCBs, solder paste, and flux. Even small amounts of sulfur on the PCB surface can penetrate the LED package during reflow, causing silver plating to darken and leading to failure.
To mitigate this, PCBs should be tested for sulfur content before being used in SMT. Ideally, the sulfur level should not exceed 5 parts per thousand. Pre-cleaning the PCBs in a high-temperature oven before reflow can also help reduce sulfur buildup. After reflow, cleaning the board and solder joints with sulfur-free solvents further minimizes the risk.
In addition, using sulfur-free glues—like phenyl-based encapsulation materials instead of methyl-based ones—can greatly improve resistance to vulcanization. Phenyl-based glues have a higher refractive index and better barrier properties, making them more effective at blocking moisture and gases.
### Prevention During Use
Once an LED product is in the hands of the end-user, it's still susceptible to vulcanization. Sulfur is a common element found in many everyday items, including fertilizers, chemicals, and even some household products. It can also be present in the air, especially in industrial or coastal areas.
To protect LEDs during use, it's important to store and handle them in environments free from sulfur-containing substances. Products should be kept in sealed packaging, away from acidic or humid conditions. For applications where exposure is unavoidable, choosing LEDs with superior encapsulation materials—like phenyl-based glues—can provide long-term protection against vulcanization.
By understanding the causes and implementing proper preventive measures throughout the entire lifecycle of an LED, manufacturers and users can significantly reduce the risk of product failure due to vulcanization.