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LEDs often face the issue of vulcanization during both production and usage, which can lead to product failure and cause significant financial losses for both customers and manufacturers. Especially with SMD LEDs and high-power LEDs, a large amount of polymer materials such as silicone and modified silicone are used in the manufacturing process, making them particularly vulnerable to vulcanization. As a result, preventing LED vulcanization has become a major challenge for many LED manufacturers.
Vulcanization in LEDs refers to the chemical reaction that occurs when sulfur (S) or sulfur-containing substances react under certain temperature and humidity conditions, forming black silver sulfide. This happens when -2 valence sulfur reacts with +1 valance silver. Since LEDs encapsulated in silicone materials are highly hygroscopic and permeable to oxygen, they are especially prone to severe failures due to vulcanization.
The main stages where LED vulcanization occurs include:
1. **During the manufacturing process**: Raw materials and finished products may be exposed to sulfur-containing elements, though the risk is relatively low.
2. **During SMT (Surface Mount Technology)**: This is the most critical stage, as a lot of sulfur-containing materials like PCBs, solder pastes, and cleaning agents are involved. These materials can enter the reflow oven with the LED and cause vulcanization.
3. **During use**: Once the LED is in the field, exposure to environmental sulfur sources can still trigger vulcanization.
**Preventing Vulcanization During Production**
In the manufacturing stage, vulcanization primarily occurs during the solid crystal and dispensing processes. The main causes are silver-containing materials and silicone-based materials. Therefore, it's crucial to control contamination from sulfur-containing tools, gloves, fixtures, and masks.
Common sources of sulfur contamination include:
- Sulfur-containing gloves and finger cots
- Sulfur-containing masks
- Sulfur-containing PCBs
- Sulfur-containing cleaners
- Vulcanized ovens and cartridges
To prevent this, manufacturers should use:
- Desulfurized gloves and finger sleeves
- Sulfur-free masks
- Non-sulfur cleaning agents
- Separate, sulfur-free ovens
- Dedicated sulfur-free fixtures and tooling
Maintaining a clean, dust-free environment and isolating sulfur-containing materials can significantly reduce the risk of vulcanization during production.
**Preventing Vulcanization During SMT**
The SMT process is the most vulnerable stage for vulcanization. Sulfur-containing materials such as PCBs, solder pastes, and cleaning agents can easily contaminate the LED. To minimize this risk, PCBs should be tested for sulfur content, and those with sulfur levels above 5 parts per thousand should be avoided.
Pre-cleaning the PCB in a high-temperature reflow oven before SMT can help reduce surface sulfur. After reflow, a final cleaning with non-sulfur and non-acidic solvents is recommended. It’s also important to avoid using sulfur-containing fixtures and machines. Establishing a dedicated sulfur-free SMT line is ideal to ensure product integrity.
Another key factor is the type of potting glue used. Methyl-based glues have lower refractive indices and higher permeability, making them more susceptible to vulcanization. In contrast, phenyl-based glues have a higher refractive index and better resistance to moisture and gas, offering superior protection against vulcanization.
**Preventing Vulcanization During Use**
Sulfur is a common element found in various forms in daily life. It is used in fertilizers, chemicals, and even in some medical products. Due to its widespread presence, it's essential to store and handle LED products in environments free from sulfur and acidic substances.
When storing semi-finished and finished LED products, it’s important to keep them sealed and away from sulfur sources. Exposure to moisture, heat, and acidic environments can accelerate the vulcanization process. Users should also be cautious about the surrounding conditions where LED products are installed, especially in industrial or outdoor settings.
In summary, preventing LED vulcanization requires a comprehensive approach that includes careful material selection, strict process control, and proper storage and handling. By addressing each stage of the product lifecycle, manufacturers and users can significantly reduce the risk of failure due to vulcanization.