Figure 1 Internal discoloration
Figure 1 shows the discoloration of a 3528 LED. The LED is a soldering iron to the aluminum plate, the lighting done a short time, followed by placing the white wrapped about a year, the interior appears black observed (particularly when it is not clear what happens, because a year After re-discovering to see). The silver layer on the exposed portion of the LED holder did not turn black. It seems that the colloid seems to be transparent.
Second, case analysis
Figure 2 is a photograph of Figure 1 after dissection. It can be seen that the discolored portion only occurs on the underlying metal with the fluorescent glue, and the metal under the white plastic case that is not in contact with the fluorescent glue is substantially silvery white. Even the exposed metal surface of the stent does not change color, see Figure 3. The surface of the exfoliated fluorescent glue that was in contact with the metal did not see discoloration, see Figure 4.
Figure 2 Anatomical observation
Figure 3 External bracket discoloration
Figure 4 No discoloration found in fluorescent glue
The color change portion is removed with a needle tip to reveal a silvery white color. Obviously, it is not that the silver is corroded and consuming light to expose the base copper. If silver is consumed, the surface should not be copper, but should be a silver compound.
From the color of the color-changing substance, there are roughly blue and reddish-brown colors. The substances of these colors are not silver compounds. There are no substances in the silver that can exhibit blue color. The blue substance is a compound of copper. The red material is copper, which traverses the silver layer due to the migration of copper atoms.
According to relevant information, the cause of blue matter may be some by-products produced by the vulcanization of certain organic silica gels, such as hydrazine, hydroxy acid, ammonia, low-volatile small molecule siloxanes, and the like. Among them, low volatility small molecule siloxanes have an effect on the metal of the electrical appliance, and bismuth has a corrosive effect on copper. From this it can be inferred that the blue substance is produced by the reaction of some of the by-products produced by certain silica gels with copper.
Third, the conclusion
1. The copper of the stent metal substrate migrates to the top of the silver plating layer, resulting in an internal "blackening" from the appearance, and a nickel layer of sufficient thickness should be added between copper and silver.
2. The colloidal material used is not suitable, resulting in a reaction with copper.
3. The internal metal is discolored, and the causes under different conditions are more complicated. This case is only one of many situations. It should be avoided to rely on experience to make a partial judgment.
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