The reliability of LED products is receiving increasing attention from manufacturers and users. The quality of the LED lamp bead, which is the basic component of the product, directly affects the reliability of the finished LED product. In practical applications, the malfunction or even complete failure of the finished product due to the failure of the LED lamp bead often occurs. In recent years, a series of reliability problems such as color temperature drift, reduced lumens, and poor light output have been increasing due to discoloration of lamp beads, which has caused serious economic losses to many LED product manufacturers and users.
This article analyzes the root cause of the discoloration failure of LED lamp beads through several LED lamp bead failure cases.
Encapsulation reason
(1) Residual foreign matter in the encapsulant
The appearance of the failed lamp bead is partially discolored and blackened. Uncovering the encapsulant, it was found that a black foreign matter was trapped in the encapsulant. The composition of the foreign matter was analyzed by scanning electron microscopy and energy dispersive spectroscopy (SEM & EDS) [5-6], confirming that the main components were aluminum (Al) and carbon (C). ), oxygen (O) element, also contains a small amount of impurity elements, the test results are shown below. Combined with the failure background of user feedback, the foreign matter is introduced during the packaging process.
(2) The encapsulant is chemically eroded to produce colloidal discoloration
The failure product is a glass tube lamp, and the inner LED lamp strip is fixed on the glass tube by using a one-component room temperature curing silicone rubber, and the LED lamp bead on the lamp strip of the solid glue portion appears yellowish and dark. The material of the failed lamp bead is silicone rubber. The elemental composition of the encapsulant was tested by SEM&EDS. It was found that the sulfur (S) element was detected more than the normal lamp bead package. The test results are shown in the figure below.
Generally, sulfur-containing substances such as sulfur, organic disulfides and polysulfides can be used as a vulcanizing agent to cause a vulcanization crosslinking reaction of the rubber, thereby changing the structure of the rubber, exhibiting a yellowish color and a darkening of the thermal decomposition temperature. .
Through the thermal decomposition temperature of the TGA test lamp encapsulant colloid, the temperature of the failed lamp bead encapsulant at the weight loss of 2%, 5%, 10%, 15% and 20% is the same as the weight loss of the same batch of good package. Above 25 °C, the thermal decomposition curve of the encapsulant is shown in the figure below, which confirms that the thermal decomposition temperature of the encapsulant is increased due to vulcanization cross-linking. Further chemical composition analysis of the fixed-component one-component cured silicone rubber was carried out using ICPOES, and it was found to contain about 400 ppm of sulfur (S) element.
It can be seen that the cause of the yellowing and darkening of the LED lamp bead is that the sulfur-containing (S) gas volatilized during the curing process of the one-component room temperature curing silicone rubber adhered and fixed in the glass lamp tube intrudes into the LED encapsulant. , the vulcanization cross-linking reaction occurs in the encapsulant, and the re-vulcanization cross-linking causes the encapsulant to turn yellow and dark. Subsequent users switched to plastic tubes that did not use one-component cured silicone rubber, and there was no discoloration of the lamp beads. Therefore, LED manufacturers should consider the matching of different materials used in each part of the product in product design and manufacturing, and avoid the subsequent reliability problems caused by material incompatibility.
Phosphor sedimentation
When the lamp bead is assembled into an LED luminaire and stored in the warehouse, the color temperature drift fails. The package rubber of the failed LED lamp bead changes from orange to light yellow. The IV characteristic test is performed, and the lamp bead can be normally lit, and the IV curve Normal, only the brightness of the light changes. Take some failed lamp beads and remove the encapsulant by mechanical unsealing. It is found that there are transparent particles on the surface of the stent. The particle composition is tested by SEM&EDS. The results show that it contains high content of strontium (Sr) element, as shown in the figure below.
High levels of strontium (Sr) and barium (Ba) were also detected on the interface between the encapsulant and the holder, as shown in the figure below.
In contrast, after the opening of the good lamp bead, the surface of the bracket is relatively clean, the main component of the surface is silver (Ag) and a small amount of carbon (C) elements, no sputum (Sr) element is detected, and in its encapsulant and bracket No elements of strontium (Sr) and barium (Ba) were detected on the contact surface. By testing the cross-section components of the failed product and the good lamp bead encapsulant, the phosphors used in the two have the same composition, all of which are yttrium aluminum garnet (the main components are oxygen (O), aluminum (Al) and yttrium (Y). ) A phosphor mixed with yttrium silicate (mainly composed of carbon (C), oxygen (O), silicon (Si), strontium (Sr), barium (Ba), and calcium (Ca).
Therefore, the failure of the LED lamp bead is caused by the silicate phosphor used to settle to the bottom of the encapsulant and the surface layer of the support, resulting in dispersion due to inconsistent light refraction patterns, resulting in color temperature drift and lamp bead discoloration.
Cause of bracket
(1) Foreign matter pollution bracket
The side of the failed lamp is discolored. After uncovering the encapsulant, the surface of the scaffold can be seen covered with a layer of foreign matter. The elemental composition of the foreign matter is tested to show that the main components are tin (Sn) and lead (Pb) elements. The measured results are shown in the figure below.
The white plastic on the periphery of the lamp-changing part was uncovered, and tin (Sn) and lead (Pb) components were also detected on the surface of the holder that was in contact with the white plastic. Since the bracket of the foreign matter covering portion is connected to the pin on the side of the lamp bead, the lead is soldered with tin-lead. Obviously, if the lamp is surface-mounted with excess solder paste on the pins, the solder will climb along the pins to the surface of the bracket to form a cover layer during soldering. Therefore, the reason for the LED lamp bead failure in this case is that the LED lamp bead is soldered to the surface of the bracket when the assembly is soldered, forming a cover, which causes the lamp bead to change color.
(2) Stent corrosion
The middle part of the failed LED lamp was discolored and blackened. After opening it, it was observed under an optical microscope. The surface of the whole stent was clearly blackened. The composition of the black stent was tested using SEM&EDS. The results showed that, in addition to the normal material composition, The black stent also has a high content of corrosive sulfur (S) elements, and the silver-plated layer on the surface of the stent also exhibits a loose corrosive appearance, as shown in the following figure.
Usually, in the production process, LED lamp beads are introduced into corrosive elements such as sulfur (S) and chlorine (Cl) due to impure materials or process contamination, etc. under certain conditions (such as high temperature, water vapor residue, etc.) The metal bracket is highly susceptible to corrosion, resulting in discoloration, leakage, and the like of the lamp bead.
(3) Poor plating quality
LED lamp beads appear to be discolored and black after aging, and the failure rate is as high as 30%. After removing the encapsulant on the surface of the lamp bead, it was found that the silver plating on the surface of the bracket lost its original brightness and appeared gray. The SEM was used to observe the surface topography of the stent. It was found that the surface of the LED failure lamp bead was loose and had more holes than the unassembled semi-finished stent. The semi-finished stent and the failed LED were sliced ​​to observe the quality of the cross-section coating. It was found that the stent plating structure was nickel-plated and then silver-plated. Compared with the semi-finished product, the nickel coating of the failed stent was thinned, and the surface silver layer became loose. The nickel-nickel plating boundary becomes blurred.
Using AES to test the superficial composition of the failed LED holder, it was found that there was nickel (Ni) element. The test results are shown in the figure below. Obviously, the nickel coating spread to the surface of the silver layer. It is concluded that the LED lamp bead is discolored due to the poor plating of the stent. After the aging, the silver layer is loose and pores are generated. The nickel layer diffuses through the pores of the silver layer to the surface of the silver layer, causing the silver layer to become black and the lamp bead to change color.
In the case of numerous LED discoloration failures, the proportion of failures due to discoloration or corrosion of the stent is the highest. Therefore, LED or bracket manufacturers should take some measures to prevent product failure.
For example: select a good quality, corrosion-resistant stent substrate; adopt appropriate plating process conditions to ensure the formation of fine grain, dense structure coating, uniform coating thickness and meet the protection requirements; for the silver plating of the surface coating, select effective The silver protection process improves the anti-tarnishing ability of the silver bracket; in the process of LED assembly and assembly, the introduction of external pollution or corrosive substances should be prevented to ensure the tightness of the LED package to reduce the water vapor and oxygen in the environment. The possibility of intrusion causing various corrosion.
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