The reason why the positive and negative lamp pins of LED lamp beads turn black
Yesterday, a customer asked us that the LED lamp bracket they were storing was black and asked us what caused it. The black LED bracket is a problem that LED packaging manufacturers encounter from time to time. The editor of Hailongxing will tell you about the reasons for the blackening of the bracket: the silver plating layer is too thin, the silver plating layer is vulcanized, the silver plating layer is oxidized, the plating quality is poor, organic matter pollution, nozzle materials, etc., resulting in LED lamp beads There are signs of blackening of the bracket. Next, Hai Longxing will give you a brief analysis of some reasons for blackening.
Existing LED light sources on the market choose copper as the base material of the lead frame. In order to prevent copper from oxidizing, the surface of the bracket is generally electroplated with a layer of silver. If the silver plating layer is too thin, the bracket will easily turn yellow under high temperature conditions. The yellowing of the silver plating layer is not caused by the silver plating layer itself, but is affected by the copper layer under the silver layer. At high temperatures, copper atoms will diffuse and penetrate into the surface of the silver layer, causing the silver layer to turn yellow. The oxidizability of copper is the biggest drawback of copper itself. Once copper becomes oxidized, its thermal conductivity and heat dissipation performance will be greatly reduced. Therefore, the thickness of the silver plating layer is very important. At the same time, both copper and silver are susceptible to corrosion by various volatile sulfides, halides and other pollutants in the air, causing their surfaces to darken and discolor. Studies have shown that discoloration increases surface resistance by about 20 to 80% and increases power loss, which greatly reduces the stability and reliability of LEDs and even leads to serious accidents.
Silver plating layer vulcanization
LED light sources are afraid of sulfur. This is because sulfur-containing gases will pass through the silica gel or bracket gaps of its porous structure and undergo a sulfurization reaction with the silver-plated layer of the light source. After a vulcanization reaction occurs in the LED light source, the functional area of the product will turn black, the luminous flux will gradually decrease, and the color temperature will drift significantly. The conductivity of the vulcanized silver sulfide increases as the temperature rises. During use, leakage is very easy to occur; more serious The situation is that the silver layer is completely corroded and the copper layer is exposed. Since the two solder joints of the gold wire are attached to the surface of the silver layer, when the silver layer in the functional area of the bracket is completely sulfurized and corroded, the gold ball will fall off, resulting in a dead light.
Silver plating layer oxidation
The silver coating of the LED light source has obvious signs of blackening, which may be related to silver oxidation.
Poor plating quality
The quality of the coating mainly depends on the crystal structure of the metal deposition layer. Generally speaking, the finer the crystal structure, the denser and smoother the coating, and the higher the protective performance. This kind of finely crystalline coating is called "microcrystalline deposition layer". Jin Jian pointed out that a good electroplating layer should have fine, smooth, uniform and continuous crystallization, and no pollutants, chemical residues, spots, black spots, burnt, rough, pinholes, pitting, cracks, delamination, etc. are allowed. Defects such as blistering, wrinkling, peeling of the coating, yellowing, crystalline coating, and partial lack of coating.
In electroplating production, the thickness of the metal coating and the uniformity and integrity of the coating are one of the important indicators for checking the quality of the coating, because the protective performance and porosity of the coating are directly related to the thickness of the coating. The special change is the cathode coating. As the thickness increases, the protective performance of the coating also increases. If the thickness of the coating is uneven, the thinnest part is often damaged first, and the remaining parts will lose their protective effect no matter how thick the coating is.
Because various liquids containing organic substances are used in the electroplating process, if the silver plating layer is not cleaned cleanly or poor quality or deteriorated liquids are used, these residual organic substances will be destroyed by light, heat and electricity once they are in an environment where the light source is lit. Under the action of organic matter, chemical reactions such as oxidation and reduction may occur, causing the surface of the silver plating layer to change color.
The material of the plastic is the key to the heat conduction of the LED package bracket. If the PPA bracket is made of nozzle material, the plastic performance of the PPA will be reduced, resulting in the following problems: poor high temperature endurance, easy deformation, yellowing, low reflectivity; high water absorption rate , the stent will cause dimensional changes and a decrease in mechanical strength due to water absorption; it has poor bonding properties with metal and silicone, is picky about glue, and does not match many silicones. These potential problems make it difficult to use lamp beads at slightly higher powers. Once the power range is exceeded, the initial brightness is very high, but it decays quickly, and the lamp goes dark within a few months.
There are many welding methods for LED lamp beads on the chip, and the following is one of the commonly used methods for reference. First, use an electric soldering iron to heat some soldering tin on the positive and negative electrode pads of the lamp bead (the soldering tin must not be too much, otherwise, the positive and negative electrode pads will be connected once heated with a hot-air gun), then use a hot-air gun to heat the positive and negative electrode pads at the same time, after the tin melts, use tweezers to place the positive and negative electrodes of the lamp bead on the corresponding pad.
LED is the light emitting diode, which is a semiconductor solid light emitting device that converts electric energy into light energy. Its core is the PN junction. In addition to the forward conduction, reverse cut-off and breakdown characteristics of the general PN junction, it also has the light emitting characteristics under certain conditions. Its structure mainly includes the following parts: lead, bracket, encapsulation glue, bonding wire, LED chip, solid crystal glue and phosphor. The discoloration failure of LED lamp bead is closely related to its material, structure, packaging process and service conditions. The following will analyze the causes of its discoloration through specific cases.
LED lamp beads are also called light emitting diodes. It has a wide range of applications. Its luminous colors include red light, emerald green light, yellow green light, yellow light, purple light, blue light, white light and orange light. The above is monochromatic light. It can also be made into two or three colors.
The gold wire used to package LED lamp beads contains more than 99.99% gold. It is produced by drawing gold from this material. In addition to 99.99% gold, this gold also contains less than 1% other trace elements. One of the core components of LED lamp bead packaging is gold wire, which is the bridge connecting the luminous chip and the welding point, and plays a decisive role in the service life of LED lamp bead. So how to identify the purity of gold thread?