In the SMT production process for LED applications, vulcanization is most likely to occur in reflow soldering. Because Jin Jian has seen various adverse cases, the intensity and speed of vulcanization of the stent silver layer is directly related to the sulfur content, temperature and time. The reflow soldering process is a typical high temperature and high humidity environment, and it is also possible to encounter a large amount of sulfur-containing materials. During the testing process, Jinjian found that there were traces of sulfur in PCB boards, solder paste, wash water, and other filling glues. During the SMT operation, these materials will be reflowed together with the LED. After being left for a period of time after reflow, the silver plating of the stent will be blackened. The sulfur-containing substance will penetrate into the LED during the reflow process, causing the silver to be vulcanized. , causing the product to fail.
LED application product SMT production process
For example, Jinjian detection has analyzed the composition of MCPCB sheets in many cases in the past. Many sheets produced on the market have different levels of sulfur remaining, although MCPCB manufacturers will clean the sheets during the process to eliminate them. Residues containing sulfur and acid-containing chemical solvents, but the ordinary production process is difficult to completely eliminate. In view of the fact that sulfur is active in high temperature environments, in the case of SMT operation, the MCPCB board can be pre-prepared by a high-temperature reflow oven (230 degrees or so) before surface mounting to perform surface cleaning (medical alcohol, etc.) to reduce MCPCB. The sulfur content of the pads and the surface layer. If the process is not allowed, the surface of the PCB can be directly cleaned before SMT. After the reflow is completed, the surface of the product should be cleaned once. The surface of the board and the solder joints are cleaned to reduce the chance of vulcanization. In the cleaning, it is necessary to choose no sulfur and no. Contains other acidic cleaning agents.
Pay special attention to solder paste here. Solder paste is a complex system that is a paste of solder powder, flux, and other additives. Solder paste at the high temperature of soldering, its solvent and some additives will volatilize, these solvents will penetrate through the gap of LED lamp beads or the pores of silica gel. The flux composition of the solder paste is complex, and the activator component usually contains a small amount of halogen or organic acid component, which can quickly eliminate the oxide film on the surface of the soldered metal, reduce the surface tension of the solder, and rapidly spread the solder on the soldered metal. surface. Halogen is usually used for chlorine and bromine. The presence of chlorine and bromine is highly susceptible to chlorination, bromination and chemical incompatibility of the lamp beads.
Therefore, Jinjian Testing recommends that LED lighting factories regularly inspect incoming materials for PCB boards, solders, and other supporting materials to avoid residual sulfur residues on the PCB. After the PCB reflow soldering is completed, the surface residue is removed or reduced by cleaning the solder joint locations. The cleaning agent avoids the use of acidic sulfur-containing adhesives and solvents.
In addition, it is necessary to prevent the use of sulfur-containing materials to vulcanize LEDs during SMT operations. For example, during the welding and processing of LEDs, please do not use accessories containing sulfur or halogen such as rubber gloves, rubber finger cots, rubber bands, rubber anti-static tablecloths, filling glue, glass glue, hot melt adhesive, etc. Also prevent the use of vulcanized fixtures and machines; it is best to establish a separate sulfur-free SMT line or production workshop to ensure that the product is not contaminated by vulcanization during the SMT of LED products; clean reflow ovens regularly and Dehumidifying ovens; and controlling the LED or LED components in the workshop environment for soldering, handling, and application, helping to reduce or even eliminate the risk of sulfur or halogen damage to the LED.
As a professional third-party LED analysis and testing organization, Jinjian Testing LED Quality Laboratory has a large number of professional microstructure inspection and analysis equipment and a large number of LED sulfur emission case analysis experience, can undertake the inspection of the major lighting factories, and quickly LED In the application product SMT production process, sulfur/chlorine/bromine pollution sources may be investigated and identified, and an optimization scheme is given to reduce the million losses for lighting manufacturers.
Edit: Lemon
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