Solder bead is a welding defect often encountered in reflow oven welding, which occurs in the rapid heating process of welding process, or the temperature of preheating area is too low, and suddenly enter the welding area, it is also easy to produce solder bead. Now the common causes of tin beads are summarized as follows.
1. Improper setting of reflow oven temperature curve. First of all, if the preheating is not sufficient, not to meet the temperature or time requirements, flux not only low activity, and very little volatilization, not only can not remove the oxide film on the surface of the pad and solder particles, but also can not rise to the solder paste powder surface, can not improve the wettability of the liquid, easy to produce tin beads. The solution is to prolong the preheating temperature at 120 ~ 150℃. Secondly, if the temperature of the preheating zone rises too fast and the time to reach the flat top temperature is too short, the water and solvent in the solder paste are not completely volatilized out. When it reaches the reflow welding temperature zone, it may cause water and solvent boiling and splash out tin beads. Therefore, attention should be paid to the heating rate, the wettability of preheated solder is affected, easy to produce tin beads. As the temperature increases, the wettability of the liquid solder will be significantly improved, thus reducing the production of tin beads. However, if the reflow welding temperature is too high, the components, printed boards and pads will be damaged. Therefore, appropriate welding temperature should be selected to make the solder have better wettability.
2. Flux failed to take effect. The function of flux is to remove the oxide film on the surface of the pad and solder particles, thereby improving the wettability between the liquid solder and the pad and the component pins (solder ends). If after the solder paste is coated, the placement time is too long, the flux is easy to volatilize, the oxidation of the flux is lost, the wettability of the liquid solder is poor, and the tin beads will inevitably be produced when the reflow welding. The solution is to choose the working life of more than 4h solder paste, or as far as possible to shorten the placement time.
3. The opening of the template is too large or seriously deformed. If the beads are always in the same place, it is necessary to check the design of the metal plate. The template opening size accuracy is not up to the requirements, for the pad is too large, and the surface material is soft (such as copper template), will cause the outline of the leakage paste is not clear, bridge each other, this situation appears in the pad leakage of fine spacing devices, after the re-flow welding will inevitably cause a large number of tin beads between the pins. The solution is to select suitable template material and template making process according to the different shape and center distance of the pad graphics to ensure the printing quality of solder paste, reduce the size of template opening, strictly control the template making process, or use laser cutting and electric polishing method to make the template.
4. The placement pressure of the SMT machine is too large. Too much placing pressure can squeeze the solder paste out of the pad. If the solder paste is coated thick, too much placing pressure is easier to squeeze the solder paste out of the pad, and tin beads will inevitably be generated after reflow welding. The solution is to control the thickness of solder paste and reduce the placing pressure of patch head.
5. The solder paste contains moisture. If you take out the solder paste from the refrigerator and use it directly with the lid open, the temperature difference is large and the water vapor condenses. When reflow welding, it is easy to cause the boiling splash of water and the formation of tin beads. The solution is: after the solder paste is taken out from the refrigerator, it should usually be placed at room temperature for more than 4h. After the temperature of the pad in the sealing cylinder reaches the ambient temperature, it can be opened and used again.
6. PCB cleaning is not clean, so that solder paste residue in the PCB surface and through hole. The solution is to strengthen the sense of responsibility of operators and process personnel in the production process, strictly follow the process requirements and operating procedures for production, and strengthen the quality control of the process.
7. Non-contact printing or printing pressure is too large. In non-contact printing, there is a certain gap between the template and PCB. If the scraper pressure is not well controlled, it is easy to make the welding paste under the template into the non-welding area on the PCB surface, and tin beads will inevitably be generated after reflow welding. The solution is: if there is no special requirements, it is appropriate to use contact printing or reduce printing pressure.
8. Flux failure. If the patch to reflow welding time is too long, due to the oxidation of solder particles in the solder paste, flux deterioration, reduced activity, will lead to solder paste no longer flow, solder ball will be generated. The solution is to choose a longer life of the solder paste (at least 4h).
