Huawei Silkroad Electronic Technology Co., Ltd.
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author： click：：296 Publication time：2022-06-17
Wave soldering is the melted solder, through the electric pump or electromagnetic pump jet into the design requirements of the solder peak, so that the pre-installed electronic components of the printed board through the solder peak, to achieve component solder ends or pins and printed board pads between the mechanical and electrical connection of the soft brazing. Wave soldering has been used for more than 20 years of history in printed board assembly, and now it has become a very mature electronic assembly process technology, which is mainly used for welding through-hole insertion components and plug-in components in mixed assembly mode. Nitto wave soldering training textbook (official) Second, wave soldering process technology introduction wave soldering has single wave soldering and double wave soldering. When single-wave peak welding is used, due to the masking effect of solder, more serious quality problems are likely to occur, such as leakage of welding, bridging and incomplete welds. The double wave peak is a good way to overcome this problem, greatly reducing the leakage of welding, bridging and weld is not full and other defects, so the current double wave soldering process and equipment wave soldering process (short angle operation) are widely used in surface assembly: fixture installation→ spray flux system→ preheating→ primary wave → secondary wave → cooling are introduced below Step content and function: 2.1 fixture installation fixture installation refers to the PCB board to be welded to install clamping fixture, which can limit the degree of heat deformation of the substrate, prevent the occurrence of phenomena such as tin, and thus ensure the stability of the tin immersion effect. 2.2 Flux system Flux system is the first link to ensure the quality of soldering, its main role is to remove the oxide layer of the PCB and component solder surface and prevent re-oxidation during the soldering process. The coating of the flux must be uniform, try not to produce accumulation, otherwise it will lead to a short circuit or open circuit in the welding. Fluxes can be applied in a variety of ways, including spray, jet and foam. At present, spray welding systems are generally used, using no-clean flux. This is because the solids content in the no-clean flux is very small (only 1/5 to 1/20 of the non-volatile and no content), so the spray welding system must be used to coat the flux. In flux systems, an anti-oxidation system is generally added to prevent oxidation and avoid insufficient welding dose during soldering, resulting in solder bridging and tip pulling. There are two ways to spray: one is to use ultrasonic shock flux to make its particles smaller, and then spray it on the PCB board; The second is the use of micro-nozzles, spray flux at a certain air pressure, this spray uniformity, small particle size, easy to control spray height / width can be automatically adjusted, is the mainstream of future development. 2.3 Preheating system 2.3.1 The solvent component in the flux of the preheating system will be volatilized by heat when passing through the preheater. In this way, the high temperature gasification of the solvent component when passing through the liquid level is avoided, resulting in a bursting phenomenon, and the quality hazard of producing tin particles is finally eliminated. The slow heating of the parts to be impregnated in the tin product through the preheater can avoid damage caused by the physical action of the sudden heat during the peak of the wave. The preheated part or terminal will not significantly reduce the soldering temperature of the solder joint due to its low temperature when passing through the peak, so as to ensure that the welding meets the temperature requirements within the specified time. 2.3.2 There are three common preheating methods in wave soldering machines: air convection heating; Infrared heater heating; The method of combining hot air and radiation is heated. 2.3.3 The preheating temperature is generally 110 °C ~ 150 °C, and the preheating time is 1min ~ 3 min. The preheating temperature is well controlled, which can prevent virtual welding, tip pulling and bridging, reduce the thermal impact of the solder wave peak on the substrate, and effectively avoid the problems of PCB board warpage, delamination and deformation during the soldering process. 2.4 Welding system Welding system generally adopts double peaks. When the wave peak is soldered, the PCB board first contacts the first peak and then the second peak. The first wave peak is a turbulent wave flowing out of a narrow nozzle, with a fast flow rate and a high vertical pressure on the component, so that the solder has better permeability to the solder ends of components with small size and high insertion density; Scrubbing the surface of the component in all directions with turbulent molten solder, thereby improving the wettability of the solder and overcoming the problems caused by the complex shape and orientation of components; At the same time, it also overcomes the masking effect of solder. The upward spray force of the turbulent wave is enough to discharge the flux gas, so even if the exhaust hole is not set on the printed board, there is no effect of flux gas, which greatly reduces welding defects such as leakage welding, bridging and unfilled welding, and improves welding reliability. After the first crest of the product, due to the short dipping time and the heat dissipation of the part itself, there are many short circuits, tin, abnormal finish of solder joints and insufficient welding strength after dipping. Therefore, the correction of the poor tin immersion must be carried out immediately after this action, which is carried out by the secondary jet with a flat and wide jet surface and a more stable peak. This is a smooth crest with a slow flow rate that facilitates the formation of a full weld, while also effectively removing excess solder from the weld ends and wetting the solder on all weld surfaces well, correcting the welding surfaces, eliminating possible tipping and bridging, obtaining a full, defect-free weld, and ultimately ensuring the reliability of component welding. 2.5 Proper cooling after cooling the dipped tin helps to enhance the bonding strength of the solder joints, and at the same time, the cooled product is more conducive to the operation of the operator after the furnace. Therefore, the product needs to be cooled after tin immersion. Third, the methods and measures to improve the quality of wave soldering are discussed from the three aspects of quality control before welding, production process materials and process parameters, respectively, the effective methods of improving the quality of wave soldering are discussed. 3.1 Control of the quality of the printed board and components before welding When designing the plug-in component pad, the size and size of the pad should be appropriate. The pad is too large, the solder spread area is large, the solder joints formed are not full, and the pads are too small, and the solder joints formed are non-infiltrating solder joints. The gap between the aperture and the component lead is too large, easy to virtual weld, when the pore diameter is 0.05mm-0.2mm wider than the lead, the pad diameter is 2 to 2.5 times the aperture is the ideal condition for welding. When designing the SDD component pad, the following points should be considered: 1. In order to minimize the shadow effect, the component solder end or pin should be facing the direction of the tin flow to facilitate contact with the tin flow and reduce false soldering and leakage soldering. 2. Wave soldering is not suitable for fine-pitch QFP, PLCC, BGA and small-pitch SOP device soldering, that is to say, try not to arrange such components on this side of the wave soldering. 3. Smaller components should not be ranked behind larger components, so as not to prevent larger components from obstructing the tin flow from contacting the pads of smaller components, resulting in solder leakage. 3.1.2 PCB flatness control wave soldering has high requirements for the flatness of the printed board, and generally requires warpage to be less than 0.5mm. If it is greater than 0.5mm, it should be flattened. In particular, the thickness of some printed boards is only about 1.5mm, and its warpage requirements are higher, otherwise the welding quality cannot be guaranteed. 3.1.3 Proper preservation and shorten the storage cycle In the welding, the copper foil and component leads without dust, grease and oxide are conducive to the formation of qualified solder joints, so the printed board and components should be stored in a dry and clean environment, and the storage cycle should be shortened as much as possible. For the printed board with a long placement time, its surface is generally cleaned, which can improve the solderability, reduce the virtual welding bridge, and remove the surface oxide layer of the component pins that have a certain degree of oxidation on the surface. 3.2 Quality control of production process materials In wave soldering, the production process materials used are: flux and solder. Discussed separately as follows: 3.2.1 Flux quality control Flux plays an important role in the control of welding quality, and its role is: • remove oxides from the welding surface; • Prevent the reoxidation of solder and welding surfaces during welding; • Reduce the surface tension of the solder; • Help transfer heat to the welding area. At present, wave soldering mostly uses no-clean fluxes. The following requirements are required when selecting flux: • The melting point is lower than the solder; the infiltration diffusion rate is faster than the melted solder; the viscosity and specific gravity are smaller than the solder; • Stable storage at room temperature. 3.2.2 Quality control of solder Tin solder at high temperature (250 ° C) tin will continue to be oxidized, so that the tin solder content in the tin pot continues to decline, deviating from the eutectic point, resulting in poor fluidity, continuous welding, virtual welding, insufficient strength of solder joints and other quality problems. The following methods can be used to solve this problem: (1) add a redox agent to reduce the oxidized SnO to Sn, and reduce the production of tin slag; (2) Continuous removal of scum; (3) Add a certain amount of tin before each welding; (4) Use antioxidant (phosphorus-containing) solder; (5) Nitrogen protection is used to isolate the solder from the air and replace ordinary gas, so as to avoid the generation of scum. This method requires a modification of the equipment and the provision of nitrogen. The best approach at present is to use phosphorus-containing solder in a nitrogen-protected atmosphere, which can control the dross rate to a minimum, with the fewest welding defects and the best process control. 3.3 The process parameters in the welding process control the influence of welding process parameters on the quality of the welding surface is more complicated and involves more technical scope. 3.3.1 The role of controlling preheating of the preheating temperature: 1 Make the solvent in the flux fully volatilize, so as not to affect the wetting of the printed board and the formation of solder joints when the printed board passes through the solder; 2 So that the printed board reaches a certain temperature before welding, so as to avoid warpage deformation caused by thermal shock. 3 The general preheating temperature is controlled at 110 °C ~ 150 °C, and the preheating time is 1min ~ 3 min. 3.3.2 The influence of the inclination of the welding track on the welding effect is more obvious, especially when welding high-density devices. When the inclination angle is too small, it is more likely to appear bridging, especially in soldering, and the shielding area of the device is more prone to bridging; The inclination angle is too large, although it is conducive to the elimination of bridging, but the amount of tin eaten by the solder joint is too small, which is easy to produce virtual welding. The orbital inclination should be controlled between 56°. 3.3.3 The height of the peak height of the peak will have some changes due to the passage of welding working time, and appropriate corrections should be made during the welding process to ensure that welding is carried out at the ideal height; The depth of tin pressing is 1/2 to 2/3 of the PCB thickness. 3.3.4 Welding temperature Welding temperature is an important process parameter that affects the quality of welding. When the soldering temperature is too low, the expansion rate and wetting performance of the solder become poor, so that the pad or component solder end cannot be fully wettened, resulting in false welding, tip pulling, bridging and other defects; When the soldering temperature is too high, it accelerates the oxidation of pads, component pins and solder, and is prone to virtual soldering. The welding temperature should be controlled at 255 °C± 5 °C. Common welding defect analysis see the next list: Wave Soldering Common Welding Defect Analysis (Nitto Official) Nitto Wave Soldering Training Textbook Official Edition is shared by collation