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The printed circuit board connects the electronic components together by soldering to play their best roles. This article will specifically introduce four precautions for soldering printed circuit boards.
The biggest effect of printed circuit board soldering is to solder all the electronic components together and let them play their own roles. The solder joints also protect the components from oxidation and peeling off. The following precautions determine the quality of the soldering process of the printed circuit board, thereby affecting its specific electronic functions.
Forms good intermolecular bonds by dipping tin
When the hot liquid solder dissolves and penetrates the metal surface welded by the printed circuit board, it is called metal dip tin or metal dip tin. The molecules of the mixture of solder and copper from a new alloy partly made of copper and partly soldered. This solvent action is called tin dip, which forms an intermolecular bond between each part to form a metal alloy eutectic. The formation of good intermolecular bonds is the core of the PCB soldering process, which determines the strength and quality of the PCB solder joints. Only the surface of the copper is not polluted, and there is no oxide film formed by exposure to the air to be wetted with tin, and the solder and the working surface need to reach an appropriate temperature.
Reduce surface tension
Everyone is familiar with the surface tension of water. This force keeps the cold water droplets on the grease-coated metal plate in a spherical shape. This is because in this example, the adhesion force that makes the liquid on the solid surface tend to spread is less than its cohesive force. Wash with warm water and detergent to reduce its surface tension. The water will infiltrate the grease-coated metal plate and flow outward to form a thin layer. This will happen if the adhesion force is greater than the cohesion force. The cohesion of tin-lead solder is even greater than that of water, making the solder spheres minimize their surface area (under the same volume, the sphere has the smallest surface area compared with other geometric shapes to meet the needs of the lowest energy state). The effect of the flux is similar to that of the cleaner on the grease-coated metal plate. In addition, the surface tension is highly dependent on the cleanliness and temperature of the surface. Only when the adhesion energy is much greater than the surface energy (cohesion) an ideal adhesion occur tin.
Maintain the thickness of the metal alloy layer
The intermetallic bond between copper and tin forms crystal grains. The shape and size of the crystal grains depend on the duration and strength of the temperature during PCB soldering. Less heat during PCB soldering can form a fine crystalline structure, forming an excellent PCB solder joint with the best strength. The reaction time is too long, whether it is due to the long soldering time of the printed circuit board or the high temperature, or both, it will result in a rough crystalline structure, which is gritty and brittle, and has low shear strength. Copper is used as the metal substrate and tin-lead as the solder alloy. Lead and copper will not form any metal alloys. However, tin can penetrate into the copper. The intermolecular bond between tin and copper forms a metal on the joint surface of the solder and the metal. Alloy eutectic Cu3Sn and Cu6Sn5. The metal alloy layer (n phase + ε phase) must be very thin. In laser PCB welding, the thickness of the metal alloy layer is on the order of 0.1mm. In wave soldering and manual soldering, the thickness of the intermetallic bond in good PCB soldering joints is mostly more than 0.5 μm.
Since the shear strength of PCB solder joints decreases as the thickness of the metal alloy layer increases, it is often tried to keep the thickness of the metal alloy layer below 1 μm. This can be achieved by making the PCB soldering time as short as possible. The thickness of the metal alloy layer depends on the temperature and time of forming the PCB solder joints. Ideally, the PCB soldering should be completed within 220't about 2s. Under this condition, the chemical diffusion reaction of copper and tin will produce an appropriate amount The thickness of the metal alloy bonding materials Cu3Sn and Cu6Sn5 is about 0.5μm. Insufficient intermetallic bonds are common in cold PCB soldering points or PCB soldering points that are not raised to the proper temperature during PCB soldering, which may cause the PCB soldering surface to be cut off. On the contrary, a too-thick metal alloy layer is common in PCB solder joints that are overheated or PCB soldering for too long, which will cause the PCB solder joints to have very weak tensile strength.
The meniscus stretched to less than 30 degrees
When the dip angle is about 35°C higher than the eutectic point of the solder, when a drop of solder is placed on the hot flux-coated surface, a meniscus is formed. To some extent, the metal surface is stained. The ability of tin can be estimated by the shape of the meniscus. If the solder meniscus has an obvious undercut edge, shaped like a water drop on a grease-coated metal plate, or even tends to be spherical, the metal is not PCB solderable. Only when the meniscus is stretched to a small angle less than 30 degrees can it have good PCB solderability.
The above factors have a great influence on the soldering effect of the printed circuit board, and you must pay attention to it during the soldering process. If you still have doubts about the soldering of printed circuit boards after reading the above content, you can contact us and we will provide you with a comprehensive solution.
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