Basic principle of SMT vacuum reflow soldering furnace

With the continuous miniaturization of components, the degree of chip integration is getting higher and higher. Whether it is notebook, smart phone, medical equipment, car electronics, military and aerospace products, BGA, CSP and other equipment encapsulated by array in products are used more and more, and there are more and more requirements for product quality. This requires us to continuously advance SMT process ability, add high-quality equipment and ensure high reliability products through high-quality welding.

Generally, after SMT patch welding, the solder joints in the equipment will remain empty, which poses a certain potential risk to the reliability of product quality. Although there are many reasons for these emptiness, such as solder paste, PCB pad surface treatment method, reflow curve setting, reflow environment, pad planning, micropores, disk emptiness, etc., the primary reason is often composed of the residual gas of molten solder in welding. When the melted solder agglomerates, these bubbles are frozen down to form an empty phenomenon. Emptiness is a common phenomenon in welding. It is difficult to have no emptiness in all solder joints of electronic assembly products. Due to the influence of vague elements, the quality and reliability of most solder joints are uncertain, which constitutes the decline of mechanical strength of solder joints, and will seriously affect the thermal and conductive functions of solder joints, and then seriously affect the electrical functions of equipment. Nitrogen reflow soldering

In view of the X-ray content of all the solder joints in the PCB, it shall not exceed 5% of the total power of the solder joint. The minimum area ratio of this order cannot be achieved by optimizing the existing process, which means that new welding processes, such as vacuum reflow furnace welding skills, are required. Vacuum reflow welding process is a skill of welding in vacuum environment. In this way, it can be fundamentally handled in the SMT chip proofing or processing and production process. Because of the oxidation of solder in non vacuum environment and the pressure difference in the solder joint table, the bubbles in the solder joint simply overflow from the solder joint, and then reach the solder joint. The bubble rate is very low or even no bubbles, reaching the expected intention.

Vacuum reflow soldering technology provides the possibility of preventing gas from falling into the solder joint and then forming an emptiness, which is particularly important in large-area welding, because these large-area solder joints need to conduct high-power electric energy and heat energy, so reducing the emptiness in the solder joint can fundamentally improve the thermal conductivity of the device material. Vacuum welding is sometimes mixed with restorative gas and hydrogen to reduce oxidation and remove oxides.

The basic principle of vacuum reflow furnace to reduce the emptiness in the welding process can be analyzed from four aspects. Here is a brief explanation and analysis.

1. The vacuum reflow furnace can supply very low oxygen concentration and appropriate restorative atmosphere, so that the oxidation degree of solder can be greatly reduced;

2. Because the oxidation degree of solder decreases, the gas of oxide and flux reaction is greatly reduced, which reduces the possibility of void generation;

3. Vacuum can make the activity of molten solder better and the activity resistance smaller. In this way, the buoyancy of bubbles in molten solder is much greater than the activity resistance of solder, and bubbles are simply discharged from molten solder; Lead free reflow soldering

4. Because there is a pressure difference between the bubble and the external vacuum environment, the buoyancy of the bubble will be very large, which makes the bubble very simply out of touch with the constraint of molten solder. After vacuum reflow welding, the reduction rate of bubbles can reach 99%, the void rate of a single solder joint can be less than 1%, and the void rate of the whole plate can be less than 5%. On the one hand, it can strengthen the reliability and bonding strength of solder joints and the wetting function of solder. On the other hand, it can reduce the application of solder paste in the process of application, and it can make solder joints get used to different environmental requirements, especially high temperature and high humidity, low temperature and high humidity.