From the actual operation of welding, it is very important to have less welding defects. In particular, poor defects such as bridging and pulling are closely related to the wettability of the solder. In the case of reflow soldering, in order to improve the deoxidation of the solder, the active material must be removed by using an active flux, but this will cause residue and corrosion and electromigration. At the same time, in the flow soldering, the tin oxide slag generated by the wave soldering is also a problem, which not only causes an increase in the solder joint defect rate, but also increases the cost. In addition, the preservability of solder paste is a necessary condition for good printing. Since the flux in the solder paste reacts with the alloy during storage, it deteriorates, resulting in an increase in viscosity, poor printing, and the like. All of the above are issues that must be considered for lead-free solders. It is very important to select an appropriate lead-free solder from these viewpoints. Table 1 shows the comparison of characteristics of lead-free eutectic alloys developed since 1980. On this basis, the current practical tin-silver-copper and other mainstream lead-free solders have been developed.
From tin-lead solder to tin-silver-copper solder, there will be great changes in actual operation, mainly:
(1) The most commonly used tin-3.0 silver-0.5 copper has a melting point of 217 ° C -219 ° C. When reflow soldering, the minimum process temperature that can be operated should be liquid phase temperature plus 10 ° C, which is better than tin lead The melting point of the eutectic solder is 40 ° C higher. It is not difficult to see that the difference between the operating temperature rise and the heat-resistant temperature of the component (240 ° C) will be greatly reduced, so it is necessary to have more accurate process temperature management than before. In addition, due to the variety of printed boards, components with different heat capacities have a temperature difference of 10 ° C, so the preheating temperature and time must be increased. Reflow soldering equipment must be heated in multiple temperature zones to reduce temperature errors and become an effective measure. See Figure 2. In this way, due to the rise of the melting point, the welding process and equipment will undergo major changes. For this reason, the lead-free tin-silver-copper solder is implemented, and lowering the melting point thereof will become a concern.
(2) It is generally believed that tin-silver-copper has lower wettability than tin-lead, and its diffusivity is 75%-80%, which is about 15% lower than that of tin-lead. In order to improve the solderability, it is necessary to add an active agent to the flux, but it may cause an undesirable phenomenon such as an increase in viscosity. In addition, since the surface tension of the lead-free solder is higher than that of the lead-containing solder, the wettability is also deteriorated under the same conditions.
(3) Since the melting point of the lead-free solder is high, the adaptability of the peak temperature to the heat-resistant temperature of the component (230 ° C - 240 ° C) must be considered, so the preheating end temperature is high. The temperature of components with different heat capacity can be made uniform. In addition, due to the oxidation of the components and the base material, the loss of solder paste activity is likely to cause solder balls. When the flux of tin-lead solder paste is used for tin-silver-copper solder paste, the preheating temperature and the preheating time must be increased, Temperature changes will cause concern, so fluxes for lead-free solder paste must be developed.
(4) During the printing process, due to the reaction between the flux and the powder in the solder paste, organic metal compounds and organic metal salts are precipitated on the surface of the powder, resulting in a decrease in fluidity, an increase in viscosity, and an influence on printing performance, resulting in poor soldering. the reason.
Performance characteristics of lead-free no-clean solder paste
Lead-free no-clean solder paste is suitable for high-speed printing and placement production lines. It should have excellent rheology, thermal collapse resistance and high stability. However, it is often found that some brands of solder paste are unstable in quality. The common phenomenon is product samples. Performance can be achieved, but once the batch is used, quality fluctuations often occur. For example, the viscosity of the solder paste becomes large or even dry during use, the printing quality decreases with time, and the welding defect rate fluctuates severely. Many people believe that this fluctuation may be caused by the lack of control of the solder paste manufacturer during the production process. In fact, the fluctuations caused by production control problems account for only a small part. Others such as tin powder quality and flux stability are important reasons for the fluctuation of solder paste quality. Among them, flux stability is the most important of all factors. of. The stability of the flux means that its physical and chemical properties are relatively stable at normal temperature, and it is difficult to crystallize or react with metals. Well-known brands of solder paste such as Alpha, Tamura and other flux systems have higher stability, so their products are relatively stable during use. A domestic brand of lead-free solder paste Bright's flux system uses a special activation system. The activation system is stable at room temperature and does not react with metals and their oxides. Therefore, the viscosity changes little during continuous printing. In reflow soldering, its activity begins to gradually work after the temperature rises until the activity reaches a maximum at the soldering temperature. The activation system also prevents excessive loss of activity of the solder paste during preheating and heat preservation, thereby greatly widening the reflow window and facilitating the user to adjust the optimum process. Another important role of flux stabilization at ambient temperature is its relatively high reliability. All solder pastes will leave some flux remaining around the solder joints after reflow, so the stability of the remaining flux will have a direct impact on the electrical properties of the surface insulation resistance. Bright's long-term reliability is no worse than Alpha, Tamura's same type of product.
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