Discussion on the development of reflow soldering process Discussion on the development of reflow soldering process (1)
In recent years, SMT production technology has undergone tremendous changes, including: changes in production standards, the use of new solder pastes, the emergence of different substrates, and the innovation of materials and designs of components themselves have led to the development of heat treatment processes. The design power of new components is driven by the miniaturization of products. These new component packages include: BGA (Ball Grid Array), COB (Bare Chip), CSP (Micro Package), MCM (Multi-Chip Module), and flip chip (flip chip). The product's miniaturized reflow soldering makes the components smaller and smaller, and the number of pins increases, making the pitch smaller. In addition, in order to reduce costs, no-clean and low-residue solder pastes are used more widely, and the use of nitrogen is also increased.
The constant demand for handheld electronic products in the market is always a powerful driving force, and the packaging process must adapt to the technical requirements of these products. Therefore, smaller, denser, lighter assembly techniques, as well as shorter product cycles, more and more dense I/O leads, and greater controllability - all refer to reflow soldering technology The level of discussion. At the same time, new requirements are put forward for the control methods and equipment of the heat treatment process.
With these pressures in mind, we present a simple vision diagram, some of which can answer the challenges that the reflow process will encounter in the future.
Nitrogen inert protection
The use of an inert gas, typically nitrogen, has been used in the reflow process for quite some time, but its price is still a problem. Because inert gases can reduce oxidation during soldering, this process can use less active solder paste materials. This is especially important for low residue solder pastes and no-cleansing. In addition, it is also critical for multiple welding processes. For example, in the welding of double-panel, nitrogen protection has great advantages in many reflow processes for boards with OSPs, because under the protection of N2, the solderability of copper pads and lines on the board is obtained. Very good protection. Another benefit of using nitrogen is the increased surface tension, which allows manufacturers to have more room to choose devices (especially for ultra-fine pitch devices) and to increase the surface finish of solder joints, making thin materials less susceptible to fading.
The real biggest benefit is lowering costs. The cost of nitrogen protection depends on a variety of factors, including the location of the nitrogen used in the machine, the availability of nitrogen, and the like. Of course, we usually feel that nitrogen consumption is an extra cost in the process, so we always try to reduce the consumption of nitrogen. At present, the chemical composition of the solder paste is constantly improving and improved so that nitrogen protection is no longer used in future processes; or at least at a higher O2 concentration (for example, 1000 ppm compared to the current 50 ppm), so that a good soldering effect can be obtained. Reduce the amount of nitrogen used. For the protection of nitrogen, we must consider a number of issues, including the quality level required for production, and the cost of each corresponding nitrogen consumption. The use of nitrogen is costly, but if it is calculated for its benefits in terms of increased yield and quality, its cost is relatively negligible.
If the welding furnace is not the one that is forced to recirculate, and the gas flow is in a stratified state, the consumption of nitrogen is relatively easy to control. However, most of the furnaces currently work in a large-capacity circulation forced convection heating, and the airflow in the furnace is constantly flowing, which poses a new problem for the control and consumption of nitrogen. In general, we use these methods to reduce the amount of nitrogen. First, the size of the furnace inlet must be reduced, especially in the vertical direction, using a baffle, a curtain curtain, or some other thing to block the pores of the inlet and outlet. The isolation zone formed by the shutter and the curtain curtain can block the leakage of nitrogen and prevent the outside air from entering the inside of the furnace. Some reflow ovens use an automatic sliding door to isolate the air. Another method is based on the scientific concept that heated nitrogen will float above the air and the two gases will not mix. Therefore, the heating chamber of the reflow furnace is designed to be higher than the position of the inlet and outlet because nitrogen is naturally stratified with air, so that a relatively low concentration can be maintained with a small supply of nitrogen.
Double-sided processing
Double-panel processes are increasingly being adopted and become more complex. This is because it gives designers a larger, more flexible design space. The dual panel greatly enhances the actual utilization of the PCB, thus reducing manufacturing costs. So far, the process often used in double-panel is to pass the reflow furnace above, and the wave soldering furnace below. Today, everyone is gradually inclined to go through the reflow oven on both sides is a better way. However, there are still some problems in the process. For example, when reflowing, the larger component at the bottom may fall, or the solder joint at the bottom will partially re-melt, which will affect the reliability of the solder joint.
Several methods have been developed to perform secondary reflow, one of which sizing the first side of the component so that it does not fall off the plate during the second reflow and remains in the correct position. . Another method is to use solder pastes with different melting points, in which the solder paste used in the second reflow has a lower melting point than the first one. However, there are some serious problems to be aware of: the first one is that the final product has a "too low" melting temperature during maintenance; the second is that if a higher level of reflow temperature is used, it will cause a pair of elements. Thermal shock of the device and substrate.
For most components, the surface tension of the solder joints during secondary reflow is sufficient to maintain the adhesion of the components at the bottom, allowing the components to be firmly attached to the substrate. Here is a proportional relationship between the weight of the component and the tension of the pin (pad). It can be calculated whether the component can be pasted on the bottom of the substrate without falling down during the secondary reflow, so that it is not necessary to make actual implementation for each component. test. 30g/in2 is a conservative value and can be used as a design standard.
Another method is to use a concept that the cold gas is blown through the bottom of the substrate so that the temperature at the bottom never reaches the melting point during the secondary reflow, but the potential temperature difference between the upper and lower sides of the substrate may cause potential stress. Although the process of secondary reflow is not simple, many problems are being solved. In the next few years, we can be sure that there will be a high-density double-panel in both quantity and complexity. Long-term development.
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