What are the common temperature curves in PCBA production?
Generally divided into 3 categories: triangular temperature curve, heating-heat preservation-peak temperature curve, low peak temperature curve.
(1) Triangular temperature curve suitable for simple PCBA products
For simple products, since the PCB is relatively easy to heat, the temperature of the component and the printed board are relatively close, and the temperature difference of the PCB surface is small, a triangular temperature curve can be used.
When the tin slip has a proper formula, the triangular temperature curve will get a brighter solder joint. But the flux activation time and temperature must
Adapt to the higher melting temperature of lead-free solder paste. The heating rate of the triangular curve is controlled as a whole, generally 1-1.5℃s. Compared with the traditional heating-heat preservation-peak curve, the energy cost is lower. This kind of curve is generally not recommended.
(2) Recommended temperature rise-heat preservation-peak temperature curve
The temperature rise-heat preservation-peak temperature curve is also called the tent-shaped curve. The figure is the recommended temperature rise-hold-hold-peak temperature curve (same as Figure 1), where curve 1 is the temperature curve of Sn37Pb soldering, and curve 2 is the temperature curve of lead-free Sn-Ag-Cu solder paste. It can be seen from the figure that the limit temperature of the component and the traditional FR4 printed board is 245℃, and the process window for lead-free soldering is much narrower than that of Sn-37Pb. Therefore, lead-free soldering needs to increase the temperature slowly, fully preheat the PCB, and reduce the temperature difference △ of the PCB surface to make the PCB surface temperature uniform, so as to achieve a lower peak temperature (235～245℃) and avoid damage to the components and FR-4 base. Material PCB. The requirements for the temperature rise-heat preservation-peak temperature curve are as follows.
1. The heating rate should be limited to 0.5～1℃/s or below 4℃/s, depending on the solder paste and components.
2. The formula of the flux composition in the solder paste should conform to the curve. Excessive heat preservation temperature will damage the performance of the solder paste.
3. The second temperature rise slope is at the entrance of the peak zone. The typical slope is 3℃/s and the time above the liquidus line requires 50～60s, and the peak temperature is 235～245℃.
4. In the cooling zone, in order to prevent the growth of crystal particles in the solder joints and prevent segregation, the solder joints are required to cool down quickly, but special attention should be paid to reducing stress. For example, the maximum cooling rate of ceramic chip capacitors is -2 to -4°C/s.
(3) Low peak temperature curve
The low peak temperature curve is to first add slow heating and full preheating to reduce the PCB surface temperature difference. In the reflow zone, large components and large heat capacity positions generally lag behind small components to reach the peak temperature. Figure 3 is a schematic diagram of the low peak temperature (230-240℃) curve. In the figure, the solid line is the temperature curve of the small component, and the dashed line is the temperature curve of the large component. When the small component reaches the peak temperature, keep the peak temperature low, and the peak time is wide, let the small component wait for the large component; the large component also reaches the peak temperature and keeps it for a few seconds, and then cools down. This measure can prevent damage to the components.
The low peak temperature (230～240℃) is close to the peak temperature of Sn-37Pb, so the risk of damage to the device is small, and the energy consumption is low; but the PCB layout, thermal design, reflow soldering process curve adjustment, process control, and equipment The requirements for lateral temperature uniformity are relatively high. The low peak temperature curve is not applicable to all products. In actual production, the temperature curve must be set according to the specific conditions of PCB, components, solder paste, etc., complex boards may require 260°C.
Through the study of welding theory, it can be seen that the welding process involves physical reactions such as wetting, viscosity, capillary phenomenon, heat conduction, diffusion, and dissolution, and chemical reactions such as flux decomposition, oxidation, and reduction. It also involves metallurgy, alloy layer, Metallography and aging are very complicated processes. In the SMT patch process, welding theory must be used to correctly set the reflow temperature curve. In PCBA manufacturing, it is necessary to master the correct process method, and through process control, as far as possible, SMT can achieve zero (no) defects or zero (no) defects through printing solder paste, mounting components, and finally SMA pass rate from the reflow soldering furnace. The quality of reflow welding is close to zero defects, and all the solder joints are required to achieve a certain mechanical strength. Only such products can achieve high quality and high reliability.