Reflow soldering is a key process in SMT chip processing. The reflow soldering process is to dry, preheat, melt, cool and solidify the PCB coated with solder paste and mounted components after reflow soldering welding process. In the soldering process, defects such as bridges, tombstones, and lack of soldering or less soldering often occur. The causes of such soldering defects are not only the factors of the reflow soldering process, but also other external factors. Next, it is revealed 4 factors that affects SMT reflow soldering processing quality. In the welding process, defects such as bridges, tombstones, and lack of soldering or less soldering often occur. The reasons for such soldering defects are not only the factors of the reflow soldering process, but also other external factors. Electronics will reveal the 4 major factors that affect the quality of SMT processing by reflow soldering.
1. PCB pad design
The soldering quality of reflow soldering is directly related to the PCB pad design. If the PCB pads are designed correctly, a small amount of skew during placement can be corrected during reflow due to the surface tension of the molten solder (called self-alignment or self-correction effects); conversely, if the PCB pads are not designed correctly, even if The placement position is very accurate, but after reflow soldering, there will be soldering defects such as component position deviation and suspension bridges.
2. The Quality of Solder Paste
Solder paste is a necessary material for the reflow soldering process. It is a paste solder that is uniformly mixed with alloy powder (particles) and a paste flux carrier. Among them, alloy particles are the main components to form solder joints, and flux is to remove the oxide layer on the solder surface and improve wettability. Ensuring the quality of solder paste has an important impact on soldering quality.
3. Quality and Performance of Components
As an important component of SMT placement, the quality and performance of components directly affect the pass-through rate of reflow soldering. As one of the objects of reflow soldering, the most basic point is high temperature resistance. Moreover, the heat capacity of some components will be relatively large, which also has a great impact on welding. For example, usually PLCC and QFP have a larger heat capacity than a discrete chip component, and it is more difficult to weld large-area components than small components.
4. The welding Process Process Control
4.1. Establishment of Temperature Curve
The temperature curve refers to the curve of the temperature of a certain point on the SMA over time when the SMA passes through the reflow oven. Temperature profiles provide an intuitive way to analyze the temperature changes of a component throughout the reflow process. This is useful for obtaining optimum solderability, avoiding damage to components due to overtemperature, and ensuring solder quality. The temperature curve is tested with a furnace temperature tester, such as SMT-C20 furnace temperature tester.
4.2. Preheating Section
The purpose of this area is to heat the PCB at room temperature as soon as possible to achieve the second specific goal, but the heating rate should be controlled within an appropriate range. If it is too fast, thermal shock will occur, and both the board and components may be damaged; too slow , the solvent volatilization is not sufficient, affecting the welding quality. Due to the fast heating rate, the temperature difference in the SMA in the rear section of the temperature zone is large. In order to prevent damage to components due to thermal shock, the maximum speed is generally specified at 4°C/s. However, the rate of rise is usually set at 1-3°C/s. A typical ramp rate is 2°C/s.
4.3. Insulation Section
The holding section refers to the area where the temperature rises from 120°C to 150°C to the melting point of the solder paste. Its main purpose is to stabilize the temperature of each component in the SMA and minimize the temperature difference. Sufficient time in this area allows the temperature of the larger components to catch up with the smaller components and to ensure that the flux in the solder paste is fully evaporated. By the end of the holding period, the oxides on the pads, solder balls and component pins are removed, and the temperature of the entire circuit board reaches equilibrium. It should be noted that all components on the SMA should have the same temperature at the end of this section, otherwise entering the reflow section will cause various bad soldering phenomena due to the uneven temperature of each part.
4.4. Return Section
The heater is set to the highest temperature in this area, allowing the component temperature to rise rapidly to the peak temperature. In the reflow section, the peak soldering temperature varies depending on the solder paste used. Generally, it is recommended to add 20-40°C to the melting point temperature of the solder paste. For 63Sn/37Pb solder paste with a melting point of 183°C and Sn62/Pb36/Ag2 solder paste with a melting point of 179°C, the peak temperature is generally 210-230°C, and the reflow time should not be too long to prevent adverse effects on the SMA. The ideal temperature profile is one where the “tip zone” above the melting point of the solder covers the smallest area.
5. Cooling Section
The lead and tin powder in the solder paste in this section has melted and fully wetted the surface to be connected. It should be cooled as fast as possible, which will help to obtain bright solder joints with good shape and low contact angle. Slow cooling causes more decomposition of the board into the tin, resulting in dull, rough solder joints. In extreme cases, it can cause poor wetting and weaken solder joint adhesion. The cooling rate of the cooling section is generally 3-10 °C/s, and it can be cooled to 75 °C.
Reflow soldering is a complex and critical process in the SMT chip processing process. It involves a variety of relatively deep sciences such as automatic control, materials, and metallurgy. There are many reasons for soldering defects. In order to obtain better soldering quality, it is also necessary to study in depth and summarized in practice.
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