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Reflow soldering, also called reflow soldering, is a key process of SMT chip processing. The process of reflow soldering is to dry, preheat, melt, cool and solidify the PCB coated with solder paste and mounted components through reflow soldering. The welding process. In the soldering process, bridging, tombstones and lack of soldering or lack of soldering defects often occur. The reasons for such soldering defects are not only the reflow soldering process factors, but also other external factors. Next, I will reveal the influence of reflow soldering on SMT. 4 major factors of processing quality. In the soldering process, bridging, tombstones and lack of soldering or lack of soldering defects often occur. The reasons for such soldering defects are not only the reflow soldering process factors, but also other external factors. Next, I will reveal the influence of reflow soldering on SMT. 4 major factors of processing quality.
One, PCB pad design
The soldering quality of reflow soldering is directly related to the PCB pad design. If the PCB pad design is correct, a small amount of skew during mounting can be corrected due to the surface tension of the molten solder during reflow soldering (called self-positioning or self-correction effect);
on the contrary, if the PCB pad design is incorrect, even The placement position is very accurate, and soldering defects such as component position offset and suspension bridges may appear after reflow soldering.
Second, the quality of solder paste
Solder paste is a necessary material for the reflow soldering process. It is a cream solder that is uniformly mixed with alloy powder (particles) and a paste flux carrier. Among them, the alloy particles are the main components that form the solder joints, and the flux is to remove the oxide layer on the soldering surface and improve the wettability. Ensuring the quality of solder paste has an important impact on soldering quality.
Third, the 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 must be high temperature resistance. In addition, the heat capacity of some components will be relatively large, which also has a large impact on welding. For example, PLCC and QFP usually have a larger heat capacity than a discrete chip component. It is more difficult to weld large-area components than small components.
Four, welding process process control
1, the establishment of temperature curve
Temperature curve refers to the curve that the temperature of a certain point on the SMA changes with time when the SMA passes through the reflow oven. The temperature curve provides an intuitive method to analyze the temperature change of a component during the entire reflow soldering process. This is very useful for obtaining the best solderability, avoiding damage to components due to over-temperature, and ensuring soldering quality. The temperature curve is tested with a furnace temperature tester, such as SMT-C20 furnace temperature tester.
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 the circuit board and components may be damaged; too slow, The solvent volatilization is insufficient, which affects the welding quality. Due to the faster heating rate, the temperature difference in the latter section of the SMA is larger. In order to prevent thermal shock from damaging the components, the maximum speed is generally specified as 4°C/s. However, the ascent rate is usually set to 1-3°C/s. The typical heating rate is 2°C/s.
3, insulation section
Holding section refers to the area where the temperature rises from 120 degree Celsius-150 degree Celsius to the melting point of solder paste. Its main purpose is to stabilize the temperature of each element in the SMA and minimize the temperature difference. Sufficient time in this area allows the temperature of the larger component to catch up with the smaller component, and to ensure that the flux in the solder paste is fully volatilized. At the end of the heat preservation section, 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, reflux section
In this area, the heater temperature is set to the highest, so that the temperature of the component rises quickly to the peak temperature. In the reflow section, the peak soldering temperature varies depending on the solder paste used. Generally, the melting point temperature of the solder paste plus 20-40°C is recommended. For 63Sn/37Pb solder paste with a melting point of 183 degree Celsius and Sn62/Pb36/Ag2 solder paste with a melting point of 179 degree Celsius, the peak temperature is generally 210-230 degree Celsius, and the reflow time should not be too long to prevent adverse effects on SMA. The ideal temperature profile is the smallest area covered by the “tip area” that exceeds the melting point of the solder.
5, cooling section
In this section, the lead-tin powder in the solder paste 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 appearance and low contact. angle. Slow cooling will cause more decomposition of the circuit board into the tin, resulting in dull and rough solder joints. In extreme cases, it can cause poor soldering and weaken the bonding force of solder joints. The cooling rate in 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 technology. It involves a variety of deep sciences such as automatic control, materials, and metallurgy. There are many reasons for soldering defects. If you want to obtain better soldering quality, Need to study in depth, and continue to summarize in practice.
