What are the factors that affect SMT through reflow soldering?

2024-08-27 17:10

Reflow soldering process is the soldering process of drying, preheating, melting, cooling, and solidifying a PCB coated with solder paste and mounted with components through a reflow soldering furnace.
1、 PCB pad design
If the PCB pad design is correct, a small amount of mounting misalignment can be corrected during reflow soldering due to the surface tension of the molten solder (known as self positioning or self correction).

2、 Quality of solder paste
Solder paste is an essential material for reflow soldering process, which is a paste solder made by uniformly mixing alloy powder (particles) and paste like flux carrier. Alloy particles are the main component that forms solder joints, and the function of flux is to remove the oxide layer on the surface of the solder and improve its wettability.
3、 Quality and performance of electronic components
The quality and performance of electronic components directly affect the conductivity of reflow soldering. As one of the objects of reflow soldering, the most basic requirement is high temperature resistance. Some components have a relatively large heat capacity, which also has a significant impact on soldering. For example, compared to discrete chips, PLCC and QFP components have a larger heat capacity, making it much more difficult to solder large-area components than small-sized components.
4、 Process control of welding process
1. Establishment of temperature curve
The temperature curve provides an intuitive method to analyze the temperature changes of a component throughout the entire reflow soldering process. This is very useful for achieving optimal solderability, avoiding damage to components due to overheating, and ensuring soldering quality.
2. Preheating section
The purpose of this paragraph is to quickly heat the PCB at room temperature to the second specific target. The heating speed should be controlled within an appropriate range. If it is too fast, it may cause thermal shock and damage to the board and components. Too slow, insufficient solvent evaporation, affecting welding quality. Due to the fast heating rate, there is a significant temperature difference in the later temperature zone within SMA. To prevent thermal shock damage to components, the maximum heating rate is generally set at 4 ℃/s. However, the usual heating rate is set at 1-3 ℃/s. A typical heating rate is 2 ℃/s.

3. Insulation section
The insulation section refers to the area where the temperature rises from 120 ℃ -150 ℃ to the melting point of the solder paste. Its main purpose is to stabilize the temperature of the SMA internal components and minimize the temperature difference. Stay in this area for sufficient time to catch up with the temperature of larger components and ensure that the flux in the solder paste evaporates fully. At the end of the insulation section, the oxide on the solder pads, solder balls, and component pins is removed, and the temperature of the entire circuit board reaches equilibrium.
4. Reflux section
Set the heater temperature in this area to the maximum temperature, causing the component temperature to quickly rise to the peak temperature. The peak temperature for soldering in the reflow section depends on the type of solder paste used, and it is generally recommended to add 20-40 ℃ to the melting point temperature of the solder paste. For 63Sn/37Pb solder paste with a melting point of 183 ℃ and Sn62/Pb36/Ag2 solder paste with a melting point of 179 ℃, the peak temperature is generally 210-230 ℃, and the reflow time should not be too long to prevent adverse effects on SMA. The ideal temperature curve is the minimum area covered by the "tip region" that exceeds the melting point of the solder paste.
5. Cooling section
In this section, the lead and tin powder in the solder paste have melted and fully wetted the surface to be connected. It should be cooled as quickly as possible to obtain bright solder joints with good shape and low contact angle. Slow cooling speed can lead to more decomposed tin entering the board, resulting in gray and rough solder joints. In extreme cases, it can lead to poor welding and weaken the bonding force.

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The influence of temperature curve of SMT reflow soldering furnace temperature controller on SMT soldering quality!