Uncover the four factors influencing SMT processing by reflow soldering
Reflow soldering, also known as reflow soldering, is a key process of SMT. The process of reflow soldering is to dry, preheat, melt, cool and solidify PCB coated with solder paste and mounted components through reflow soldering. In the welding process, defects such as bridging, erecting, lack of welding or lack of welding often occur. The causes of such welding defects include other external factors besides reflow process factors. Next, we will reveal the four major factors that reflow affects the processing quality of SMT. 1、 PCB pad design reflow soldering quality is directly related to PCB pad design. If PCB pad design is positive
Reflow soldering, also known as reflow soldering, is a key process of SMT. The process of reflow soldering is to dry, preheat, melt, cool and solidify PCB coated with solder paste and mounted components through reflow soldering. In the welding process, defects such as bridging, erecting, lack of welding or lack of welding often occur. The causes of such welding defects include other external factors besides reflow process factors. Next, we will reveal the four major factors that reflow affects the processing quality of SMT.
The quality of reflow soldering is directly related to 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 molten solder during reflow (called self positioning or self correction effect); On the contrary, if the PCB pad design is incorrect, even if the mounting position is very accurate, there will be component position offset, suspension bridge and other welding defects after reflow soldering.
2、 Quality of solder paste
Solder paste is a necessary material for reflow soldering process. It is a paste solder made by evenly mixing alloy powder (particles) and paste flux carrier. The alloy particles are the main components to form the solder joints, and the flux is used to remove the oxide layer on the welding surface and improve the wettability. Ensuring the quality of solder paste has an important impact on the welding quality.
3、 Quality and performance of components
As an important component of SMT mounting, the quality and performance of components directly affect the pass rate of reflow soldering. As one of the objects of reflow welding, the most basic thing must be high temperature resistance. In addition, the thermal capacity of some components will be relatively large, which also has a great impact on welding. For example, the thermal capacity of PLCC, QFP is generally larger than that of a discrete chip element, and it is more difficult to weld large area components than small components.
4、 Welding process control
1. Establishment of temperature curve
Temperature curve refers to the curve of temperature change of a point on SMA with time when SMA passes through the reflux furnace. The temperature curve provides an intuitive method to analyze the temperature change of a component in the whole reflow process. This is very useful for obtaining the best weldability, avoiding damage to components due to overheating, and ensuring the welding quality. The temperature curve is tested by furnace temperature tester, such as SMT-C20 furnace temperature tester.
2. Preheating section
The purpose of this area is to heat the room temperature PCB 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; If it is too slow, the solvent will not volatilize sufficiently, which will affect the welding quality. Due to the fast heating speed, the temperature difference in the SMA at the rear section of the temperature zone is large. In order to prevent damage to components caused by thermal shock, the maximum speed is generally 4 ℃/s. However, the rise rate is usually set at 1-3 ° C/s. The 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 solder paste. Its main purpose is to stabilize the temperature of each element in SMA and minimize the temperature difference. Sufficient time is allowed in this area to make the temperature of the larger element catch up with that of the smaller element, and to ensure that the flux in the solder paste is fully volatilized. By the end of the insulation section, the oxides on the bonding pads, solder balls and component pins are removed, and the temperature of the entire circuit board is balanced. It should be noted that all components on the SMA should have the same temperature at the end of this section, otherwise entering the reflux section will cause various undesirable welding phenomena due to uneven temperature of each part.
4. Reflux section
In this area, the temperature of the heater is set to the highest, so that the temperature of the components can quickly rise to the peak temperature. In the reflow section, the peak welding temperature varies depending on the solder paste used. 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 that the "tip area" beyond the solder melting point covers the minimum area.
5. Cooling section
The lead tin powder in the solder paste in this section has melted and fully wetted the connected surface. It should be cooled as fast as possible, which will help to get bright solder joints with good shape and low contact angle. Slow cooling will cause more decomposition of the circuit board into the tin, resulting in gray and rough solder joints. In extreme cases, it can cause poor tin adhesion and weaken the bonding force of solder joints. The cooling rate of the cooling section is generally 3-10 ℃/s, and it can be cooled to 75 ℃.
Reflow welding is a complex and key process in SMT process, which involves many deep sciences such as automatic control, materials, metallurgy, etc. There are many reasons for welding defects. In order to obtain better welding quality, in-depth research is needed, and constantly summarized in practice.