I. The SMT reflow oven process overview
SMT reflow oven is a pre-printed on the circuit board pads through the solder paste, melted in a heated environment, so that the SMD and the circuit board to form a reliable electrical connection and mechanical connection welding technology. In this process, the temperature is the key factor affecting the quality of welding, which directly determines the melting state of the solder paste, the heat of the components and the quality of the final solder joint.
II. The SMT reflow oven temperature of different temperature zones on the impact of circuit boards
1. Preheating zone
1.1 Temperature range and purpose
The temperature in the preheating zone is generally between 150 – 200°C. The main purpose of this stage is to allow the circuit board to be heated up. The main purpose of this stage is to allow the circuit board and components to slowly and evenly warm up.
1.2 Effects on the board
Removal of moisture and solvents: The preheating process allows moisture and solvents in the flux to slowly evaporate.
Reduces thermal shock: Proper preheating reduces the amount of thermal shock that components are subjected to in subsequent high temperature zones. If there is no preheating or if the preheating temperature is too low, these components may be damaged due to thermal stress when they suddenly enter the high temperature zone.
2. Holding Temperature Zone
2.1 Temperature range and purpose
The temperature in the holding zone is usually maintained at about 180 – 220°C. In this temperature zone, the temperature in the solder paste is maintained at a constant level. In this zone, the flux in the solder paste is in full effect, removing oxides from the component pins and the surface of the circuit board pads, and at the same time keeping the solder paste in a suitable viscosity state, ready for the subsequent reflow soldering.
2.2 Effects on the circuit board
Optimization of the role of the flux: the appropriate holding temperature to ensure that the flux is fully activated, effectively remove the oxide layer.
Regulation of solder paste performance: temperature affects the viscosity and fluidity of the solder paste.
3. Reflow zone
3.1 Temperature range and purpose
The reflow zone is the critical temperature zone for reflow soldering and is typically between 220 – 260°C. In this zone, the solder paste is completely melted. In this zone the solder paste is completely melted and a good solder joint is formed.
3.2 Effects on the board
Critical factor for the quality of the solder joint: If the temperature is too low, the solder paste does not melt completely and cold soldering occurs. Cold soldered solder joints have a rough appearance, loose internal structure, and insufficient strength of the solder joints, making them prone to open-circuit failures during subsequent use.
Oxidation and thermal stress problems: Excessively high temperatures can cause excessive oxidation of the solder, reducing the reliability of the solder joint. At the same time, too high a temperature will also cause greater thermal stress on the board and components. For some components that are less heat resistant, such as ceramic capacitors, problems such as cracking may occur. Moreover, excessive thermal stress may also cause deformation of the board, affecting the dimensional accuracy of the board and the mounting accuracy of the components.
4. Cooling zone
4.1 Temperature range and purpose
The purpose of the cooling zone is to allow the solder joints to cool and solidify quickly in order to form a stable crystal structure and improve the strength of the solder joints. The cooling speed is usually controlled at 2 – 5°C/s.
4.2 Effects on the board
Improvement of solder joint performance: The appropriate cooling speed can make the solder joint form a fine and uniform grain structure, improve the mechanical and electrical properties of the solder joint. If the cooling speed is too slow, it may lead to coarse crystallization of the solder joint and reduce the strength of the solder joint.
Impact on the overall performance of the circuit board: For some high-density circuit boards, due to the close layout of components, heat dissipation is relatively difficult. Reasonable cooling zone temperature settings are particularly important, otherwise it may be due to local overheating and affect the performance and reliability of the entire board.
III. The relationship between reflow soldering temperature and circuit board materials and components
1. Circuit board materials
1.1 The influence of the characteristics of the substrate
Commonly used circuit board substrates such as FR – 4, at different temperatures have different performance. During the high temperature process of reflow soldering, its glass transition temperature is a key indicator. If the reflow temperature is too high, close to or exceeding the glass transition temperature of the substrate, the substrate will become soft and deformed.
1.2 The effect of temperature on circuit board wiring
Circuit board wiring also generates thermal expansion when the temperature changes. If the reflow temperature is not properly controlled, the thermal expansion of the wiring may cause the wiring to break or short out with neighboring wiring.
2. Components
2.1 Temperature tolerance of different types of components
There are various types of components on the circuit board, such as chips, capacitors, resistors and so on. Different components have different temperature tolerances.
2.2 Effect of Temperature on Component Pins and Packages
For example, gold-plated pins and tin-plated pins react differently at high temperatures. Meanwhile, the encapsulation materials of components, such as plastic encapsulation or ceramic encapsulation, may be deformed and cracked at high temperatures, affecting the normal function of components.
IV. Conclusion
The impact of SMT reflow oven temperature on the circuit board is multifaceted, from the quality of the solder joints to the performance of the circuit board material, and then to the reliability of the components are closely related to it. In the actual circuit board welding process, according to the specific structure of the circuit board, component types and welding requirements, precise control of the temperature of the reflow soldering temperature of each temperature zone, in order to ensure high-quality welding results, improve the overall performance and reliability of the circuit board.
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Post time: Jan-06-2025