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Industry Dynamics | 2025-09-16
SMT reflow process core analysis
1. Abstract
The purpose of this document is to provide a systematic introduction to convection reflow soldering (hereinafter referred to as reflow soldering), the core process in surface mount technology (SMT). It covers its basic principles, key equipment components, analysis of core process parameters (temperature profiles), and application areas. This document is the basis for understanding and optimising the reflow process and is suitable for process engineers, production managers and all related technical practitioners.
2. Introduction to reflow soldering
Reflow soldering is through the re-melting of pre-printed in the PCB pads on the solder paste, so as to achieve the surface assembly components solder end or pin and PCB pads between the mechanical and electrical connection of a welding process. It is a key link in the SMT process, which directly determines the welding quality and reliability of the final product.
Basic principle: will be mounted components of the PCB board in the reflow soldering furnace, conveyor belt driven PCB sequentially through the furnace chamber temperature zones, through the preheating, holding, reflux and cooling of the four stages, so that the paste through the heating, melting, wetting, cooling and solidification process, and ultimately the formation of permanent solder joints.
3. Basic structure of a reflow oven
A typical convection reflow oven consists of the following components:
Heating System: Contains multiple independent heating temperature zones (typically 6-13), each of which can be accurately temperature controlled by heat pipes and fans.
Conveyor system: consists of a high temperature resistant Mesh mesh belt or chain guide that conveys the PCBs through the entire oven chamber at a constant speed.
Cooling System: Located at the exit end of the furnace, it provides fast and controllable cooling of the finished soldered PCBs to form a fine grain structure of solder joints by means of a forced air-cooled or water-cooled heat exchanger.
Control system: PLC or industrial computer for setting and storing parameters such as temperature of each temperature zone, speed of conveyor belt, gas flow rate, etc., and monitoring the operation status of the equipment.
Nitrogen protection system (optional): High purity nitrogen (N₂) is injected into the furnace chamber to reduce the oxygen content, in order to prevent oxidation during the soldering process and improve the wettability and surface finish of the solder joints.
4. Reflow Temperature Profile Analysis
Temperature Profile (Temperature Profile) is the soul of reflow soldering process, it describes the curve of temperature change with time at a certain point on the PCB. An optimised profile is a prerequisite to ensure soldering quality. It can be divided into four key stages:
Typical Reflow Temperature Profile
Temperature. (°C) |
| Preheat Soak Reflow Cooling
| /-->---|------------->|peak temperature |
| / | / |
| / | / |
|/ | / |
|----------|----------|-------|-------------|-----> times (s)
Heating zone Holding zone Reflow zone Cooling zone
Preheat Zone:
Purpose: To heat up the PCB and components uniformly, activate the flux in the solder paste and evaporate some of the solvent.
Key parameters: the heating rate is usually controlled at 1.0-3.0 ° C / s. Too fast will lead to component thermal stress cracking and solder spattering; too slow will make the flux excessive volatility.
Holding/Activation Zone (Soak/Activation Zone):
Purpose: to make the PCB of different sizes, quality components temperature stabilisation, reduce the temperature difference. Flux in this stage to completely remove the pad and component pin surface oxides.
Key parameters: Temperatures are typically maintained between 120-160°C for approximately 60-120 seconds.
Reflow Zone：
Purpose: To heat the solder paste above its melting point so that it melts and wets the pads and component pins to form intermetallic compounds (IMCs) and make metallurgical connections.
Key Parameter: Peak Temperature: Typically 20-30°C above the melting point of the paste (e.g. for SAC305 paste with a melting point of 217°C, a peak temperature of 240-250°C is required).
Time Above Liquidus (TAL): Typically 30-90 seconds is required. Too short a time may result in inadequate soldering; too long a time may result in thick IMC, increased brittleness, and possible damage to components or PCBs.
Cooling Zone:
Purpose: To solidify and mould the molten solder to form reliable solder joints.
Key parameter: Cooling rate is usually controlled at 2-4°C/s. Appropriate cooling rates allow for the formation of fine microscopic solder joint structures and improved mechanical strength.
5. Application of nitrogen (N₂) protection
Injecting nitrogen into reflow soldering (keeping the oxygen content in the furnace below 1000 ppm, or even <100 ppm) can provide significant benefits:
Reduced oxidation: Significantly improved wetting of the solder joints and improved solder quality.
Improves solder joint brightness and consistency.
Allows the use of less active fluxes and reduces residues from subsequent cleaning.
Although it increases operating costs, nitrogen protection is almost essential when soldering BGAs, CSPs, fine pitch components or when using low solid content solder pastes.
6. Advantages and application areas
Advantage:
Mature and stable: very mature technology, wide process window, easy to control.
Highly efficient and productive: continuous production is possible with very high efficiency.
Cost-effective: equipment costs and operating costs are more advantageous than vacuum reflow soldering.
Wide range of applications: covers most application scenarios such as consumer electronics, home appliances, automotive electronics, industrial control, etc.
Limitations:
It is difficult to completely eliminate soldering voids when soldering components with "air pockets" effect (e.g. QFN, large size BGA, power modules).
For extremely heat sensitive or oversized PCBs, thermal uniformity control is a major challenge.
7. Conclusion
Convection reflow soldering is an indispensable cornerstone process in the electronics manufacturing industry. A deep understanding of the principles of the equipment and precise control of the temperature profile is the key to guaranteeing the yield and reliability of SMT production. It continues to support the majority of the world's electronics manufacturing with its excellent price/performance ratio and stability.
For most routine applications, optimised convection reflow soldering is sufficient to provide perfect and reliable solder joints. When faced with higher reliability requirements (e.g., ultra-low void rates), consider our vacuum reflow solutions as a technology upgrade.
