Manufacturing Insights: Reflow Soldering Principles and Precautions

Reflow Soldering Technology in the Electronic Assembly Process is a key step to ensure a strong bond between components and the PCB. This article will explore the working principles of reflow soldering, helping everyone to better understand the importance of this technology in enhancing product quality and production efficiency.  

What is Reflow Soldering?  

Reflow soldering, also known as reflow, is a soldering technique used in Surface Mount Technology (SMT) to attach electronic components to printed circuit boards. It utilizes hot airflow to melt and reflow solder paste at high temperatures, forming good solder joints.  

(Reflow Oven)  

Common Reflow Soldering Equipment  

Hot Air Reflow Soldering: Primarily uses heating wires for heating, with hot air circulating layer by layer within the furnace to transfer the required heat for soldering. It provides the advantage of uniform and controllable temperature and high solder joint stability.  

(Hot Air Reflow Soldering)  

Nitrogen Reflow Soldering: Based on hot air reflow, nitrogen is added during heating to reduce oxygen levels in the oven, preventing component pin oxidation during soldering, improving solder wetting, reducing bubble formation in solder joints, and enhancing reliability.  

(Nitrogen Reflow Soldering)  

Working Principle of Reflow Soldering  

Solder paste expands and contracts with heat. Utilizing this property, the solder paste pre-applied to PCB pads is heated to a liquid state and forms a permanent connection between component pins and PCB pads upon cooling.  

The Four Temperature Zones of Reflow Soldering  

A reflow oven is divided into four zones: preheat, soak, reflow, and cooling. Each zone has a distinct role in the reflow process, and therefore, each zone is set to a different temperature.  

(Four Temperature Zones of a Reflow Oven)  

Preheat Zone: Typically set between 60°C - 130°C, it preheats the PCB and components. If the PCB warms too quickly from room temperature, thermal shock can damage both the board and components. Preheating the PCB effectively prevents thermal stress caused by rapid temperature changes, allows moisture and volatile elements in the solder paste to evaporate, reduces bubble formation in solder joints, and ensures good solder quality in the next stages.  

Soak Zone: Typically set between 120°C - 160°C, it further heats the PCB, ensuring moisture from the solder pads and component pins completely evaporates. This step ensures that the board and components reach a uniform temperature before entering the reflow zone, preventing defects caused by thermal shock in the reflow zone.  

Reflow Zone: This is the most critical step in the reflow soldering process. When entering the reflow zone, the temperature quickly rises to around 245°C to melt the solder paste on the PCB pads (the reflow zone temperature depends on the melting point of the solder paste).  

The molten solder dissolves and spreads between the PCB pads and component pins, achieving good wetting of the pads and component leads. Capillary action draws the solder into the gaps between component pins and pads.  

Cooling Zone: The cooling zone rapidly cools the solder joint temperature to solidify and form a stable metallic joint. Upon entering the cooling zone, the temperature drops quickly, and as the solder joint cools and solidifies, a permanent connection forms between component pins and pads. (The cooling speed must be controlled to prevent thermal stress from rapid cooling.)  

The temperature of the reflow oven should be determined by the melting point of the solder paste, which varies depending on the type of solder used. For instance, low-temperature solder paste melts around 138°C, requiring a reflow oven temperature of approximately 180°C ±5°C; medium-temperature solder paste melts around 178°C, requiring a temperature of around 215°C ±5°C; high-temperature solder paste melts around 217°C, requiring a temperature of about 245°C ±5°C.  

Typically, to ensure complete melting of the solder paste for good soldering results, the oven temperature is set slightly higher than the solder paste's melting point.  

Precautions  

Double-Sided Soldering: In general, for double-sided PCBs, solder the side with fewer or smaller components first. After cooling, solder the other side. During the, the temperature may soften solder joints on the underside, causing large components to fall off. Therefore, it is recommended to place large components on the same side of the PCB and solder the side with large components first.  

(PCB with large components)  

Component Heat Tolerance: The selected solder paste must be suitable for the components' heat tolerance, ensuring that the melting point of the solder paste is within the safe temperature range for the components.  

Pad Spacing: Due to surface tension in the solder on the pads, close spacing between pads can cause solder bridging. It is recommended to keep a pad spacing of at least 0.3mm.  

(Pads with close spacing)