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Reflow Soldering

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Reflow Soldering Process

1. What Is Reflow Soldering?

With the development of the whole SMT technology, is becoming more and more perfect, a variety of SMD components (SMC) and SMD devices (SMD) appear. As part of the placement technology, reflow soldering process technology and equipment have also been developed. Accordingly, its application is becoming more and more extensive, and almost all areas of electronic products have been applied. Reflow soldering is surface mount technology (SMT). The most commonly used method is when the electronic components are glued to the printed circuit board.

The reflow process aims to avoid rapid heating and damage to electronic components, gradually melting the solder and slowly heating the connection interface.

2. Reflow soldering Process Differentiation

Professionals usually divide the traditional reflow soldering process into preheating, immersion heat, Welding stage, and cooling.

Preheat stage: Preheat is the first stage of reflow soldering. The temperature of the entire component group board climbs to the target immersion temperature. The primary purpose is to enable the component board to safely and gradually reach the working temperature of the immersion heat (pre-reflow temperature) and allow the volatile solvent in the solder paste to vaporize and evaporate away.

Heat soak phase: Typically, 60 to 120 seconds of heat to remove volatiles from the solder paste and activate the flux, which will begin a redox reaction on the component wires and pad.

Welding stage: This is where the highest temperature is reached during the whole process. The most important thing is the peak temperature, the maximum temperature allowed across the entire process. A typical peak temperature is 20 to 40°C above the liquefaction temperature of the solder.

Cooling phase: The treated component group is gradually cooled and solidified with the solder joint. Proper cooling can inhibit the formation of metal intercalation or avoid thermal shock to the component. Typical cooling zones are set between 30 and 100°C, and rapid cooling produces a good grain structure and achieves the desired structural strength.

3. Reflow Soldering Technology Commonly Used In SMT

Infrared (IR) Reflow oven: This type of IR oven is also mostly conveyor belt type, but the conveyor belt only plays the role of support, the transmission of the substrate. Its heating mode is mainly according to the infrared heat source to radiation heating, the temperature in the furnace chamber than the previous uniformity. The mesh is larger, and suitable for reflow soldering heating of the substrate for double-sided assembly. This type of reflow oven can be the primary type of reflow oven. Used in China a lot, and the price is also relatively low.

Hot air reflow soldering: Hot air reflow furnace through the laminar flow movement of hot air transfer of thermal energy, the use of heaters and fans, so that the furnace air constantly warming and circulating, the parts to be soldered in the furnace by the incandescent gas heating, to achieve welding. Hot air reflow oven with uniform heating, temperature stability, the temperature difference between the top and bottom of the PCB, and the temperature gradient along the length of the furnace direction is not easy to control, generally not used alone.

Since the 1990s, with the continuous expansion of SMT applications and the further miniaturization of components, equipment development manufacturers have improved the distribution of heaters, air circulation flow, and increased temperature zones to 8, 10. Therefore, it can further precisely control the temperature distribution of each part of the furnace, making it more convenient for the ideal adjustment of the temperature profile. Professionals have regarded the full hot air forced convection reflow oven as the mainstream equipment for SMT soldering.

Infrared + hot air reflow soldering: In the mid-1990s, reflow soldering in Japan has transferred to the infrared + hot air heating method. It foot according to 30% infrared, 70% hot air as a heat carrier for heating. The infrared hot air reflow oven effectively combined the strengths of infrared RS and forced convection hot air reflow soldering, an ideal heating method in the 21st century.

It makes full use of the characteristics of solid penetration of infrared radiation, high thermal efficiency, and power saving. While effectively overcoming the temperature difference and affecting infrared RS, to make up for the impact of hot air reflow soldering on the gas flow rate requirements too fast. Infrared + hot air reflow soldering furnaces had been commonly used internationally.

Hot gas reflow soldering: Hot gas reflow soldering refers to welding a specially designed heating head through air or nitrogen, using hot airflow. This method requires processing different size nozzles for various size solder joints. The speed is relatively slow, used in rework or development.

Poly-infrared RS: Focused infrared RS (reflow soldering) is suitable for rework workstations for rework or partial soldering.

RS process flow: RS processing for the surface mount board, its process is more complicated, can be divided into two kinds: single-sided mount, double-sided mount.

  • Single-sided mount: Pre-application of solder paste → placement (Include Hand placement and Automatic placement by machine) → , reflow soldering → inspection, and electric test.
  • Double-sided mounting: It is a side pre-painted solder paste, SMD (Include Hand placement and Automatic placement by machine),  RS, B side pre-painted solder paste, SMD (Include Hand placement and Automatic placement by device). Reflow soldering, and inspection and electric test.
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