Introduction to the principle and process flow of reflow soldering

Reflow soldering is one of the three main processes in SMT mounting technology. Reflow soldering is mainly used to weld circuit boards that have already been installed with components. By heating, the solder paste is melted to fuse and weld the SMT components with the circuit board solder pads. Then, through the cooling of reflow soldering, the solder paste is cooled to solidify the components and solder pads together. Below, JEENOCE will provide a detailed introduction to the principle and process of reflow soldering.

1、 Reflow soldering principle

Due to the continuous miniaturization of electronic product PCB boards, chip shaped components have emerged, and traditional welding methods are no longer suitable for the needs. In the assembly of hybrid integrated circuit boards, reflow soldering is used, and most of the components assembled and soldered are chip capacitors, chip inductors, SMT transistors, and diodes. With the increasingly perfect development of SMT technology and the emergence of various surface mount components (SMCs) and surface mount devices (SMDs), the reflow soldering process technology and equipment as part of the mounting technology have also been correspondingly developed. Its application is becoming increasingly widespread, and it has been applied in almost all electronic product fields.

Reflow, also known as reflow soldering, is a process of remelting pre allocated solder paste onto printed circuit board pads to achieve mechanical and electrical bonding between surface assembled component solder joints or pins and printed circuit board pads. Reflow soldering is the process of soldering components onto a PCB board, which involves surface mounted components. Reflow soldering relies on the action of hot air flow on the solder joint, and the adhesive flux undergoes physical reactions under a fixed high-temperature air flow to achieve SMD welding; So it is called "reflow soldering" because the gas circulates in the welding machine to generate high temperature and achieve the welding purpose.

2、 The principle of reflow soldering can be described in several ways:

A. When the PCB enters the heating zone, the solvents and gases in the solder paste evaporate. At the same time, the flux in the solder paste wets the solder pads, component terminals, and pins. The solder paste softens, collapses, and covers the solder pads, isolating them from oxygen.

B. When the PCB enters the insulation area, sufficient preheating is provided to the PCB and components to prevent them from suddenly entering the high-temperature welding area and damaging them.

C. When the PCB enters the soldering area, the temperature rapidly rises, causing the solder paste to reach a molten state. Liquid solder wets, diffuses, diffuses, or mixes with the solder pads, component ends, and pins of the PCB to form solder joints.

D. PCB enters the cooling zone to solidify the solder joints; The reflow soldering has been completed.

3、 Reflow soldering process requirements

Reflow soldering technology is not unfamiliar in the field of electronic manufacturing. The components on various boards used in our computers are soldered to circuit boards through this process. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and manufacturing costs are also easier to control. This device has an internal heating circuit that heats nitrogen to a sufficiently high temperature and blows it towards the circuit board where the components have already been attached, allowing the solder on both sides of the components to melt and bond with the motherboard.

1. It is necessary to set a reasonable reflow soldering temperature curve and regularly conduct real-time testing of the temperature curve.

2. Welding should be carried out according to the welding direction specified in the PCB design.

3. Strictly prevent vibration of the conveyor belt during the welding process.

4. It is necessary to inspect the welding effect of the block printed board.

5. Whether the welding is sufficient, whether the surface of the solder joint is smooth, whether the shape of the solder joint is half moon shaped, the situation of solder balls and residues, and the situation of continuous welding and virtual welding. Also, check for color changes on the surface of the PCB. Adjust the temperature curve based on the inspection results. During the entire production process, welding quality should be regularly checked.

4、 Factors affecting reflow soldering process:

1. Usually, PLCC, QFP and individual discrete sheet metal components have a larger heat capacity, making it more difficult to weld large-area components than small ones.

2. In the reflow soldering furnace, the conveyor belt not only performs reflow soldering on the conveyed products, but also becomes a heat dissipation system. In addition, the heat dissipation conditions at the edges and centers of the heating part are different, and the temperature at the edges is generally lower. In addition to different temperature requirements in each temperature zone, the temperature on the same load surface also varies in the furnace.

3. The impact of different product loading capacities. The adjustment of the temperature curve for reflow soldering should consider achieving good repeatability under no-load, load, and different load factors. The load factor is defined as: LF=L/(L+S); Where L=length of assembled substrate, S=spacing of assembled substrate. The greater the load factor, the more difficult it is to achieve good repeatability in reflow soldering process. The typical range of high load factor for reflow soldering furnaces is 0.5~0.9. This depends on the product situation (component welding density, different substrates) and the different models of the reflow furnace. To achieve good welding results and repeatability, practical experience is crucial.

5、 What are the advantages of reflow soldering technology

1) When using reflow soldering technology for welding, it is not necessary to immerse the printed circuit board in molten solder, but to complete the welding task through local heating; Therefore, the soldered components are less susceptible to thermal shock and will not be damaged due to overheating.

2) Due to the fact that welding technology only requires the application of solder at the welding site and local heating to complete the welding, welding defects such as bridging are avoided.

3) In reflow soldering technology, the solder is only used for secondary purposes and there is no possibility of reuse. Therefore, the solder is very clean and free of impurities, ensuring the quality of the solder joints.

6、 Introduction to reflow soldering process flow

The reflow soldering process involves surface mounted boards, which have a complex process and can be divided into two types: single-sided mounting and double-sided mounting.

A. Single sided mounting: pre coated solder paste → mounting (divided into manual mounting and machine automatic mounting) → reflow soldering → inspection and electrical testing.

B. Double sided mounting: A-side pre coated solder paste → SMT (divided into manual mounting and machine automatic mounting) → Reflow soldering → B-side pre coated solder paste → SMT (divided into manual mounting and machine automatic mounting) → Reflow soldering → Inspection and electrical testing.

The simple process of reflow soldering is "screen printing solder paste patch reflow soldering, the core of which is the accuracy of screen printing. The yield of the patch is determined by the PPM of the machine, and reflow soldering is to control the temperature rise, high temperature, and low temperature curves."

7、 Maintenance system for reflow soldering equipment

The maintenance work that we must do after using reflow soldering; Otherwise, it is difficult to maintain the service life of the equipment.

1. Daily inspection of various components should be carried out, with special attention paid to the conveyor belt to prevent it from getting stuck or falling off

When repairing the machine, the power should be turned off to prevent electric shock or short circuit

3. The machine must remain stable and must not tilt or exhibit any instability

When encountering situations where heating stops in individual temperature zones, it is necessary to first check whether the corresponding fuse tube has been remelted by re melting the paste pre allocated to the printed circuit board solder pads

8、 Precautions for reflow soldering machine operation

To ensure personal safety, operators must remove the factory label and decorations, and the sleeves must not be too loose.

During operation, attention should be paid to high temperatures to avoid burns and maintenance

3. The temperature range and speed of reflow soldering cannot be set arbitrarily

4. Ensure indoor ventilation, and the smoke exhaust pipe should lead to the outside of the window.