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3 SMT Welding Processes – Reflow Soldering, Wave Soldering and Through-hole Reflow Soldering

This article introduces three SMT (Surface Mount Technology) welding processes, including reflow soldering, wave soldering, and through-hole reflow soldering. Keep reading!

Table of Contents

Reflow soldering

Reflow Soldering
Reflow Soldering

1. Definition
By heating and melting the solder paste pre-coated on the pads, the pins of the electronic components pre-mounted on the pads are electrically interconnected with the pads to achieve the purpose of soldering the electronic components to the PCB board.

Specifically, reflow soldering is to solder components to PCB boards and surface mount devices. It relies on the effect of hot air flow on solder joints to cause the colloidal flux to physically react under a certain high-temperature air flow, thereby achieving the purpose of welding SMD (surface mount devices).

The entire soldering process of reflow soldering is as follows:

2. Reflow soldering structure
Reflow soldering is divided into a pre-heat zone, a soak zone, a reflow zone and a cooling zone.

Pre-heat zone: Its purpose is to raise the temperature of the printed circuit board from room temperature to the active temperature (above 135°C) required for the flux in the solder paste to function. The heating rate in the temperature zone should be controlled at 1~3°C per second, and the time is around 90s.

Soak zone: Its purpose is to maintain the printed circuit board in a specific temperature range for a period of time, make the components in each area of the printed circuit board have the same temperature, reduce their relative temperature difference, and allow the flux inside the solder paste to fully exert its effect, removing the oxides on the surface of the component electrodes and pads, thereby improving the welding quality. When entering the heat preservation area, the PCB and components are fully preheated to prevent the PCB from suddenly entering the welding high-temperature area and damaging the PCB and components.

Reflow zone: Its purpose is to raise the temperature of the printed circuit board above the melting point of the solder paste and maintain it for a certain welding time so that it can form an alloy and complete the welding of the component electrodes and the pads. In the reflow zone, the temperature rises rapidly and the solder paste reaches a molten state. The liquid solder wets, diffuses, overflows or reflows the PCB pads and component pins to form solder joints → the PCB enters the cooling zone to solidify the solder joints. At this point, reflow soldering is completed.

Cooling zone: Its purpose is to cool down the printed circuit board and solidify the solder joints. It is usually set to 3-4°C per second and the time is about 70 seconds. If the speed is too high, cracks will appear in the solder joints, and if the speed is too slow, the oxidation of the solder joints will be aggravated. The ideal cooling curve should be in a mirror image relationship with the reflow zone curve. The closer to this mirror image relationship, the tighter the solid structure of the solder joints, the higher the quality of the solder joints, and the better the bonding integrity.

What Is Reflow Soldering

Wave soldering

Wave Soldering
Wave Soldering

1. Definition
Wave soldering is used to solder dual in-line package (DIP) type devices. The soldering process of wave soldering is shown in the figure below:

Wave soldering uses an electric pump or electromagnetic pump to spray the solder wave required by the design, and the molten liquid solder forms a specific shape of solder wave on the surface of the solder tank; finally, the plug-in device on the PCB is immersed at a specific angle through the conveyor chain. The process of deep penetration of this solder wave to achieve solder joint welding.

2. The structure of wave soldering
The wave soldering machine is mainly composed of a spray system, preheating system, welding system, cooling system and transportation system.

Transportation system: After the circuit board is fixed on the jig, the jig is sent to the wave soldering machine through the conveyor chain, and the PCB is transported smoothly during the process. The transportation system consists of conveyor belts, motors, and speed regulators. The conveyor chain has an inclination angle, and its purpose is to facilitate tin removal. The smaller the angle, the larger the solder joints.

Spray system: Add flux to the welding area of DIP devices to be welded, commonly used foaming agents, sprays, etc. The spray system consists of a photoelectric sensor, rod cylinder, sprayer (nozzle), and PLC.

Preheating system: It consists of heating pipes and is designed to provide sufficient temperature to form good solder joints. Preheating time is usually <3min.

Welding system: The most commonly used one on the market is the double-wave system.

Cooling system: Use external cooling facilities to cool the PCB after soldering, reducing thermal stress on electronic devices and improving the reliability of solder joints.

3. Features of wave soldering

  • It saves labor and materials. Compared with manual welding, it improves production efficiency and reduces production costs.
  • The circuit board is exposed to high-temperature solder for a short time, which can reduce the warpage and deformation of the circuit board.
  • It eliminates the interference and influence of human factors on product quality and improves the quality and reliability of solder joints.
  • The wave soldering machine has sufficient solder, which is beneficial to improving the quality of solder joints.

Difference between reflow soldering and wave soldering

1. Different welding equipment
Reflow soldering and wave soldering use different machines to complete soldering.

2. Different welding techniques and processes
The reflow soldering process melts the solder through the circulating air flow in the equipment. Before entering the reflow oven, you need to apply solder paste through a steel mesh (it is required to apply it evenly to ensure uniform heating). After entering the oven, the solder paste on the substrate is melted to achieve contact between the device and the solder; while wave soldering heats the welding rod inside the machine to bring the device into contact with the solder (liquid tin) before soldering.

3. Different scope of application
Reflow soldering is an SMT mounting process and is suitable for SMD mounting devices; wave soldering is a DIP plug-in process and is suitable for plug-in devices.

4. Different welding order
Reflow soldering first and then wave soldering. Usually, the size of mounted components is smaller than that of plug-in components, and circuit board assembly is completed in order from small to large. After the double-sided mounting device is completed, a tooling jig is made to fix the single board, and the mounted device is covered, and then put into the oven for wave soldering.

Through-hole reflow soldering (PIHR)

Through-hole Reflow Soldering(PIHR)
Through-hole Reflow Soldering(PIHR)

1. Definition
The through-hole reflow soldering process is not commonly used. It refers to inserting the pins of electronic components into plug-in holes filled with solder paste and using the reflow soldering process, through-hole devices and surface mount components can be processed simultaneously. Then through the reflow soldering process, the through-hole parts and SMT surface mount devices are reflowed and soldered to the PCB board at the same time.

2. Main features
Compared with wave soldering, through-hole reflow soldering has the following advantages:

  • The PCB board surface is clean and the appearance is obviously better than that of wave soldering.
  • The process is simplified and labor intensity is reduced.
  • There are fewer welding defects such as false soldering and tin connection, and the workload of repairing the board is reduced.

Wave soldering is particularly suitable for scenarios where there are many surface mount components and very few plug-in components. However, to achieve through-hole soldering reflow soldering, the plug-in device must meet the following requirements:

  • Heat-resistant material.
  • Welding leg length.
  • Good wettability of solder legs.
  • Enough space is required for solder paste printing.
  • Suitable for automatic placement.
  • The vertical and horizontal gaps around the pins must be large enough to provide adequate space for printed solder paste.

Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or thousands of tiny electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state, creating permanent solder joints.

Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies and surface mount.

Through-hole reflow (THR) technology is a process in which through-hole components are soldered to a printed circuit board (PCB) by using reflow technology. It allows soldering the through-hole components and surface-mount components at the same time and with the same reflow profile.

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