In the traditional electronic assembly process, wave soldering technology is generally used for the soldering of the printed board assembly with the through-hole plug-in component (PTH).

Wave soldering has many shortcomings:

1. High-density, fine-pitch SMD components cannot be distributed on the welding surface;

2. There are more bridging and missing welding;

3. Flux spraying is required; the printed board is warped and deformed due to greater thermal shock.

As the current assembly density of circuits is getting higher and higher, the soldering surface will inevitably be distributed with high-density, fine-pitch SMD components. The traditional wave soldering process has been unable to do this. Generally, the soldering surface SMD components can only be reflow soldered separately. , And then manually repair the remaining plug-in solder joints, but there is a problem of poor solder joint quality consistency.

Five new types of mixed welding technology emerge

1. Selective welding

In selective soldering, only some specific areas are in contact with the solder wave. Since the PCB itself is a poor heat conduction medium, it will not heat and melt the solder joints of adjacent components and the PCB area during soldering. The flux must also be pre-coated before soldering. The flux is only applied to the parts to be soldered on the lower part of the PCB, but selective soldering is not suitable for soldering SMD components.

2. Dip soldering process

Using the immersion selective soldering process, solder joints of 0.7mm to 10mm can be soldered. The soldering process of short leads and small-sized pads is more stable, and the possibility of bridging is small. The distance between the edges of adjacent solder joints, devices and soldering tips should be More than 5mm. Select the dip soldering process, you can use the following parameter settings:

2.1. Soldering tin temperature 275℃～300℃

2.2. Immersion speed 20mm/s～25mm/s

2.3. Immersion time 1s～3s

2.4. Speed after immersion 2mm/s

2.5. The shock pump speed is determined by the number of welding tips.

3. Through hole reflow soldering

Through-hole Reflow (THR), is the use of reflow soldering technology to assemble through-hole components and special-shaped components. As products pay more and more attention to miniaturization, increased functionality and increased component density, many single-sided and double-sided panels are mainly surface-mounted components. However, due to factors such as inherent strength, reliability, and applicability, in some cases through-hole devices are still superior to SMC, especially connectors at the edge of the PCB.

The disadvantage of using through-hole devices on circuit boards with surface-mounted components is that the cost of a single solder joint is very high. For this type of assembly, the key is to be able to provide through-hole and surface-mounted components in a single integrated process. Provide synchronous reflow soldering.

Compared with the general surface mount process, the amount of solder paste used in the through-hole reflow process is more than that of the general SMT, which is about 30 times. Currently, the through-hole reflow process mainly uses two solder paste coating technologies, including solder paste printing and automatic solder paste dispensing.

3.1. Solder paste printing

Screen printing is the preferred method of depositing solder paste on the PCB. The thickness of the stencil is a key factor, which will affect the amount of solder paste that is missed onto the PCB. A stepped mesh board can be used, where the thicker areas are designed for through-hole devices. This stencil design can meet the requirements of different solder paste volumes.

3.2. Automatic solder paste application

Auto-dot solder paste successfully deposits the correct volume of solder paste for through-holes and irregular-shaped components. It provides the flexibility and ability to deposit large amounts of solder paste that may not be possible with screen printing. When applying solder paste to the exposed plated through hole (PTH), it is recommended to use a nozzle with a slightly larger diameter than the PTH. In this way, when the solder paste is applied, the solder paste is forced to close to the hole wall of the PTH, and the material is slightly extruded from the bottom of the PTH, and then the component is inserted from the opposite direction of the solder paste. If you use a nozzle smaller than the PTH diameter, the solder paste will be discharged from the hole and cause serious solder paste loss.

Through-hole reflow soldering can replace wave soldering in many aspects to realize the soldering of plug-in components, especially the soldering of plug-in solder joints with high-density SMD components (or fine-pitch SMD) distributed on the soldering surface. Improve the welding quality, which is enough to make up for the lack of expensive equipment. The emergence of through-hole reflow soldering is of great help to enriching soldering methods, improving circuit board assembly density, improving soldering quality, and reducing process flow.

4. Use shielding die wave soldering technology

Because traditional wave soldering technology cannot cope with the soldering of fine-pitch, high-density SMD components on the soldering surface, a new method came into being: the use of a shielding die to shield the SMD components to achieve wave soldering of the soldering surface plug-in leads.

4.1. Advantages of using shielded die wave soldering technology

1) Realize double-sided mixed PCB wave soldering production, which can greatly improve the production efficiency of double-sided mixed PCB and avoid the problem of poor quality consistency in manual soldering.

2) Reduce the preparation time for pasting solder mask, improve production efficiency and reduce production costs.

3) The output is equivalent to traditional wave soldering.

4.2. Shielding mold material

1) The mold must be anti-static. Common materials are: aluminum alloy, synthetic stone, fiberboard. In order to avoid the wave soldering sensor not sensing when using synthetic stone, it is recommended not to use black synthetic stone.

2) Make the thickness of the mold base material. According to the thickness of the components on the back side of the machine disc, a base material with a thickness of 5mm to 8mm is selected to make a mold.

4.3. Mold process size requirements:

1) Dimensions of the mold: The length and width of the mold are respectively equal to the length and width of the PCB plus the width of the 60mm carrier side and the mold width must be ≦350mm. When the PCB width is less than 140mm, you can consider placing two PCBs in a mold for welding.

2) The process side is 8mm away from the edge, and the other two sides are close to the edge with 10mm wide and 10mm high bakelite strips to increase the strength of the mold and reduce the deformation of the mold.

3) Each reinforcement bar must be fixed with screws, and the distance between the screw and the screw is less than 150mm.

4) After the mold is made, the press buckles (fix the PCB on the mold) must be installed around the circumference and within 100mm, and the following points must be noted:

① Do not touch the parts after one revolution;

②Does not affect DIP plug-in;

③It can stabilize the PCB in the mold.

5) The four corners of the mold should be chamfered with R5.

6) When the PCBA on the mold passes through the tin furnace, some parts will rise due to the impact of the tin wave. Therefore, the pressing method is used to solve some parts that are easy to float.

The main methods currently used:

①Pressed pieces of metal iron block;

②Install pressing parts on the mold;

③Making fixtures for anti-floating high-pressure parts.

5. Automatic soldering machine process technology

Because the traditional wave soldering technology cannot cope with the welding of double-sided SMD, high-density SMD components and components that are not resistant to high temperatures, a new method has emerged: the use of automatic soldering machines to solder the soldering surface plug-ins.

Advantages of using automatic soldering machine technology:

5.1. Small investment and quick results.

5.2. Realizing automated welding production can improve the consistency of solder joints, save manpower, improve production efficiency, reduce manufacturing costs, and avoid the problem of poor solder joint consistency in manual welding.

Which welding technology to choose depends on the characteristics of the product:

1) If the batch of products is small and the variety is large, then selective wave soldering technology can be considered, no special mold is required, but the equipment investment is relatively large.

2) If the product type is single, the batch is large, and you want to be compatible with the traditional wave soldering process, you can consider using the shielded die wave soldering technology, but you need to invest in the production of special molds.

These two welding techniques are relatively easy to control, so they are being widely used in the current electronic assembly production.

3) Due to the difficulty of process control, through-hole reflow soldering has fewer applications than the former, but it is very helpful to improve soldering quality, enrich soldering methods, and reduce process flow. It is also a very promising soldering method.

4) The process technology of automatic soldering machine is easy to master. It is a new type of welding technology that has developed rapidly in recent years. It has the characteristics of flexible application, low investment, and low maintenance and use cost. It is also a very promising welding technology.