Precautions for Manual Welding of PCBA

PCBA is the abbreviation for Printed Circuit Board+Assembly, which means that the entire process of PCB blank board passing through SMT loading or DIP plug-in, abbreviated as PCBA. The standard writing in Europe and America is PCB ‘A’ with the addition of ”, which is known as the official idiom.

How to improve the welding method of PCBA board

1. Surface tension of PCBA board

The cohesive force of tin lead solder is even greater than that of water, making the solder appear as a sphere to minimize its surface area (under the same volume, the sphere has the smallest surface area compared to other geometric shapes to meet the minimum energy state requirements). The effect of soldering flux is similar to that of cleaning agents on metal plates coated with grease. In addition, surface tension is highly dependent on the cleanliness and temperature of the surface. Only when the adhesion energy is much greater than the surface energy (cohesion) can ideal tin deposition occur.

2. Production of PCBA plate metal alloy eutectic compounds

The intermetallic bonds between copper and tin form grains, and the shape and size of the grains depend on the duration and strength of the welding temperature. Less heat during welding can form fine crystalline structures, forming excellent welding points with optimal strength. The reaction time of SMT processing is too long, whether due to long welding time, high temperature, or both, which can lead to rough crystalline structure, which is sandy and brittle with low shear strength.

3. PCBA board solder corner

When the eutectic temperature of solder is about 35 ℃ higher than that of solder, a meniscus is formed when a drop of solder is placed on a hot surface coated with flux. To some extent, the ability of the metal surface to adhere to tin can be evaluated by the shape of the meniscus. If the solder meniscus has a clear undercut, shaped like water droplets on a greased metal plate, or even tends to be spherical, then the metal is not solderable. Only the meniscus is stretched into a shape less than 30. Only a small angle can have good weldability.

4. PCBA board tin coating effect

When hot liquid solder dissolves and penetrates into the surface of the metal being welded, it is called tinning of the metal or tinning of the metal. The molecules of the mixture of solder and copper form a new type of alloy with copper as the component and solder as the component. This solvent action is called tinning, which forms intermolecular bonds between the various components and generates a metal alloy eutectic. The formation of good intermolecular bonds is the core of the welding process, which determines the strength and quality of the welding points. Only the surface of copper is free from pollution, and there is no oxide film formed due to exposure to air in order to adhere to tin, and the soldering and working surfaces need to reach an appropriate temperature.

5. PCBA metal substrate

The PCBA board uses copper as the metal substrate and tin lead as the solder alloy. Lead and copper do not form any metal alloy co compounds, but tin can penetrate into copper. The intermolecular bonds between tin and copper form metal alloy co compounds Cu3Sn and Cu6Sn5 at the junction surface of the solder and metal.

Metal alloy layer (n-phase+ ε The phase must be very thin. In laser welding, the thickness of the metal alloy layer is on the order of 0.1mm. In PCBA plate wave soldering and manual soldering, the thickness of the metal to metal bonds at excellent welding points mostly exceeds 0.5 μ M. Due to the decrease in shear strength of welding points as the thickness of the metal alloy layer increases, it is often attempted to maintain the thickness of the metal alloy layer at 1 μ Below m, this can be achieved by minimizing the welding time as much as possible.

Precautions for manual welding of PCBA

1. It is necessary to operate with an electrostatic ring. The human body can generate static electricity of over 10000 volts, and the IC will be damaged when the voltage is above 300 volts. Therefore, human static electricity needs to be discharged through the ground wire.
2. Wear gloves or fingertips for operation, and do not directly contact the machine board and components with bare hands.
3. Weld at the correct welding temperature, welding angle, and welding sequence, while maintaining appropriate welding time.
4. Correctly remove the PCB: When removing the PCB, hold the edge of the PCB by hand and do not touch the components on the board.
5. Try to use low-temperature welding as much as possible: high-temperature welding will accelerate the oxidation of the soldering iron head and reduce its lifespan. If the temperature of the soldering iron head exceeds 470 ℃. Its oxidation rate is twice that of 380 ℃.
6. Do not apply excessive pressure during welding: Do not apply excessive pressure during welding, as it may damage or deform the soldering iron head. As long as the soldering iron tip can fully contact the solder joint, heat can be transferred. (Choose different soldering iron heads based on the size of the solder joint, which can also improve heat transfer of the soldering iron head).
7. Do not strike or swing the soldering iron nozzle when soldering: knocking or swinging the soldering iron nozzle can damage the heating core and splatter the solder beads, shortening the service life of the heating core. If the solder beads splash onto the PCBA, they may form a short circuit and cause poor electrical performance.
8. Use a wet water sponge to remove the oxide and excess tin residue from the soldering iron tip. The moisture content of the cleaning sponge should be appropriate. A high moisture content not only cannot completely remove the solder shavings on the soldering iron head, but also can cause poor soldering such as solder leakage and false soldering due to the sharp decrease in the temperature of the soldering iron head (this thermal shock causes significant damage to the heating elements inside the soldering iron head and soldering iron). The water on the soldering iron head sticking to the circuit board can also cause corrosion and short circuits on the circuit board. If the water content is too low or wet water treatment is not carried out, It will cause damage and oxidation to the soldering iron head, resulting in no soldering, and it is also easy to cause poor soldering and other defects. It is necessary to regularly check the appropriate moisture content in the sponge, and clean the tin slag and other debris in the sponge at least 3 times a day.
9. The amount of tin and soldering flux should be appropriate during welding. Excessive soldering can easily cause soldering or mask welding defects. Too little soldering not only has low mechanical strength, but also can lead to solder joint failure due to the gradual deepening of the surface oxide layer over time. Excessive flux can contaminate and corrode PCBA, which may lead to electrical defects such as leakage, and too little flux will not be effective.
10. Regularly maintain tin on the soldering iron head: This can reduce the oxidation opportunity of the soldering iron head and make it more durable.
11. The incidence of flux splashing and solder balls is related to the proficiency of soldering operations and the temperature of the soldering iron head; The problem of flux splashing during welding: When using a soldering iron to directly melt the soldering wire, the flux will rapidly heat up and splash. During welding, the method of not directly contacting the soldering iron with the soldering wire can reduce the flux splashing.
12. When welding, be careful not to burn the plastic insulation layer of the surrounding wires and the surface of the components with the electric soldering iron, especially for products with compact welding structures and complex shapes.
13. After welding, self inspection is required:
    • Is there any solder leakage.
    • Is the solder joint smooth, full, and shiny.
    • Is there any residual flux around the solder joint.
    • Whether there is tin connection.
    • Is there any detachment of the solder pad.
    • Check for cracks in the solder joints.
    • Is there any pinching phenomenon at the solder joint.
14. When welding, it is also necessary to pay attention to some safety precautions, wear a mask, and equip with ventilation equipment such as an exhaust fan to maintain ventilation at the welding station.

When carrying out PCBA manual welding, pay attention to some basic precautions, which can greatly improve the welding technology and product welding quality.