What are the main factors affecting the weldability of PCBA?

Date：2022-05-26 16:30:04 Views：1117

Welding process is a very important process in PCBA processing process. Once the welding reliability is not strong, it will have a great impact on the final welding quality, which is easy to cause bad welding phenomena such as false welding, tin bead and bridging. There are many factors affecting the welding reliability in PCBA processing process, such as solder factor, Material oxidation factors and welding process control factors are the main reasons affecting the reliability of PCBA welding. Next, let's learn in detail why these factors affect the weldability of PCBA welding.

Weldability refers to the evaluation of welding difficulty in the process of PCBA welding. The stronger the weldability means that it is easier to weld and less prone to welding quality problems, while the worse the weldability means that it is more difficult to weld and more prone to welding quality problems; Solder refers to welding materials, which are usually divided into lead-free solder and lead solder. The solder used for reflow soldering is solder paste, the solder used for wave soldering is tin strip, and the solder used for manual soldering is mostly tin wire; The factors that lead to the decrease of solderability in the welding process due to solder include: solder composition, solder pollution and solder use. Take solder paste for example, if the composition proportion of solder paste is wrong, such as the proportion of tin lead is wrong, the proportion of tin and flux is wrong, or the solder paste is polluted by oil pollution and moisture oxidation, or the solder paste is fully warmed when used Stirring will affect the reliability of welding.

1. False soldering: there is an obvious black boundary between the solder and the lead of components or copper foil, and the solder is concave to the boundary.

Hazard: the circuit board cannot work normally.

Cause: 1) the component leads are not cleaned, tin plated or oxidized; 2) The printed board is not cleaned well, and the quality of the sprayed flux is not good.

2. Solder accumulation: the solder joint structure is loose, white and matte.

Hazard: insufficient mechanical strength, possible false welding.

Reasons: 1) poor solder quality; 2) Insufficient welding temperature; 3) Loose component leads.

3. Excessive solder: the solder surface is convex.

Hazard: solder is wasted and may contain defects.

Cause: solder was evacuated too late.

4. Too little solder: the welding area is less than 80% of the pad, and the solder does not form a smooth transition surface.

Hazard: insufficient mechanical strength of circuit board.

Causes: 1) poor solder fluidity or premature solder withdrawal; 2) Insufficient flux; 3) Welding time is too short.

5. Rosin welding: Rosin slag is sandwiched in the weld.

Hazard: insufficient strength, poor continuity, and possible on and off.

Cause: 1) there are too many welding machines or they have failed; 2) Insufficient welding time and heating; 3) The surface oxide film is not removed.

6. Overheating: the solder joint is white, without metallic luster, and the surface is rough.

Hazard: the pad is easy to peel off and the strength is reduced.

Cause: the power of soldering iron is too large and the heating time is too long.

7. Cold welding: the surface becomes bean curd slag particles, and sometimes there may be cracks.

Hazard: low strength and poor conductivity.

Cause: the solder shakes before solidification.

8. Poor wetting: the interface between solder and weldment is too large and not smooth.

Hazard: low strength, no connection or on-off.

Cause: 1) the weldment is not cleaned; 2) Insufficient flux or poor quality; 3) The weldment is not fully heated.

In addition to solder factors, PCB circuit board and electronic components are also one of the reasons affecting solderability, and the main reason is the oxidation problem caused by material moisture or pollution. If the PCB circuit board pad or electronic component pin is oxidized, tin rejection will occur during welding, which will affect the reliability of welding and cause defects such as false welding and false welding. In addition, improper welding process operation will also affect the reliability of welding, For example, the temperature during welding is too high or too low, and the welding time is too short or too long.