What are the reasons for cold welding in PCBA processing

The reasons for cold welding

Abstract: During the reflow soldering process of PCBA processing, the welding defects caused can be mainly divided into two categories. The first category is related to metallurgy, including cold welding, non-wetting, silver migration, etc.; the second category is related to abnormal solder joint morphology. , including tombstoning, offset, wicking, bridging, voids, solder balls, insufficient solder paste volume and weak or open circuits, grainy surfaces, etc. Today SZTech-SMT will explain to you the reasons and solutions for cold welding in PCBA processing. Cold soldering refers to the phenomenon that the solder paste on the soldering end of the PCBA is not fully melted and reflowed. If granular solder joints appear, the surface of the solder joints is not smooth and irregular, or the metal powder is completely melted, it is referred to as cold welding in SMT processing. The cause analysis and solutions of cold welding are as follows:

Reasons

1. The reflow temperature is too low or the residence time at the reflow soldering temperature is too short, resulting in insufficient heat during reflow and incomplete melting of the metal powder.

2. In the cooling stage, the strong cooling air, or the movement of the uneven conveyor belt disturbs the solder joints, and presents uneven shapes on the surface of the solder joints, especially at a temperature slightly lower than the melting point, when the solder is very soft.

3. Surface contamination on and around pads or leads can inhibit flux capability, resulting in incomplete reflow. Sometimes unmelted solder powder can be observed on the surface of the solder joint. At the same time, insufficient flux capacity will also result in incomplete removal of metal oxides and subsequent incomplete condensation.

4. The quality of solder metal powder is not good; most of them are formed by the encapsulation of highly oxidized powder particles.

Solutions for cold welding of SMT processing

1. According to the PCBA board’s size and the board’s thickness, combined with the heat coefficient of the components, set the appropriate reflow temperature and soldering time.

2. Pay special attention to the stability of the transmission of reflow soldering equipment.

3. Use highly active solder paste or appropriately increase the amount of solder paste flux; the incoming inspection system should be strictly controlled, and the storage environment of components and PCB should be paid attention to.

4. Do not use inferior solder paste, formulate a management system for the use of solder paste to ensure the quality of solder paste.

The above content is shared by SZTech-SMT for you about the causes and solutions of cold welding in SMT processing. I hope it will be helpful to you. To learn more about SMT chip processing, welcome to visit www.sztech-smt.com

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