How does "welding knowledge" solve the problem of cold welding in PCBA processing?

Date：2022-05-17 15:27:57Views：1012

For chip components, the appearance of good solder joints shall be smooth, bright and continuous, the edges shall gradually become thin and straight, and the bottom layer of the front end shall not be exposed or have sharp protrusions. There is no deviation in part position, part crack, notch and damage, and port electrodeposition. In order to help you have an in-depth understanding, the following contents are sorted out by Chuangxin testing network for your reference.

In the process of PCBA reflow welding, the welding defects can be divided into two categories. The first category is related to metallurgy, including cold welding, non wetting, silver migration, etc; The second type is related to abnormal solder joint shape, including monument, offset, wicking, bridging, cavity, solder ball, insufficient amount of solder paste, false welding or open circuit, granular surface, etc.

First of all, cold welding refers to the phenomenon that the solder paste on the PCBA welding end has incomplete and sufficient melting and reflow. If there are granular solder joints, the surface of solder joints is not smooth and irregular, or the metal powder is completely melted, it is referred to as cold welding in SMT processing. How to solve the problem of cold welding in PCBA processing?

1、 Causes of cold welding in SMT processing

1. Too low reflow temperature or too short residence time at reflow welding temperature leads to insufficient heat during reflow and incomplete melting of metal powder.

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

3. Surface contamination on and around pads or pins can inhibit the ability of flux, resulting in incomplete reflow. Non molten solder powder can sometimes be observed on the surface of the solder joint. At the same time, insufficient flux capacity will also lead to incomplete removal of metal oxides and subsequent incomplete condensation.

4. The quality of solder metal powder is poor, and most of it is encapsulated by highly oxidized powder particles.

2、 Solution of cold welding in SMT processing

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

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

3. Use solder paste with high activity or appropriately increase the amount of solder paste flux; The incoming inspection system should be strictly controlled, and attention should be paid to the storage environment of components and PCBs.

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

I believe that by reading the above content, you have a preliminary understanding of the problem of cold welding in PCBA processing. At the same time, I hope you can make a summary in the learning process, so as to continuously improve your professional level.