Understand the Im-Ag process in the pcb board

- Jan 07, 2019-

During the manufacturing process, the board turns yellow or black after being stored in the air for a period of time.

After a reflow soldering furnace, it will turn yellow in a short time and become a spot in six months.

The surface of the Im-Ag discolors in the air, mainly because of the presence of pores in the silver-containing surface of the components in the circuit, as a result of the reaction with the halides in the air. There is a lot of research in this color change industry, and it will not be explained here.

For the problem of discoloration of the Im-Ag single-sided pcb board, FASTPCBA conducted a detailed study. Through the DOE test statistical analysis, there are two main factors of Im-Ag discoloration after reflow soldering, namely coating thickness and exposure time. Tests have shown that increasing the thickness helps to improve the ability to resist discoloration; reducing exposure time helps to reduce the degree of discoloration.

electronic board

The thickness of the sinking silver layer is generally 100 ~ 500nm, and the range of EDX analysis is um level. It is impossible to accurately test the depth of the sinking silver layer. XPS (photoelectron spectroscopy) can be used to analyze the shallow indication of nm level. Silver is a displacement reaction on the copper surface, depositing a thin layer of silver directly on the copper surface. At normal temperature, the migration and diffusion speed between copper and silver atoms is very stable; but under high temperature conditions, such as reflow soldering, the diffusion between copper and silver atoms is accelerated. When there is no reflow soldering furnace, there is no copper peak, indicating that there is no copper in the surface layer; as the number of furnaces increases, the peak value of copper increases continuously, and the peak value of silver decreases continuously, indicating that copper elements gradually diffuse to the surface, shallow surface Silver is constantly being replaced by copper. As the number of welds increases, copper continues to spread to the surface. When the thickness of the silver layer is insufficient, once it is diffused to the surface, it is oxidized in high-temperature air, causing discoloration.

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