With most countries banning the use of hazardous substances such as lead, the electronics industry is now abandoning the use of tinblast sheets for PCB surface treatment. The current trend is towards other finishes such as OSP, immersion gold, immersion tin, and immersion silver. Among them, immersion silver surface treatment technology has excellent performance and reasonable cost, so it has become a popular choice.
What is immersion silver pcb?
Immersion Silver PCB is a type of PCB circuit board that the manufacturer provides an exposed copper pad with a surface finish of Immersion Silver. The thickness of the immersion silver surface treatment is 04 to 1 micron, widely used by chip manufacturers for baseline bonding, EMI shielding, and metal dome contacts. Under proper storage conditions, immersion silver PCBs should be stored for a minimum of 12 months. However, once the user removes the board from the storage condition, they must solder it within 24 hours.
Immersion silver is an environmentally friendly surface treatment process in which the manufacturer platens the copper surface with 0Layers 4 to 1 microns thick. This ensures that the copper surface does not tarnish, providing reliable soldering for the electronic components on the circuit board.
The displacement reaction is the basis of the silver immersion technology. In solution, silver ions replace the metallic copper on the surface of the circuit board pads. First, the manufacturer uses a micro-etching solution to micro-roughen the copper surface. They then form a uniform layer of immersion silver at a slow, controlled deposition rate.
The slow rate of leaching silver deposition helps to establish a dense crystal structure, resulting in a high-density layer of silver on the surface of the copper. The slow rate of deposition also helps to avoid particle growth due to agglomeration and precipitation. This technology uses a very stable immersion silver solution that has a long service life and is not sensitive to trace halides or light.
Advantages of immersion silver
Immersion Silver is cheaper to manufacture PCBs with a surface finish than PCBs that use Immersion Gold. If the PCB requires functional connectivity, and the budget is limited, then immersion silver is a worthwhile option. Users choose immersion silver because of its flat surface and low contact resistance. Immersion silver is also suitable for soldering fine-pitch components such as BGAs and other smaller components.
The presence of silver on the solder surface allows for better diffusion of the solder. This makes the physical strength of the solder bond stronger. As a RoHS-compliant PCB surface treatment, immersion silver is an environmentally friendly substance. The surface treatment of the material immersed in silver is also able to withstand multiple reflow soldering. In addition, immersion silver is not affected by the fracture of the black pad interface.
Many industries use immersion silver in their products. These include computer peripherals, automobiles, and communication systems. The excellent electrical conductivity of immersion silver makes it suitable for use in devices with high-speed signal designs.
Disadvantages of immersion silver
The trend of using immersion silver as a surface treatment has not yet caught on in large numbers. The loss of silver surface luster and the formation of solder joint voids are the two main reasons why the process has not become more popular. When exposed to the environment and unprotected, the sulfur in the environment reacts with the leached silver, forming silver sulfide on the surface of the pads.
Comparison with other surface treatments
The characteristics of the immersion silver surface treatment make it intermediate between OSP and immersion gold. Even when exposed to moisture, heat, and pollution, immersion silver continues to provide good solderability and electrical connection, although it can tarnish. Immersion gold has a layer of nickel underneath it, which gives it physical strength. Since immersion silver does not have a nickel layer underneath, its physical strength is lower than that of immersion gold.
Immersion silver requires more storage and handling as compared to immersion tin. However, immersion silver is safer for the environment than immersion tin.
Immersion silver is easier to use compared to organic solder preservatives or OSP.
Immersion silver provides a flatter surface than sprayed tin.
Handling and storage of immersion silver PCBs
It is recommended to wear gloves when handling immersion silver PCBs. The oils and acids on the human hands react with the surface of the silver, causing it to tarnish it.
The shelf life of immersion silver PCBs under dry storage conditions is 6 to 12 months, which is the same as that of tinspray plates. However, once the immersion silver PCB is removed from storage, it must be soldered within the next 24 hours.
If the immersion silver PCB is stored for more than 12 months, it is necessary to perform a solderability test before proceeding with assembly. Placing the solder paste and reflowing the bare board is sufficient for testing.
Solder paste for silver-impregnated PCBs
Solder paste manufacturers often offer solder paste formulations that are specifically designed for immersion in silver. The oxide and acid content in the solder paste can affect how easy it is to assemble. It is recommended to use a low-activity and no-clean solder paste when soldering PCBs with immersion silver. It is preferable to use regular-shaped particles in solder paste, as the degree of sphericity of the solder powder affects the oxide level. Particles with irregular shapes exhibit higher oxide levels.
Conclusion
As a surface treatment, the advantages of immersion silver far outweigh its costs and drawbacks. With the introduction of the RoHS and WEEE directives, immersion silver technology has gained widespread popularity.
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