Advantages of Immersion Silver PCBs

All printed circuit boards manufactured by Rush PCB have a surface finish to protect the solderable copper pads from oxidizing and deteriorating, making the PCB unusable. The surface finish is a critical interface between the pad and the component. Essentially, the surface finish protects the exposed copper pads and provides a solderable surface during the assembly process.

With most countries banning the use of hazardous substances like lead, the electronics industry is now moving away from the surface finish of PCBs using HASL. The trend is towards other surface finishes such as OSP, immersion gold, immersion tin, and immersion silver. Among them, the surface treatment technology of immersion silver offers excellent performance at a reasonable cost, and is therefore a popular choice.

What is Immersion Silver PCB?

Immersion silver PCB is a printed circuit board where the fabricator has provided exposed copper pads with a surface finish of immersion silver. Popularly used by chip-makers for aluminum wire bonding, EMI shielding, and metallic dome contacts, immersion silver surface finish has a thickness of 5 to 18 micro inches. Under proper storage conditions, it is possible to store immersion silver PCB for a minimum period of 12 months. However, once users remove the boards from storage conditions, they must solder them within 24 hours.

Immersion silver being an environmentally friendly surface treatment process, fabricators plate a 5 to 18 micro inches thick layer on the copper surface. This ensures the copper surface does not tarnish, rather providing reliable soldering of electronic components on the circuit board.

A displacement reaction is the basis of the Immersion silver technology process. In the solution, silver ions displace metal copper from the pad surface of the circuit board. Initially, the fabricator micro-roughens the copper surface with a micro etching solution. Then they allow a uniform layer of immersion silver to build up with a slow controlled speed of deposition.

The slow speed of immersion silver deposition helps to build a dense crystalline structure, thereby forming a high-density layer of sliver on the copper surface. The slow speed of deposition also helps avoid particle growth from agglomeration and precipitation. The technique 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

Rush PCB fabricates PCBs with surface finish using Immersion silver is cheaper than one with immersion gold is. If the requirements of a PCB demand functional connections, and there is a budget constraint, immersion silver is a worthwhile choice. Users choose immersion silver as it offers a flat surface and a low contact resistance. Immersion silver is also good for soldering fine pitch components like BGA and other smaller components.

The presence of silver on the soldering surface allows solder to spread better. This allows the solder bond to be physically stronger. As an RoHS compliant PCB surface finish, immersion silver is an environmentally friendly substance. The material surface finish with immersion silver also has the capability to survive multiple soldering reflows. In addition, immersion silver is immune to black-pad interfacial fractures.

Many industries use immersion silver in their products. These include computer peripherals, automobiles, and communication systems. The excellent conductivity of immersion silver makes it suitable of use in equipment using high-speed signal design.

Disadvantages of Immersion Silver

The trend of using immersion silver as a surface finish has not taken off in high numbers. Tarnishing of the silver surface and formation of solder joint voids are the two main reasons for the process not becoming more popular. When exposed and left unprotected in the environment, sulfur from the environment reacts with the immersion silver, generating silver sulfide on the pad surface.

Comparison with other Surface Finishes

• The features of immersion silver as surface finish place it in between OSP and immersion gold. Even when exposed to humidity, heat, and pollution, immersion silver continues to provide good solderability and electrical connection, although it will tarnish. Immersion gold has a layer of nickel under it, which provides it with physical strength. As there is no nickel layer under immersion silver, its physical strength is lower than that of immersion gold.
• Immersion silver requires more care for storage and handling than immersion tin does. However, immersion silver is safer for the environment than immersion tin is.
• Immersion silver is easier to use compared to Organic Solder Preservative or OSP is.
• Immersion silver offers a more planar surface than HASL does.

Handling and Storage of Immersion Silver Finish PCBs

Rush PCB recommends wearing gloves when handling immersion silver PCBs. Oil and acids from human hands can react with the silver surface, tarnishing it.

Shelf life of immersion silver PCBs under dry storage conditions is 6 to 12 months, the same as for HASL boards. However, after removing immersion silver PCBs from storage, they must undergo soldering within the next 24 hours.

If immersion silver PCBs have been in storage for more than 12 months, it is necessary to perform a solderability test before moving them to assembly. Placing solder paste and reflowing a bare board is enough for the test.

Solder Paste for use with Immersion Silver PCBs

Paste manufacturers usually provide solder paste formulations specifically for use with immersion silver. Oxide levels and acid levels in a solder paste affects the ease of assembly. Rush PCB recommends using low-activity and no-clean solder paste when soldering PCBs with immersion silver. Using regularly shaped particles in the paste is preferable, as the degree of sphericity of the solder powder affects oxide level. Particles with irregular shapes exhibit higher oxide levels.

Conclusion

As a surface finish, the benefits of immersion silver far outweigh its costs and disadvantages. With the onset of RoHS and WEEE directives, immersion silver has gained widespread popularity. With its immersion silver surface finish, Rush PCB provides a good alternative to the more expensive ENIG surface finish for fine-pitch and flat-pack components. Additionally, the finish is stable and has a moderate shelf-life.