It is a standard that printed circuit board manufacturers cover the circuit board’s surface with copper finishes. These coatings form a layer between the empty PCB and the electrical components mounted on it. The surface finishes perform dual functions as follow. First, they protect the copper coatings from getting exposed to external factors such as humidity. In the same vein, they ensure excellent assembling of the components to the board.
A cautionary note here, though. The immersion silver printed circuit boards will become useless when left without protection as the copper coatings will oxidize and even deteriorate.
Therefore, these circuit boards pass through various kinds of surface finish treatments including HASL (Hot Air Solder Leveling) and ENIG (Electroless Nickel Immersion Gold). Still, the ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold and Gold – Hard Gold are not left out. The OSP (Organic Solderability Preservative), Immersion Tin, and Immersion Silver are the remaining types of finishes available.
The electronics industry is now dropping the option of using HASL as a surface finish for PCBs. It is so because the HASL finish contains hazardous substances such as lead. The trend is moving towards other forms of surface finishes. Among the many kinds mentioned above, the surface finish technology/treatment of immersion silver is a popular choice. It provides spectacular performance at a reduced cost.
Follow me along as we discuss the treatment of circuit boards with immersion silver finish in detail.
Fundamentals of Immersion Silver PCBs
The immersion silver treatment is a chemical reaction process whereby circuit board engineers deposit a thin layer of silver on the copper surfaces of PCBs. This treatment helps to guard the exposed copper pads against oxidation and deterioration. Moreover, the awareness of this process depended on the potential difference between silver and copper. Whereas, the aim is to allow the two elements to pass through a displacement reaction.
It is amazing to know that the process is easy, straightforward, and wastes no time. Most times, producers add some quantity of organic matter to the immersion silver treatment process which helps to prevent electromigration. On a general note, it is hard to quantify the amount of this organic matter they add. However, research and analysis have shown that the weight is small, usually less than 1%.
The PCB manufacturers and fabricators commonly use immersion silver surface finish for EMI shielding and aluminum wire bonding. Apart from that, immersion silver has an average surface layer in the range of 5 to 18 microinches. Furthermore, it can last (stay) for at least one year if properly handled.
Displacement reaction between silver and copper is the basis for immersion silver technology. Thus, board specialists allow an even deposition of silver ions on copper metal slowly at a controlled speed. The slow speed of deposition helps to get rid of particle growth from precipitation.
Reasons Why Immersion Silver PCB is becoming the Number 1 Choice
Immersion silver PCB treatment is gaining popularity in the electronics industry every passing day owing to its many advantages. Here are some of the excellent features that make it an ideal surface finish.
- It provides the best conditions for the solder to spread evenly on the board’s surface. As such, creating a room for excellent assembly of electrical components and solder bonds to be strong.
- More so, immersion silver is also ideal for assembling fine components such as BGA, fine pitch, and other smaller components.
- The immersion silver PCBs are environmentally friendly. Moreover, the technology is RoHS compliant and contains no hazardous substance.
- Immersion silver treatment is a technology that has come to stay because the material finish can withstand multiple soldering reflows.
- As an innovative technology, it has excellent electrical properties and provides a flat surface as well as low contact resistance.
- In the same manner, immersion silver surface has better conductivity. Therefore, circuit board experts use it in a piece of equipment where high-frequency signal design is needed.
- The major reason why immersion silver treatment is an ideal choice is that it is cost-effective eliminating every form of budget constraint.
Drawbacks to Using Immersion Silver PCBs
Though immersion silver has these worthwhile advantages, it is more susceptible to handling, temperature, and other physical factors. It can react with sulfur when exposed to the environment forming silver sulfide on the copper pad’s surface. Consequently, it is this silver sulfide that tarnishes the surface.
It requires special packaging. When an engineer opens the package and don’t use up all the circuit boards at once, they should guard the remaining. They should reseal them immediately or allow them pass through soldering reflow within that same day. Besides, the trend of adopting immersion silver is still slow, but gradually becoming more popular.
Immersion Silver PCB and Other Surface Finishes
In terms of functional performance, immersion silver is a changing process between OSP, immeresion tin and ENIG. It compares to other surface finishes in the following ways;
- The solder joint strength is stronger than that of silver/nickel PCB finishes. However, during its application as a contact surface, the immersion silver finish is inferior to gold.
- Immersion silver PCB finishes can resist environmental aging very strongly more than the OSP counterpart.
- The best circuit board features such as flatness, electrical properties, etc are all superior in immersion silver than in HASL finish.
- In the same vein, immersion silver treatment is less durable than ENIG because it does not have a nickel layer underneath. More so, its shelf life is equal to that of HASL.
- Immersion silver is environmentally safer than immersion tin. Furthermore, it consists of a more level surface than hot air surface leveling (HASL).
Storage and Handling of Immersion Silver PCBs
Printed circuit board engineers recommend using gloves while handling immersion silver PCBs to avoid contamination. The fluids from the human head if eventually drop on the boards can react with the silver surface and tarnish it.
Moreover, it takes between 6 and 12 months to store both immersion silver PCBs and HASL boards under dry storage situations. Therefore, the handlers of immersion silver circuit boards must solder them within 24 hours of removing them from storage.
The brightly-colored appearance of immersion silver circuit boards show signs of contamination when they display yellow or black discoloration. If you suspect contamination, consult a PCB fabricator or expert for advice.
Any PCB that has been in storage for some time should undergo a solderability test before assembly. The test usually requires a solder paste and reflow of an empty PCB, especially for SMD applications. If there’s a complete solder wetting, then the test would show a satisfactory pass.
There’s a solder paste specifically meant for immersion silver surface finishes. The oxide and acid levels affect the simplicity of assembly. Therefore, it is better to use solder paste with very small concentrations when assembling an immersion silver surface finish.
Circuit board manufacturers recommend that you use only water to clean or wash immersion silver finishes. More so, storing the immersion silver surface boards in a sealed bag or container is better to prevent direct contact with air circulation.
Final Thought …
The fact that immersion silver surface has better flatness and joint strength, makes it a perfect choice in most cases. Apart from that, there are many applications of immersion silver in PCB products. They are useful in automobile and computer peripherals, communication, and high-frequency signal design. Immersion silver PCBs have excellent conductivity which other surface finishes don’t have. Therefore, they’re ideal for use in equipment requiring high-speed signals.
Circuit board experts recommend immersion silver technology because of its high-grade inspectability and simplicity in soldering. But, the disadvantages have led to slow adoption of its application. Still, the percentage is not declining as about 10 percent of PCBs use immersion silver technology.
The advantages of immersion silver surface finish are more than its drawbacks. With the RoHS compliance, it has gained a widespread advantage. The fact remains that immersion silver PCB requires more care in handling and storage than other types of surface finish. It is coming up as a standard for a good, stable, non-hazardous finish with moderate shelf life.
