What is Immersion Silver PCB?

Printed circuit boards (PCBs) need an additional polish to improve their qualities. Although there are other options, Immersion silver PCB is the most common. Even professionals could have trouble carrying out the procedure or contrasting it with other accessible solutions.

Let's examine the immersion silver method in more detail, identify its benefits, go through typical issues, and contrast it with ENIG.

PCB in immersion silver

It is one of the common finishes that may be applied to a typical PCB and is also referred to as IM silver, silver plating, and ENIG. Approximately one-tenth of all PCBs are created using this method. And that's a lot given that the majority of inexpensive components often lack any kind of surface polish.

The surface integrity of a PCB is essential. It is thus because it receives the majority of external impact. PCB is shielded by an immersion silver coating, which reduces its susceptibility to oxidation, temperature changes, mechanical strains, etc. The procedure primarily aims to increase the durability of a PCB.

The Process and Principle of Immersion Silver

Let's divide the procedure into two sections, the application process and the functioning concept, and examine each independently.

Basics of Silver Plating

Therefore, it is the chemical process that deposits a tiny coating of silver on the copper foil of the PCB. The equation is as follows: Cu2+ + 2Ag+ = Ag+ Cu2+;

Simply put, two electrons are released into the silver solution as a result of the partial dissolution of copper from the surface. The previously dissolved silver then takes electrons to return to its metallic form and settle on the copper surface of a PCB.

In case you're interested in how much silver replaces copper. The coating is only between 0.1 and 0.4 m thick. Still, it is more than enough to completely exhibit silver's qualities.

Silver Plating: The Process of Application

Now that we have that out of the way, let's go through the whole step-by-step PCB immersion silver procedure:

1. Pre-treatment, which includes pickling, cleaning, washing, and hot air drying at 80°C.
2. Placing PCBs in a bath with acid resistance.
3. Acid cleaning at 30°C using a 10% solution of acid cleaner.
4. Eliminate oxide.
5. Pollutants and clean fingerprints are still present.
6. Water cleaning
7. Washing with pure water.
8. The liquid is clear.
9. Micro-etching at 30°C for 1-2 minutes using 250 ml/L of WCD-126, 200 ml/L of 35% hydrogen peroxide, and 80 grams of sulfuric acid.
10. Washing with pure water.
11. Dry and clean PCBs.
12. Perform a preliminary dip at 40°C for 1-2 minutes using 300ml/L of MT-IMG 500A and 30ml/L of MT-IMG 500B.
13. Conduct a sterling immersion silver procedure using 300ml/L of MT-IMG 500A, 70ml/L of MT-IMG 500B, and 10ml/L of MT-IMG 500S for 2-4 minutes at 50°C.
14. Washing with deionized water.
15. Use hot, 80°C air to dry.

Your PCBs are now ready for inspection and packing whenever you're done. But the silver coating can be an issue for you. For instance, it is applied too thinly or not enough. Here are some things to address and keep in mind for the future.

• Inadequate temperature.
• An incorrect nitrate concentration
• The length of emergence.
• agitating the liquid.
• The presence of pollutants.
• Measuring technique.
• Pollution from sulfates.
• Subpar cleaning
• Out-of-date chemicals.

Immersion of silver PCB benefits

Some of the benefits of an Immersion silver PCB surface treatment were previously mentioned in the section before this one. But in this part, we'll go through each benefit in more depth.

• Economical - One advantage of employing immersion silver PCB surface treatment is that it is less expensive than some other solutions. If you're on a tight budget, this is about the greatest bargain you can get. Even though it is less costly than other choices, this surface treatment ensures an excellent functional relationship. The low contact resistance and smooth surface that immersion silver offers are the main reasons why most users favor it.
• Solder spreads more readily — Silver spreads poorly on the majority of surface finishes. But with immersion silver, the problem is solved since the silver makes it possible for the solder to spread easily. The key benefit of solder that spreads readily is that a stronger physical connection is produced.
• Environmentally friendly - When it comes to PCB coatings, immersion silver has this trait down pat. It is regarded as wholly secure and unharmful to the environment.
• Used repeatedly - In relation to several soldering reflows, an immersion silver PCB surface finish may also be used repeatedly. Multiple soldering reflows may be performed without affecting the aesthetics.
• Resistance to black pad interfacial fractures - It is important to note that immersion silver is immune to black pad interfacial fractures.
• Utilized throughout a wide range of sectors — Immersion silver has a variety of uses, including in communication systems, autos, and computer peripherals.

Immersion silver has excellent conductivity, which makes it the ideal material for high-speed signal applications.

Common Issues with Immersion Silver and Solutions

Along with the aforementioned challenges, two more frequent problems might arise while silver-plating PCBs.

Electrical Impact

The "crevice" corrosion mechanism and this problem have certain similarities. Usually, during immersion, silver oxidation (dissolving of copper) and reduction of silver ions and their depositing are carried out concurrently. Both the anode and cathode layers are made of copper. This results in a consistent solver coating on the copper surface of the PCB.

The supply of silver, however, degrades if a copper layer gap develops as a result of mechanical stress or corrosion. Additionally, the copper in the gap functions as an anode, supplying electrons for the silver-silver reactions. The copper pads, which were supposed to be covered, end up with silver deposited on them. A PCB begins to malfunction as a result.

Galvanic Effect Avoidance

You should think about the following suggestions to lessen the chance that the Galvanic Effect may occur:

• Opt for a less corrosive immersion silver procedure with a lower pH, but stay away from thickening the silver.
• Manage the micro-accuracy etching so that it is carried out in the needed quantity of solution and time.
• Redesign the PCB such that little copper wires connecting big copper surfaces are eliminated.
• To make the solder mask less susceptible to immersion silver plating chemicals, optimize the pretreatment, curing, and imaging procedures.

Inability to solder (IC Hole/PAD)

During deployment, immersion silver technology is still susceptible to flaws. Solderability is only one of them. Inadequate hole wall quality is the primary cause of difficulties with IC Holes and PADs. Usually, it has large holes or copper that isn't thick enough. The overall solderability of the components suffers as a consequence. This challenge is somewhat related to the Galvanic Effect.

It is important to carry out copper etching and silver plating correctly. So they can guarantee that a solder mask is properly created and cured, as well as fully avoid solderability issues. You should take into account the following tips in addition to the ones previously mentioned about preventing the Galvanic impact.

IC Hole/PAD Avoidance

For an effective immersion silver coating to be applied, a PCB's surface must be 100% copper. Each tank has to be large enough to successfully exchange electrons between the copper layer and the saturated silver liquid.

Controlling the pace of micro-etching is also essential. It prevents the copper layer from oxidizing too much and creating poor-quality holes. For the uniformity of micro-etching, you should avoid connecting big copper surfaces with tiny lines.

In order to avoid solderability concerns, please consider the following supplementary advice while producing immersion silver PCBs:

• Maintain PCB storage at 30°C and 40% relative humidity.
• Immersion silver PCBs should be packaged within 24 hours after production because else, they would immediately start to degrade.
• Silver PCBs shouldn't come into touch with anything or anything that contains sulfur or chloride.
• Immersion silver PCB have to be spotless and free of fingerprints. When handling them, gloves should be used.
• Make use of craft or non-sulfur paper for packing.
• Desiccants should not be used to prevent the fading of PCB surfaces

ENIG vs. Immersion silver PCB

Electroless Nickel/Immersion Gold finishing is referred to as ENIG. It is sometimes seen as a more costly but technologically superior variant of silver plating.

Simply said, ENIG plating involves protecting the copper pads on PCBs with nickel and then coating the exposed copper with a thin layer of gold. The method is much the same as immersion silver processing. However, it improves PCBs with better surface planarity, better solderability, and increased oxidation resistance.

Read more: HDI any-layer printed circuit boards

Conclusion

ENIG is somewhat more expensive than Immersion silver PCB. It is an excellent option if the PCB has to meet functional connection criteria while lowering expenses. In many circumstances, Immersion Silver is seen to be the best choice due to its superior flatness and joint strength. In terms of functional performance, Immersion Silver is a transitional technology between OSP and ENIG. The enhanced ease of assembly and inspectability of the Immersion Silver treatment are reasons why EuropePCB promotes it.