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What is Immersion Silver

Immersion silver is a surface finish that sits between organic coatings and electroless nickel/immersion gold. The process is simple and fast. Even when boards see heat, humidity, or contamination, silver keeps good solderability. The silver layer can lose shine though. Immersion silver does not have the physical strength that electroless nickel/immersion gold has. This is because there is no nickel layer under the silver.

Core Components of the Immersion Silver Bath and Their Roles

The formula of the immersion silver bath directly decides the quality of the silver layer. The main components include silver salt, complexing agent, acid regulator, and additives. These parts work together to control the reaction rate and the silver layer shape.

Silver salt

Silver salt gives the silver ions. Silver nitrate (AgNO₃) is commonly used. Typical concentration is 5–15 g/L. If the concentration is too low, the silver layer is too thin (less than 0.1 μm) and cannot protect the copper well. If the concentration is too high, the reaction can be too strong and the silver may grow as dendrites. That makes the surface rough (surface roughness Ra > 0.5 μm).

Complexing agent

Common complexers are ammonia or organic amines. They form a stable complex with silver ions, for example [Ag(NH₃)₂]⁺. This lowers the free silver ion level and slows the reaction. The molar ratio of complexing agent to silver ion should be controlled at about 2:1 to 3:1. If the ratio is too high, the deposition rate becomes too slow (less than 0.05 μm/min). If the ratio is too low, the reaction cannot be controlled well and the silver layer can have pinholes.

Acid regulator

Formic acid or acetic acid are commonly used to set the bath pH to about 3.5–5.5. If pH is less than 3.5, complex stability drops and more free silver ion appears, so the reaction becomes violent. If pH is more than 5.5, copper ions can form copper hydroxide precipitate and contaminate the silver layer. That can make black spots appear.

Additives

Additives include surfactants (for example sodium dodecyl sulfate) and corrosion inhibitors (for example benzotriazole). Surfactants reduce the solution surface tension to below 30 mN/m and help the silver cover evenly, especially inside via holes and at corner areas. Corrosion inhibitors suppress excessive copper dissolution. Copper ion level should be kept below 2 g/L to avoid voids in the silver layer.

Immersion Silver Process Steps

Immersion silver has three main steps: pre-dip, immersion, and final deionized water rinse.

A pre-dip has three purposes. First, it acts as a sacrificial solution. It prevents copper and other contaminants from being carried into the immersion bath from the micro-etch tank. Second, it gives a clean copper surface for the displacement reaction. The copper surface in the pre-dip obtains the same chemical environment and pH as the immersion bath. Third, because the pre-dip has the same composition as the immersion bath except for metallic silver, it automatically makes up the immersion tank. In the immersion reaction the only consumed material is metallic silver. The organic parts in the bath change only because some solution is carried out by the boards. If the pre-dip and the immersion solution share the same composition, the amount carried in by the pre-dip equals the amount carried out by the immersion bath. This prevents unnecessary build-up of organics in the immersion bath.

The immersion reaction occurs by a displacement reaction between copper and silver ions. If the copper surface is micro-etched by an AlphaSTAR micro-etch solution, the result is a copper surface that can slowly develop a uniform silver layer under a controlled immersion rate. A slow immersion rate helps form dense crystal structures and avoids particle growth that comes from fast precipitation and agglomeration. That yields a high-density silver layer.

This dense structure, at a moderate thickness of about 6–12 µin (microinches), not only gives good corrosion resistance but also very good conductivity. The immersion silver bath is stable and has a long service life. It is not very sensitive to light or trace halides.

Performance Comparison: Immersion Silver vs. Immersion Gold (ENIG)

Immersion gold, which is electroless nickel with immersion gold, is a common high-end PCB surface finish. The core differences between immersion gold and immersion silver sit in performance and cost.

Corrosion resistance

An immersion gold stack often uses a nickel layer of 5–10 μm plus a gold layer of 0.05–0.1 μm. The nickel layer isolates copper and gives much better corrosion resistance than immersion silver. After a 1000-hour damp heat test, the immersion gold layer shows little change, while the immersion silver layer may display slight oxidation. In very humid and polluted environments, like industrial control, immersion gold shows clear reliability advantages. In normal environments, such as consumer electronics, immersion silver can meet requirements. The shelf life of immersion silver performance is stable for about 12 months under normal storage.

Soldering performance

Immersion silver has solder wetting (spread) of about 80–85%. That is slightly better than immersion gold at 75–80% because silver has higher affinity to solder. But the solder joint strength of immersion gold averages 6–7 N, which is higher than immersion silver at 5–6 N. That is because the nickel layer bonds better with copper. For fine pitch solder joints (less than 0.2 mm), the uniformity of immersion silver can be more helpful for solder paste printing. For large solder joints (greater than 1 mm), immersion gold shows better long-term reliability.

Cost and process

Immersion silver costs about 50–60% of immersion gold. This is mainly because gold salts cost much more than silver salts. Immersion silver also needs fewer process steps because you do not need to plate nickel. The production efficiency is higher and a single batch processing time can be about 30% shorter. But immersion gold has a wider process window and tolerates parameter variation better. Immersion silver requires stricter control of solution composition. For example, silver ion concentration deviation should be less than 1 g/L.

Chemical Silver Operation Precautions

Below are operation and handling recommendations for immersion silver.

1. Handling recommendations

1. At every step after immersion silver treatment, wear clean, sulfur-free gloves when handling the boards.

2. When you inspect boards after immersion silver, place them on sulfur-free paper.

3. Avoid exposing the silver layer to sulfur or chlorine compounds at every stage.

4. The silver layer is thin and can scratch easily. Handle boards gently.

2. Punching and routing recommendations

1. The immersion silver process is usually set as a late step in PCB production. It is not recommended to do immersion silver before routing or final shaping.

2. After immersion silver and before routing, place sulfur-free paper between layers and between top and bottom boards to prevent scratches.

3. Cleaning recommendations for silver-plated boards

1. Do not use surfactants or acidic cleaners on the silver surface.

2. Do not rub the silver surface with any eraser.

3. Clean the silver surface only with pure water or by using electrostatic cleaning methods.

4. Packaging and storage recommendations

1. After boards come off the line, move them quickly to a noncorrosive environment with temperature and humidity control. Keep the storage temperature below 30°C and relative humidity below 50%.

2. After inspection, vacuum-seal the boards as soon as possible. Finish vacuum packing within 8 hours and not more than 24 hours.

3. Packaging options:
   A. Pack 10–20 boards as a unit. Use sulfur-free and chlorine-free paper between each board. Cover top and bottom with 2–3 sheets of sulfur-free paper. Vacuum-seal. This can keep boards for up to six months.
   B. Pack 10–20 boards as a unit without paper separators. The top and bottom boards touch solder side to packaging film. This method can only store for about 2 months. Use this only after agreement with the customer.

4. Do not put desiccant into immersion silver packaging because many desiccants contain sulfur.

5. Do not use adhesive tape, sticky labels, ink marks, or rubber bands on immersion silver boards or on sulfur-free paper. These items may contain sulfur.

6. Choose vacuum bags that prevent contamination and resist concentration and moisture ingress.

7. After vacuum packing, store the package at temperature below 30°C and relative humidity below 50%.

8. After opening vacuum packages for assembly, aim to finish assembly within one day.

5. Baking recommendations

1. For warped or bent PCBs, do baking and flattening before immersion silver.

2. If immersion silver boards are warped, do baking and flattening. Wrap boards tightly in aluminum foil to reduce silver oxidation during baking.

3. Use a dedicated oven for baking. If no dedicated oven is available, clean the oven thoroughly to avoid silver surface contamination.

6. Test board handling recommendations

1. When taking test boards after immersion silver, wear clean sulfur-free gloves.

2. Place immersion silver as a late-stage step in PCB production.

3. Wrap each immersion silver board in two sheets of the same size aluminum foil and then vacuum-pack.

7. Sulfur-free materials and packaging testing

1. Sulfur-free paper, sulfur-free gloves, and packaging film need surface element checks. Use EDX at 100× and double-sided scanning.

8. Production notes for immersion silver boards

1. If residues from earlier steps (green solder mask, residual film, etc.) remain, they can cause exposed copper issues.

2. If residues exist, remove or prevent them before immersion silver. Pre-treatment before immersion silver cannot always remove residues on pads.

3. On receipt, visually inspect immersion silver boards. If many exposed copper points or silver surface discoloration are found, stop production for that lot number and handle it. This prevents large scrap from large-scale exposed copper.

9. Deionized water requirements

1. The quality of rinse water after immersion silver directly affects the ion cleanliness of the finished board. Therefore, add a conductivity meter to the rinse stage after immersion silver.

2. Before topping up any immersion silver tanks, confirm water quality. Personnel must not leave when adding water.

3. Water quality requirements for immersion silver lines:

   • S.S (suspended solids): below 5 PPM

   • T.D.S (total dissolved solids): below 10 PPM

   • Total hardness: below 20 PPM

   • No detectable metal ions

   • No detectable chloride ion

   • Conductivity below 10 μS

10. Chemical additions

1. Add chemicals into each immersion silver tank using dedicated measuring cups to prevent cross-contamination.

11. Rework recommendations

1. Rework in the immersion silver process is allowed only once. Record and fully inspect reworked boards.

This is the weakly alkaline immersion silver process. It is mainly designed to solve the galvanic effect on fine PCB lines.

How Pre-treatment Affects Silver Layer Quality

The quality of pre-treatment on the PCB copper is the basis for a uniform silver layer. Steps like degreasing, micro-etch, and acid wash ensure the copper is clean and activated.

Degreasing

Use an alkaline degreaser (pH 10–12) to remove oil and dirt from copper, such as fingerprints and cutting fluid. Typical temperature is 50–60°C and time is 1–2 minutes. If degreasing is not complete, the silver layer may not deposit in some spots, causing exposed copper points. You can check by the water film test. After degreasing, the water film should stay continuous for at least 30 seconds.

Micro-etch

Use sodium persulfate or a sulfuric acid + hydrogen peroxide system to micro-etch the copper. This removes oxides and creates a microscopic roughness (Ra 0.1–0.3 μm). Control micro-etch removal at 0.5–1 μm. If micro-etch is insufficient and oxide remains, the bonding strength of the silver layer drops (peel strength < 0.5 N/cm). If micro-etch is too deep (more than 1.5 μm), the copper becomes too rough and the silver layer can trap contaminants and lower soldering reliability.

Acid wash

After micro-etch, neutralize residual etchant with 5–10% sulfuric acid for 30–60 seconds. Make sure the copper surface pH is less than 4. If acid wash is not enough, residual oxidants (like persulfate) can contaminate the immersion bath and make the silver layer turn black due to silver oxidation.