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Immersion Silver PCB

Immersion silver PCB is a printed circuit board which surface finish is coated by immersion silver, immersion siliver is most used for aluminum wire bonding, metallic dome contacts, and EMI shielding. The thickness of immserion silver is 5 micro inches to 12 micro inches, and is can be store for at least 12 months.

Immersion silver pcb

In order to meet the urgent requirements of the electronics industry for banning lead, the PCB industry is gradually moving surface treatment from HASL to other finishes, such as immersion silver, immersion tin, immersion gold, and OSP. Immersion silver is a new surface treatment technology with excellent performance at a reasonable cost, which is considered the best choice.

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As a new environmentally-friendly surface treatment process, immersion silver is plated 6-18u” thick on the copper surface to ensure reliable soldering of electronic components on the circuit board. The immersion silver process is based on the displacement reaction between the silver ions in the solution and the metal copper on the printed circuit board. After the copper surface is micro-roughened by the micro etching solution, a uniform immersion silver layer can be slowly formed at a controlled silver deposition speed.

The slow silver immersion speed is conducive to depositing a dense crystal structure, forming a high-density silver layer, and avoiding particle growth due to precipitation and agglomeration. The immersion silver solution is very stable, has a long service life, and is insensitive to light and trace halides.

Immersion silver is cheaper than immersion gold. If the PCB has functional connection requirements and needs to reduce costs, immersion silver is a good choice. What is more, immersion silver has good flatness and contact; it is better to choose immersion silver technology. It has been widely used in communication products, automobiles, computer peripherals, and high-speed signal design. It can also be used in high-frequency signal design because of its excellent electrical properties. EMS recommends the immersion silver process because it is easy to assemble and has a good inspection pass rate. However, the trend of using immersion silver grows slowly (but has not decreased) because of defects such as tarnishing and solder joint voids. It is estimated that about 10%-15% of PCBs are currently using this technology.

Features: Between OSP and immersion gold, the process is simple and fast. Even if exposed to heat, humidity, and pollution, immersion silver can still provide good electrical properties and solderability, but it will tarnish. Because there is no nickel under the silver layer, immersion silver does not have the desired physical strength of immersion gold.

Immersion Silver Process and Principle

Immersion Silver Plating

Immersion silver in the PCB process:

Electrical test → AOI → immersion silver → visual inspection/packing/shipping

Immersion Silver Process:

Pre-treatment → loading → acid degreasing → water washing → pure water washing → micro-etching → pure water washing *2 → preliminary dip → sterling immersion silver → pure water washing *2 → hot water washing → silver protection → hot water washing → pure water washing *2 → drying → IPQC inspection→ packing

Remark:

  1. Pre-treatment: The main processes are pickling, scrubbing, washing, and drying.
  2. Post-treatment: Horizontal line (the main process includes washing and drying).

Reaction principle

immersion silver Reaction principle

Reaction schematic diagram

Immersion silver Reaction schematic diagram

Process Control

StepsOperating typeTemperature ℃Time minutes
Oil removalSpray301-2
CleaningSprayIndoor temperature1-2
Micro-etchingOverflow301-2
CleaningSprayIndoor temperature1-2
Preliminary dipOverflo
w
400.5-1
Sterling immersion silverOverflow502-4
Deionized water washingSprayIndoor temperature1-2
DryingHot air801-2
Typical steps of immersion silver process

The stability of the immersion silver process is evaluated by monitoring the products in the horizontal production line (the process steps are shown in the above table). The oil removal and micro-etching in the pre-treatment step are very important for controlling the immersion silver layer’s appearance. The total micro-etching depth is about 1-2µm, which is related to the actual condition of the copper surface.

  1. Function Introduction:
  2. Acid degreasing:
  3. Remove oxide.
  4. Clear fingerprints.
  5. Remove S/M residues.
  6. Remove other pollutants.
  7. Washing:
  8. Clean the liquid medicine remaining on the board surface to prevent the liquid from polluting the follow-up process.
  9. Micro-etching:
  10. Roughen the surface to strengthen the bonding force between the coatings.
  11. Provide a flat copper surface so that the silver can be evenly applied to the plating surface.
  12. Adjust the copper surface to a highly active surface.
  13. Preliminary dip:
  14. Ensure the board is clean, which prevents pollutants from being brought into the silver tank.
  15. Ensure the board is operated at the correct pH and temperature.
  16. There is a thin and dense release layer on the copper surface, conducive to the subsequent silver layer deposition.
  17. Sterling immersion silver
  18. A uniform silver coating is produced on the copper surface by a displacement reaction.
  19. Main Parameters:
No.ProcessIngredientParameter controlProduction time
1Acid degreasingAcid degreasant: 100ml/L;Temperature: 30±5℃;Circulating pressure: ≤0.9kgf/cm2;30-60S;
2Micro-etchingSulfuric acid: 80g/L;35% Hydrogen peroxide200ml/L;WCD-126 additive:250ml/L;Temperature: 25-30℃;Micro-etching thickness: 30-60u’’;Cu2+: ≤45g/LCirculating pressure: ≤0.9kgf/cm2;30-90S;
3Preliminary dipMT-IMG 500A: 300ml/L;MT-IMG 500B: 30ml/L;Temperature: 40±5℃;Circulating pressure: ≤0.9kgf/cm2;PH:1.9530-60S;
4Sterling immersion silverMT-IMG 500A: 300ml/L;MT-IMG 500B: 70ml/L;MT-IMG 500S: 10ml/L;Temperature: 48-54℃;Circulating pressure: ≤0.9kgf/cm2Cu2+: <4g/L;2-4minute;
5Pure water washingPure water/3-5minute
6Hot water washingPure water55±5℃30-90S;

The Step of potion configuration

  •  Preliminary dip tank:
  • Fully clean the equipment with oil removing agent and rinse thoroughly.
  • Add pure water to 50% of the tank volume. Keep a record of the exact amount of water added.
  • Add MT-IMG 500A according to the ratio of 300ml/L and heat to operating temperature.
  • Add MT-IMG 500B according to the ratio of 30ml/L.
  • Add pure water to the tank to reach the final volume and heat to operating temperature.
  •  Sterling immersion silver tank
  • Fully clean the equipment with oil removing agent and rinse thoroughly.
  • Add deionized water to 50% of the tank volume. Keep a record of the exact amount of water added.
  • Add MT-IMG 500A according to the ratio of 300ml/L and heat to operating temperature.
  • Add MT-IMG 500B according to the ratio of 70ml/L.
  • Add MT-IMG 500S at a ratio of 10ml/L.
  • Add pure water to the final volume and heat to the operating temperature.
  • Ensure that the silver liquid is fully mixed. The liquid should be the operating temperature before immersing the silver.

4. Plating Bath Maintenance, Addition, and Control

  • Preliminary dip liquid
    When maintaining the liquid medicine, MT-IMG 500 A and MT-IMG 500 B should be replenished at a volume ratio of 10:1. You can change the tank when the liquid reaches the following standards:
  • Copper content ≥ 2000 ppm (2g/L).
  • When the immersion silver liquid needs to be replaced, the preliminary dip liquid must be replaced as well.
  • Immersion silver liquid:
    When maintaining the liquid medicine, MT-IMG 500 A and MT-IMG 500 B should be replenished at a volume ratio of 6:1. Add MT-IMG 500S regularly, and the silver concentration should be maintained between 600-1000 ppm by adding MT-IMG 500S (15-25ml/L).
    You can change the tank when the liquid reaches the following standards:
  • Copper content ≥4000 ppm (4g/L).
  • When the liquid reaches 20 MTO silver concentration (based on the initial 1000 ppm (1g/L) ).

5.Tank Cleaning and Maintenance

  • Cleaning of preliminary dip and silver tank:

Tank wall cleaning → water washing → alkaline washing (10g/L sodium carbonate 60℃ 1H) → acid washing (10% nitric acid 60℃ 1H) → pure water washing.

  • Micro-etching tank cleaning:

Tank wall cleaning → water washing → pure water washing.

  • Other tank cleaning:

Tank wall cleaning → water washing → alkaline washing (10g/L sodium carbonate 60℃ 1H) →  acid washing (2% sulfuric acid 60℃ 1H) → pure water washing.

How to Solve the Problem for Bad Immersion Silver?

Immersion silver
  • Black oxidation: Caused by contact between the silver surface and sulfide, which affects the solderability of the final surface.

Treatment process: Rework

Improve methods:

  1. Use non-sulfur and non-chlorine gloves for operation.
  2. Use non-sulfur and non-chlorine neutral unimpregnated liner board.
  3. Yellow oxidation: Caused by contact between the silver surface and chloride. This does not affect solderability.

Treatment process: 5% dilute sulfuric acid → water washing → drying

Improve methods:

  • Use non-sulfur and non-chlorine gloves for operation.
  • Use non-sulfur and non-chlorine neutral unimpregnated liner board.
  • Leakage plating (exposed copper): There are pollutants on the copper surface.

Treatment process: Rework. Preliminary dip→ chemical immersion silver → washing → drying. The operation time of the controlled silver tank is not more than 45sec. This method can only be used once.

Silver is insufficient or too thick:

  • The temperature is too high or too low.
  • The influence of silver ion concentration.
  • Nitrate concentration.
  • Action time.
  • Microetching on the copper surface.
  • Stirring of the liquid.
  • Measuring method.
  • There are silver deposits or other contaminants on the tank wall:
  • The liquid in the immersion silver tank is old.
  • The operating temperature is too high in the immersion silver tank.
  • The immersion silver tank is polluted by chloride ions (there are many white particles on the tank wall).
  • Sulfate pollution (Black sedimentary particles on the tank wall)
  • Poor filtering effect.
  • The content of the chelating agent is too low.

About rework:

  • Copper surface oxidation and fingerprint rework process: Return to immersion silver process:

Oil removal → water washing → water washing → 5% dilute sulfuric acid → water washing → drying.

  • Immersion silver board rework:
  • Rework of solder mask layer:
  • Dry film solder mask: Wash off the solder mask (film stripping aid) → press dry film solder mask → exposure → developing → post-baking → rework immersion silver.
  • Solder-resistant solder mask: Wash off the solder mask (film stripping aid) → printing solder resist → pre-baking → exposure → development → post-baking → rework immersion silver.

Immersion silver (such as uneven gloss, exposed copper, etc.) rework: Wash off the solder mask (film stripping aid) → silver fading.

The Common Problems and Solutions during immersion silver

  1. Galvanic Effect

The “Galvanic Effect” mechanism of copper lines is similar to the “crevice” corrosion mechanism. Under normal conditions, copper is both the anode and the cathode. In this way, the oxidation of copper and the reduction of silver ions proceed simultaneously, forming a uniform silver-plated layer on the copper surface. However, if there is a “gap” between the solder mask and copper circuit, the supply of silver ions in the gap will be limited. The copper in the gap will become a sacrificial anode, providing electrons for the reduction reaction of silver ions on the exposed copper pads (as shown in Figure 4).

Schematic diagram of the “Galvanic Effect” of copper lines

Under normal production conditions (2.5 minutes, 0.25 um), the Galvanic Effect is shown in the scanning electron microscope picture above. Before the solder mask is peeled off, it can be seen that the copper pad is completely covered by silver, even the copper wire under the side etching. After the solder mask is peeled off, Galvanic Effect cannot be seen on the left and the middle images, but we can see a slight bite etching from the right image. It should be noted that bite etching does not occur under the side etching of the solder mask, but in the area behind the solder mask/copper interface. If the residence time is doubled to 5 minutes to form a 0.48um thick silver-plated layer, the Galvanic Effect will become stronger. As shown in the figure below, there will be a Bite Etching groove with 20 um width and 10 um depth on the copper circuit under the solder mask.

Therefore, the risk of the Galvanic Effect can be prevented or reduced by the following methods:

(1)Choose a less corrosive immersion silver process (with appropriate pH) and do not increase the silver thickness to meet corrosion resistance requirements.

(2)Control the micro-etching within the required amount.

(3)Avoid connecting large copper surfaces and small copper lines in the PCB design.

(4)Improve the bonding force between the solder mask and the copper surface by optimizing pretreatment, imaging, curing, developing processes, and use chemical-resistant solder mask.

Solderability (IC Hole/PAD)

 Immersion Silver Finish for PCBs

IC hole solderability problems caused by hole wall quality (thick hole, insufficient copper thickness):

Immersion silver technology is indispensable in circuit board manufacturing, but it can also cause defects. The formulation of preventive measures needs to consider the contribution of chemicals and equipment to various defects in actual production to avoid defects and improve the yield rate. The prevention of the Galvanic Effect can be traced back to the copper plating process. For high-aspect-ratio holes and micro-vias, the uniform thickness of the electroplated layer can help eliminate the dangers of the Galvanic Effect. Excessive etching or side etching during film stripping, etching, and tin stripping processes will promote the formation of cracks, resulting in micro etching liquid or other liquids in cracks. However, the problem of the solder mask is still the main reason for the Galvanic Effect. Most of the defective boards with the Javanni effect have side etching or solder mask peeling off. This problem mainly comes from the exposure and development process. Therefore, if the solder mask shows a “positive foot” after development and is completely cured, then the Galvanic Effect problem can be almost eliminated.

In order to get a good immersion silver layer, the immersion silver position must be 100% metal copper. Each tank solution must have a good through hole capacity, so that the liquid in the through-hole can be exchanged effectively. If it is a very complicated stack up, such as an HDI board, installing ultrasonic or jet in the pretreatment process and immersion silver tank is very useful. For production management, controlling the micro-etching rate to form a smooth, semi-bright surface can also improve the Galvanic Effect. For original equipment manufacturers (OEMs), they should avoid connecting large copper surfaces or high aspect ratio through holes with thin lines in the design to eliminate the hidden dangers.

For chemical suppliers, silver immersion liquid should not be very aggressive. It is necessary to maintain an appropriate pH value, control the immersion speed, generate the expected crystal structure, and achieve the best anti-corrosion performance with the thinnest silver thickness. Corrosion can be reduced by increasing the density of the coating and reducing the porosity. Using sulfur-free material packaging and sealing to isolate the PCB board from the air also prevents the sulfur in the air from the silver surface. It is best to store the packaged boards in an environment with a temperature of 30°C and relative humidity of 40%. Although immersion board has a long shelf life, it still has to follow the first-in, first-out principle.

Matters Need Attention:

  • The silver-plated material shall be placed on site for no more than 4 hours and generally in office for no more than 1 day. The material shall be vacuum-packed within 24 hours after discharge.
  • The silver-plated parts should avoid contact with items containing sulfur and chloride. When the silver-plated part comes into contact with chloride, the board surface will turn yellow, which will not affect the solderability. However, if it comes into contact with sulfide, the board surface will turn black, and the solderability will be reduced.
  • Copper surface control: The copper surface must be immaculate and clear of fingerprints and oil stains. Clean gloves must be worn during the milling profile and final inspection.

Packaging requirements:

  • Packaging before outward processing: Separate the board with non-sulfur paper or kraft paper.
  • Packaging before shipment: Separate with sulfur-free paper, and then use vacuum packaging. No air leakage is allowed. It is forbidden to put desiccant on the board to avoid yellowing of the silver surface.