Immersion Silver PCB Finish: Solderability and Tarnish

Immersion silver deposits a thin layer of silver onto the copper by a displacement reaction, the same principle that puts gold on an electroless nickel layer. The result is a flat, solderable finish with excellent electrical properties and a cost between the organic coatings and the metal finishes.

Its reputation has suffered from tarnish, which is a surface discolouration rather than a loss of function, and from a historical association with creep corrosion. Understanding what the finish actually does, and what it needs, allows a design to use it where it is genuinely the best choice.

What Immersion Silver Is

The layer is silver deposited directly on the copper, typically 0.1 to 0.3 micrometres thick, with an organic co-deposit that slows the tarnish. There is no nickel barrier, so the silver sits on the metal it is protecting.

That makes the finish very flat, which suits fine pitch components, and it gives a low contact resistance, which suits test pads and press-fit surfaces. The absence of nickel also removes the black pad failure mode that affects some gold finishes.

Process Steps

The process begins with a clean copper surface, since any oxide or residue prevents the displacement reaction from starting evenly. The panel is microetched, rinsed, then immersed in the silver bath, and finally rinsed and dried with a controlled process that avoids water marks.

Control points are the silver thickness, the bath chemistry and the drying. A bath with an imbalance of the organic additive produces a silver layer that tarnishes quickly, and a poor drying step leaves residues that show as stains and can affect solderability in the areas where the water collected.

Solderability and Joint Quality

Silver dissolves readily in molten solder, so the joint forms quickly and wets well, which is one of the attractions of the finish. The joint itself is a copper-tin intermetallic, with the silver dissolved into the solder, and its strength is comparable to a joint on an organic coating.

The finish is compatible with both leaded and lead free processes, and it works with most flux chemistries. Where the assembly uses a water soluble flux and a wash, the silver is consumed by the soldering anyway, so the cleaning requirement is the same as for any other finish.

Tarnish and Storage

Tarnish is the formation of silver sulphide on the surface when the board is exposed to sulphur bearing air. It appears as a brown or black discolouration and it is often mistaken for a defect, although the layer beneath remains solderable.

The practical control is packaging. Boards are supplied in sulphur free paper or in sealed bags with desiccant, and the shelf life in that packaging is typically six to twelve months. Once the bag is opened, the clock runs, and the storage environment matters more than the elapsed time.

Suitability for Different Assemblies

The finish suits a general purpose assembly with fine pitch components and a conventional reflow process. It is also used on boards that are probed in test, because the contact resistance is low and stable.

It is less suitable where the board is stored in an industrial or sulphur rich atmosphere, where it is exposed to humidity for long periods before assembly, or where the assembly process involves a high temperature excursion that removes the organic co-deposit before the solder arrives.

Comparison with Other Finishes

Against an organic coating, silver offers a longer shelf life and a lower contact resistance, at a higher cost. Against electroless nickel immersion gold, it offers a lower price and no black pad risk, but it has no nickel barrier and it tarnishes.

Against tin, it avoids the whisker concern and the intermetallic growth that affects a tin finish over time. That comparison is the reason silver is often chosen for a product with a long storage interval before assembly.

Design and Handling Rules

Do not expose the finish to sulphur bearing materials during storage. Rubber bands, some paper and certain packaging films contain sulphur compounds and will tarnish a board that is left against them, which is why the packaging specification matters as much as the plating specification.

Handle with gloves, keep the interval between bag opening and assembly short, and confirm the solderability of the first article rather than assuming it from the finish. Where the board is coated after assembly, the coating covers the silver and stops further tarnish.

Inspection and Defects

Inspection covers the thickness, the coverage and the appearance. A uniform matte silver surface is normal, a blotchy or brown surface suggests tarnish, and a silver layer that is missing in places indicates a cleaning problem upstream.

The defects that reach assembly are incomplete coverage on a pad, water marks from drying and tarnish from packaging. gopcb supplies immersion silver boards with controlled thickness, sulphur free packaging and solderability data so the finish the design assumes is the finish the assembly line receives.

Choosing a Surface Finish for a Product

The decision between the available finishes is made from four inputs: the assembly process, the storage interval, the circuit impedance and the number of times the surface has to be touched. A fine pitch board assembled within weeks of fabrication is well served by an organic coating, while a product that sits in a warehouse for a year before assembly benefits from a metallic surface finish such as silver or gold.

Impedance is the input that is often overlooked. Every finish changes the thickness of the metal on the pad and, on a controlled impedance structure that runs to the surface, that changes the impedance slightly. The effect is small for a thin finish and larger for a thick one, and it belongs in the stackup calculation rather than in a separate discussion about plating.

Assembly and Rework Considerations

Immersion silver behaves well in rework because the surface does not form a hard intermetallic that has to be broken before solder will wet. A reworked joint wets in the same way as an original one, provided the area has not been tarnished and the flux is appropriate for the finish.

The finish also tolerates multiple reflow cycles, which matters for a board that is assembled on both sides or that goes through a second process for a selective assembly. Boards that require more than two reflow exposures are usually checked with a solderability test after the first, because the co-deposit that protects the silver is consumed by the heat and the surface becomes more susceptible to tarnish afterwards.

FAQ

Is tarnish a defect? It is a cosmetic change and the surface usually remains solderable, but heavy tarnish indicates exposure and should be assessed rather than ignored. The assessment is a solderability test, not a visual opinion.

How long does an immersion silver board last? Six to twelve months in sealed sulphur free packaging with desiccant, and considerably less once the bag is opened in a humid or polluted environment.

Does immersion silver work with wire bonding? It is used for some aluminium bonding processes, but gold over nickel is the more common choice where the bond has to be reliable over a wide temperature range.

Related reading: PCB gold plating types, exposed copper and pad finishes, PCB manufacturing processes, and lead free versus leaded solder.