Immersion Tin and Immersion Silver Finish Selection

Immersion tin and immersion silver are the two flat, lead free finishes that sit between the organic coating at the low cost end and the nickel based finishes at the high reliability end. Both are thin, both are deposited by a chemical displacement reaction and both are sensitive to how they are stored and handled. Choosing between them comes down to the process, the storage period and the failure mode that the product can and cannot tolerate.

Why These Two Finishes Exist

A flat finish is needed for fine pitch printing and for area array packages, and it is needed at a lower cost than the nickel and gold family. The organic coating is flat and cheap but it has limited thermal durability, and the immersion finishes fill the gap by providing a metallic surface that survives several reflow cycles.

Both immersion tin and immersion silver are deposited by a displacement reaction, in which the copper is oxidised and the coating metal is deposited in its place. The reaction is self limiting, so the layer is thin and conformal, and it follows the copper surface rather than building a thick deposit.

Immersion Tin Process

The tin is deposited over a copper surface with an inhibitor that controls the reaction and prevents the growth of intermetallic between the tin and the copper. Without the inhibitor the copper diffuses into the tin and consumes the layer, which reduces the shelf life and the solderability.

The process is straightforward, and the bath is relatively easy to maintain compared with the nickel chemistries. The control points are the temperature, the concentration and the age of the bath, and the thickness of the resulting layer. Our plating thickness guide covers how the thin layers are measured.

Immersion Silver Process

The silver is deposited in the same way, and the result is a very flat, highly solderable surface with excellent electrical properties. The layer is thin, usually measured in tenths of a micrometre, and it sits directly on the copper with a thin intermetallic region between them.

Because silver is an excellent conductor and does not form a resistant oxide, the finish works well for high frequency and for a press fit interface where a low and stable contact resistance matters. The disadvantages appear in storage and in the environment, which is where the selection usually turns.

Shelf Life and Storage

Both finishes have a limited shelf life compared with a gold finish, and the figure depends on the storage conditions as much as on the coating. Humidity and airborne contamination are the two variables that matter, and a store with sulfur or chlorine compounds will degrade a silver finish quickly.

The storage specification should therefore state the packaging, the temperature, the humidity and the maximum period, and the material should be used within it. A finish that has exceeded its storage period should be verified by a solderability test rather than assumed to be usable.

Whisker Risk

A pure tin coating can grow conductive filaments, known as whiskers, which are a reliability risk where the spacing between features is small. The mechanism is driven by stress in the deposit and is accelerated by thermal cycling, and the mitigation is the coating structure rather than the storage.

Immersion tin is a thin layer, and its whisker behaviour differs from a thicker electroplated tin, but the risk is not zero and it should be considered where the pitch and the voltage make a filament a hazard. The topic is a permanent part of the finish selection discussion for a high reliability product.

Solderability and Joint Formation

Both finishes wet readily, and the joint forms by the dissolution of the coating into the molten solder and the reaction of the solder with the copper beneath it. The layer is thin enough that it disappears into the joint, which is why the resulting interface is between the solder and the copper.

The solderability of the surface is what matters most, and it is affected by the age of the coating and by the surface chemistry. Our solderability guide covers the test methods used to verify a finish before assembly.

Creep Corrosion on Silver

An immersion silver surface exposed to sulfur bearing air forms a thin silver sulfide film, which appears as a discolouration and reduces solderability. The phenomenon is known as creep corrosion, and it is worse in an environment with a high sulfur concentration, such as an industrial or a rubber manufacturing area.

The protection is the packaging and the storage environment rather than the coating itself. A board that has been stored in an open environment can fail a solderability test even though the coating thickness was correct when it was shipped.

Choosing Between Them

Immersion silver gives the better electrical surface and the better solderability when it is fresh, and immersion tin gives a more predictable surface at a lower cost when the storage and the environment are controlled. For a product that will be assembled quickly and used in a benign environment, silver is attractive.

For a product that will be stored for a long period before assembly, or for a silver sensitive environment, tin is often the better answer. The decision should also consider the whisker question where the geometry makes a filament a real risk. Our surface finish guide compares how the finishes behave in the same applications.

Process Control Points

For both processes the control points are the bath concentration and temperature, the immersion time, the rinse quality and the drying. A rinse that leaves residue produces a surface that appears finished and does not solder, and it is the defect that most often escapes the fabrication inspection.

The thickness measurement on a coupon, the solderability check and the storage record together form the evidence that the finish is what the order specified. Our quality documentation describes how a finish defect is classified at gopcb.

Process Control and Verification

On a design of this kind, immersion silver is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Process Control and Verification

On a design of this kind, immersion silver is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

FAQ

Which finish gives the longer shelf life? Both are limited compared with gold, and the figure depends on the storage conditions. Immersion tin is generally more forgiving of humidity, while silver is more sensitive to sulfur and chlorine in the air.

Is immersion silver suitable for press fit connectors? It is, and the low contact resistance is an advantage. The surface has to be clean and the connector has to be inserted without damaging the coating.

Does immersion tin present a whisker risk? A thin immersion layer behaves differently from a thick electroplated tin, but the risk is not eliminated. Where the geometry and the voltage make a filament a hazard, the finish should be qualified for that specific product.