Immersion Tin Coating Thickness: 6 Process Controls That Matter

Immersion tin is a thin metallic coating deposited by a displacement reaction, where tin from the bath replaces copper on the surface of the pad. It gives a flat, solderable finish at a modest cost, and it is popular for fine pitch work where solder levelling would be too rough.

The finish has one structural weakness. Tin and copper continue to react after plating, growing an intermetallic layer that consumes the tin, so the coating that protects the pad today is slowly being converted into a compound that no longer solders well. Controlling thickness is really controlling how long the finish stays good.

Why Immersion Tin Is Used

The coating is deposited from solution without an external current, so it covers fine features evenly and does not bridge the gaps between them. The surface it leaves is matte grey and flat, which suits paste printing onto fine pitch pads and press-fit holes alike.

It also avoids the two costs of the alternatives. There is no gold to pay for and no hot solder to shock the laminate, which makes the process attractive for high layer count boards where a thermal excursion is unwelcome.

How the Coating Forms

The reaction is a displacement. Copper at the surface dissolves into the bath while tin deposits onto the pad, and the process stops once the surface is fully covered because there is no longer exposed copper to drive it. That self-limiting behaviour is what makes the coating uniform.

Because the reaction stops on its own, coating thickness is governed by chemistry and time rather than by current. A bath that is close to exhaustion stops earlier and gives a thinner coating, which is why the coating can drift within a specification that looks unchanged on paper.

Bath Chemistry: Tin, Acid and Additives

The bath contains a tin salt, an acid to keep the metal in solution, a complexing agent such as thiourea to control the displacement reaction, and additives that affect grain structure and appearance. Copper that has dissolved into the bath accumulates and must be controlled.

All four components drift with throughput. Tin is consumed by deposition, copper rises by dissolution, and the complexing agent breaks down over time. Analysis on a schedule tied to panel area, rather than to the calendar, is the only way to hold coating thickness inside a narrow window.

Thickness Targets and Why They Are Narrow

The usual target for an immersion tin coating is around one micrometre. Below that, coverage is incomplete and porosity allows copper to oxidise through the coating; well above it, the reservoir of tin is larger but the intermetallic growth that follows is also more extensive.

The window is narrow because both ends fail differently. Thin coatings fail quickly through porosity, while thick coatings look healthy and then lose solderability during storage as the intermetallic layer reaches the surface. Neither failure is visible at final inspection.

Intermetallic Growth With Time

The intermetallic layer grows at a rate that depends strongly on temperature, roughly doubling for every ten degrees of storage temperature. That is why a coating that lasts six months in a cool warehouse may fail in six weeks in a hot one.

Growth is also why storage instructions matter more for immersion tin than for most finishes. Because tin is consumed from the surface inward, the amount remaining to solder to shrinks continuously, and the process accelerates as the layer thickens. There is no plateau to wait for.

Rinsing, Drying and Handling

The bath is acidic and contains a complexing agent, so thorough rinsing is essential. Residue left in a hole or under a mask opening continues to attack the coating and produces discoloured patches and early oxidation that look like a chemical fault.

Handling matters because the coating is soft and thin. Gloves, edge handling and clean storage racks prevent the marks and scratches that create local thin spots, and each of those thin spots becomes the place where copper oxidation begins during storage.

Solderability and Storage

Solderability testing measures the coating’s ability to wet, and the standard method uses a wetting balance or a dip and look test defined by IPC standards. The result should be recorded at release and again after an accelerated ageing interval that represents the intended shelf life.

Where the board will be stored for an extended period, the ageing test is what proves the shelf life rather than the specification on the finish. Boards returned from storage after a failed wetting test point at the ageing behaviour, not at the quality of the coating that was shipped.

Measuring Thickness and Coverage

Thickness is measured by X-ray fluorescence on a pad, which is fast but assumes the coating composition is consistent, or by coulometric or cross section methods that give a more direct answer. Because coverage faults are local, several pads across the panel should be measured.

Coverage is checked visually for colour uniformity and for exposed copper, and the check is worth doing on the panel edges where rinse and drain effects are strongest. A section also shows the intermetallic layer, which is useful when the question is why a stored board no longer solders.

Defects and Their Causes

Patchy, dark or iridescent areas point at bath contamination or incomplete rinsing. Blistered or flaking coating points at a bath that is out of balance or at a copper surface that was not properly prepared, and poor adhesion usually traces back to an inadequate micro-etch.

Where the finish must survive a long storage period or repeated thermal excursions, the comparison with other metallic finishes is worth making. The immersion silver and hard gold options and the OSP and ENIG routes each trade cost against flatness, storage life and assembly behaviour, and the right answer depends on how the board will be used.

Points to Confirm at First Article

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

Documented Process Control

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

FAQ

How thick should immersion tin be? Around one micrometre is the usual target, with a tolerance that is deliberately tight. Thinner coatings are porous, and thicker coatings hold more tin that will be consumed into intermetallic during storage, so more is not automatically better.

Why does immersion tin lose solderability in storage? The tin reacts with the copper beneath it to form copper-tin intermetallic compounds, and this reaction continues at room temperature. Once the intermetallic reaches the surface, the outer layer is no longer metallic tin and wetting deteriorates.

Can the coating be reworked? Stripping and re-plating is possible on a bare board but is rarely economical, and repeated cycles leave the copper surface worse each time. Preventing the fault through bath control and packing is far cheaper than correcting it later.