Tin Stripping After Plating: 5 Process Checks

Tin stripping is the step that removes the tin resist after the copper has been etched, and it decides the quality of the copper surface that the solder mask will later cover. In a solder mask over bare copper flow the plated tin protects the traces through etching, then has to disappear completely without attacking the copper underneath it.

The chemistry is selective by design, but the selectivity has limits. Strip rate, temperature, dissolved tin and the condition of the rinse that follows all move the result, and a panel that leaves this stage with a thin film of tin residue or a discoloured copper surface will fail at assembly rather than at final inspection.

Why Tin Stripping Follows Copper Etching

The plated tin layer is a sacrificial mask. It covers the traces while the uncovered copper is etched away, and because tin resists the etchant it holds the trace profile underneath itself intact. Once the etch is finished the tin has no further purpose and would interfere with solder mask adhesion, so it is removed in the next tank.

The order of the two steps is what makes the process practical. Etching first and stripping second means the copper is protected for the whole etch, and it means the tin thickness can be specified for etch resistance alone. Our solder mask application notes start from the surface this stage leaves behind.

How the Tin Etch Chemistry Works

Most tin etch baths are based on nitric acid with an inhibitor, or on a proprietary mixture that oxidises tin while leaving copper passive. The inhibitor is the part that matters. Without it the bath attacks copper at a rate close to its attack on tin, and the traces lose width as fast as the resist disappears.

The reaction produces dissolved tin and a sludge of hydrated tin oxide. Both accumulate, and both change how the bath behaves: dissolved tin slows the strip rate, while sludge settles on panel surfaces and on the coil of the heater. Filtration and a controlled bleed keep both within range.

Strip Rate, Temperature and Bath Loading

Strip rate is measured as the time needed to clear a known tin thickness, usually checked with a coupon or a visual end point on a test panel. A falling rate means the bath is loaded or cold, and the usual operator response, adding more nitric acid, treats the symptom while raising the attack on copper.

Temperature is the strongest lever on rate and also on selectivity, so it belongs on the record with every analysis. Bath loading should be tracked as square metres of tin removed per litre rather than as hours of running, because two products with different tin areas load the same tank at very different speeds.

Protecting the Copper Surface During the Strip

Copper surface protection is a balance between removing all the tin and removing none of the copper. The first sign of over-strip is a colour change, from the pink of fresh copper to a dull brown or a darkened, slightly pinkish grey that no longer wets evenly.

The second sign is dimensional: a trace measured after stripping is narrower than the same trace measured after etching. Where a design has fine lines, that loss can be the difference between passing and failing, so the strip window should be set from the fine lines and checked on the copper surface preparation coupon rather than on a wide ground plane.

Sludge, Filtration and Bath Life

Tin sludge behaves like a fine abrasive when it is pumped through spray nozzles, and it wears them unevenly. Filters that are changed on a schedule keep nozzle pressure stable, while filters changed only when pressure drops leave a period in which the panel sees a weaker spray and a slower strip.

Bath life ends when the dissolved tin reaches the point at which the strip rate can no longer be held inside the window without raising the temperature or the acid strength beyond what the copper will tolerate. That point should be predicted from the loading record, not discovered on a panel with a residue defect.

Rinse, Neutralisation and Drying

The rinse after the strip has to neutralise acid carried on the panel and on the rack. Counter-flow rinses with a measured flow rate and a defined drip time do that reliably; a single still tank does not, because its pH climbs through the shift and the last panels of the lot are rinsed in the weakest water.

Drying matters for the same reason. Water trapped in a hole or under a resist edge carries dissolved salts into the solder mask cure, where they form blisters. Cleanliness after this stage is verified the same way as after assembly, using the methods in cleanliness verification.

Verification and the Defects That Escape

Verification is simple and cheap: a visual check under low-angle light for tin residue, a tape or peel check where the specification calls for one, and a measurement of trace width on a coupon. A surface that still shows a silver tint under the microscope has not been stripped, however clean it looks in normal light.

The defects that escape are the ones with no immediate symptom. Residual tin shows up as poor solder mask adhesion or as a solderability problem after surface finish application, weeks after the panel was packed. Test methods and acceptance limits for the chemistry are published by SMTA.

Checks Before Release

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Verification and Records

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Points to Confirm at First Article

Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

FAQ

Can tin stripping and copper etching run in the same tank? No. The two chemistries attack different metals and need different inhibitors, temperatures and waste routes. Running them together removes the selectivity that makes the tin resist worth plating in the first place.

Why does the copper look darker after stripping? A dark, dull surface usually means the bath has started to attack copper, or that the rinse left acid on the surface long enough to oxidise it. Both are corrected at the strip tank, not at the solder mask line.

How often should the tin etch be analysed? At least once per shift, and again after any large addition or filter change. Dissolved tin is the value that predicts the end of bath life, so it deserves the same attention as the acid concentration.