Tin Stripping and Copper Surface Protection
Tin stripping removes the tin or tin-lead layer from the copper that has been plated over, so that the underlying copper becomes the finished conductor after etching. It is used with pattern plating, where a metal resist is plated up and then stripped away, and with the recovery of panels that have been over-plated and need to be reprocessed.
What Tin Stripping Does
The bath dissolves the metal resist without attacking the copper beneath it. That selectivity is the whole purpose of the chemistry, and it is the property that a strip control programme is written to protect.
The step also prepares the copper for what follows. A panel that leaves the strip bath has bare copper that will be etched, masked or coated, and the condition of that copper decides the performance of the next process as much as the strip itself does, and a surface finish chosen for the later stages depends on the condition it leaves behind.
Chemistry of the Strip Bath
Most strip baths use nitric acid with a dissolved metal and an inhibitor, or a proprietary alkaline system for sensitive work. In the acidic system the tin is oxidised and dissolved while the copper is protected by the inhibitor, which forms a temporary film on the copper surface.
The inhibitor is the component that determines selectivity, and it is also the component most easily exhausted. As the metal content rises, more inhibitor is consumed and the bath loses its ability to distinguish between tin and copper. A bath analysed only for acid reaches its limit without warning, because the acid is replenished while the metal is not removed.
Rate, Temperature and Time
Strip rates of 10 to 25 micrometres per minute are typical at 25 to 35 degrees Celsius, so a 5 micrometre resist is removed in well under a minute when the bath is fresh. The rate falls as the dissolved metal rises, and the fall is faster than the analysis curve suggests.
Time is judged from the appearance of the panel rather than from the clock. The strip is complete when the surface is uniformly bright copper, and copper that has gone to a deep pink or lost its lustre has been exposed to the bath for longer than it needed to be. A panel left in the bath after the strip is complete loses copper at the same rate as one that still carries its resist.
Copper Attack and Selectivity
Copper attack shows as a change of colour, as a dull surface and as a slight reduction in line width at the edges of a trace. It is measured by weighing a copper coupon before and after a strip pass, and by inspecting the line width on a coupon that has been through the bath.
Selectivity is not an absolute property, since the bath cannot avoid touching the copper at all. It is a matter of rate, and the practical measure is how much copper a 30 second exposure removes compared with the tin it is meant to dissolve.
Tin Sludge and Bath Life
Dissolved tin eventually precipitates as a sludge that settles on the tank floor and lodges in the pipework. Sludge reduces the effective bath volume and raises the metal content, so the useful life of a strip bath is usually limited by dissolved metal rather than by the acid concentration.
Baths are retired on the metal content, filtered to remove suspended sludge and monitored by titration. A bath that is filtered but not analysed will reach its limit while still appearing to work, and the first sign is a panel that comes out of the strip with a faint grey film on the copper. Filtering removes suspended sludge but not dissolved metal, so filtration extends cleanliness rather than life.
Rinsing and the Following Step
The strip bath leaves acid and dissolved metal on the panel, and both have to be removed before the next process. Rinsing is done in a cascade so that the last stage is clean, and the rinse quality is judged by conductivity rather than by appearance.
Bare copper that leaves the rinse is vulnerable, and the hold time before the next step decides how much oxide forms. Where the following step is an etch, the window is short and the panels are normally processed directly, without racking or a pause for inspection. The rinse is normally a cascade whose last stage is fed with deionised water, and bare copper protection begins at that stage.
Defects and Their Causes
Incomplete stripping leaves a grey or white film on the copper, which becomes a plating or etch defect later. The plated metal acts as the etch resist during the outer layer etch, so any film left behind changes the shape of the feature the etch produces. A thin film of this kind is often invisible except at a low angle to the light, which is why the strip is checked with the panel tilted. Over-stripping leaves a dull, etched surface and a narrower line than the artwork defined, and it is usually a sign that the bath is running hot or that the conveyor has slowed.
A pink or copper coloured deposit on the panel after the strip indicates that the bath has begun to plate copper back rather than dissolve tin, which happens when the dissolved metal content is high. That failure is a bath retirement trigger rather than a condition to adjust around.
Waste Treatment and Safety
Strip baths carry dissolved tin and, in leaded processes, dissolved lead, so the waste stream is treated as heavy metal bearing effluent and is segregated from the general acid waste. Precipitation, settling and filtration are the usual treatment steps.
The process also produces fumes, and the tank is normally fitted with extraction. Operators work with acid resistant gloves and eye protection, and the spill procedure is part of the process document rather than of a general safety file. The treatment route, the effluent pH and the disposal records belong with the process documentation as well.
Process Window and Records
The window is described by acid concentration, inhibitor level, dissolved metal, temperature, immersion or conveyor time and rinse quality. The rate is verified on a coupon at each shift, and the copper loss is measured on the same coupon.
Records should carry the titration results, the metal analysis, the bath age, the coupon strip rate and the copper loss figure. The same discipline applies to the strip bath as to any other wet chemistry on the line, and a bath without its analysis record is a bath whose remaining life is unknown. An analysis more than a shift old is treated as an unknown strength for the purposes of the record.
FAQ
How fast does a tin strip bath work? Rates of 10 to 25 micrometres per minute are typical at 25 to 35 degrees Celsius, and the rate falls as dissolved metal accumulates in the bath.
Why does the bath start attacking copper? Because the inhibitor is consumed as the metal content rises, so selectivity falls and the bath begins to etch the copper as well as the tin.
What limits the life of the bath? Dissolved tin rather than the acid level. The metal content is the retirement criterion, and it is confirmed by titration and by the appearance of the stripped panel.
