Tin plating
Specifying electrodeposited tin: substrate and underlayer compatibility, appearance, solderability, whisker qualification, dimensional buildup, and acceptance.
Tags: processes, finishing, electroplating, tin
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An example 0.500 mm blade with 2 µm nickel plus 8–12 µm tin per face finishes at 0.520–0.528 mm. An uncoated 0.530 mm slot leaves 0.002–0.010 mm total clearance. Check actual local buildup and finished dimensions, including forming variation. Assess insertion and contact performance separately.
Specify the tin stack, significant surfaces, and joining or contact requirements. Control solderable shelf life, fit and whisker risk separately where required.
Scope and finish identity
ASTM B545-22, listed as active with a catalog update of 1 December 2022, covers electrodeposited tin on metallic articles. Its stated purposes include solderability, low contact resistance, corrosion protection, and anti-galling behavior. It excludes hot-dipped and other non-electrodeposited coatings and mill products. Its scope generally requires at least 99% tin, with a stated exception for deliberate alloying for special purposes. The public abstract distinguishes six service classes by minimum thickness and service severity, and three appearance types: matte, bright, and flow-brightened. Use the complete ordered edition for its designation and requirements. ASTM B545-22
Keep three identities separate: the substrate alloy, the deposited finish composition, and any solder subsequently applied. A tin finish does not identify the solder alloy or qualify a soldering cycle. Likewise, a component supplier's proprietary plating code must be resolved to that supplier's actual stack and process; it is not a universal finish designation.
Substrate, barriers, and significant surfaces
A bounded manufacturer example is PennEngineering's ET/DT electro-tin with clear preservative coating. Its guidance describes matte tin and, on SMT fasteners, tin over nickel over copper for solderable shelf life. For base materials containing more than 5% zinc, it describes copper and nickel barriers against zinc migration. Its definition of significant surfaces excludes threads, holes, deep recesses, and angle bases. Those exclusions matter: purchasing that manufacturer's finish description does not promise the same minimum deposit in every recess. The web guidance is undated; its cited ASTM class letters should be interpreted with the supplier and the ordered standard edition. PennEngineering plating guidelines
For another part, specify its actual alloy and condition, including prior heat treatment, existing coatings, and any high-strength substrate requirements. Agree the barrier material, thickness, and coverage rather than inferring them from the topcoat. Identify functional threads, spring-contact regions, solder lands, and bores explicitly. If a drawing requires coating where a supplier normally excludes coverage, resolve that difference before ordering.
Matte, bright, and solderability after aging
Appearance identifies a finish family, not a universal reliability ranking. Technic's Tin Process Selector Guide, printed Rev 1215, lists bright rack/barrel processes alongside matte and satin processes intended for wire or reel-to-reel applications. It explicitly calls itself selection guidance and directs users to the individual technical data sheets. A chemistry chosen for strip reflow is not automatically interchangeable with one qualified for finished loose parts. The printed revision code is reported as written; an upload or crawl date is not its issue date. Technic process selector
Samtec's matte-tin FAQ describes larger grain size and a duller appearance than its bright tin, and warns that bright acid tin can darken at the elevated temperatures of lead-free reflow. It presents matte tin as compatible with its lead-free processing applications. This is supplier-specific guidance, not permission to expose every tin-plated part, plastic body, or preservative coating to the same thermal profile. The page is undated. Samtec matte-tin FAQ
Define solderability at the condition that matters: as received, after specified storage, or after thermal/environmental conditioning. Record packaging, handling, finish age, solder alloy, flux, test method, and acceptance criterion. An appearance check cannot substitute for a wetting test. Conversely, discoloration should be investigated against functional requirements rather than automatically classified as loss of solderability. A successful accelerated-aging test supports its tested conditions; converting it into an unrestricted calendar shelf life requires additional evidence.
Whisker risk and qualification boundaries
Tin whiskers are conductive protrusions that can create unintended electrical paths. In an original 2009 ECTC study, Panashchenko and Osterman compared tin on copper with tin over a nickel underlayer after prolonged storage and sequential environmental exposures. Both groups developed whiskers; long whiskers appeared on nickel-underlayer specimens under the study's conditions. This establishes a limitation on claiming nickel as a guaranteed cure, rather than proving nickel is always harmful or predicting another product's lifetime. Original study, NASA-hosted IEEE reprint
Texas Instruments supplies a contrasting controlled production example. For its matte-tin leadframe packages with formed leads, TI describes annealing for one hour at 150 °C within 24 hours of plating, and a minimum 7 µm as-plated thickness with up to 15% thinning after trim/form. Its published qualification summary reports nonzero whisker-length bounds under specified cycling and humidity tests. These controls apply to TI's described materials and package processes; they are neither a universal annealing recipe nor a statement that passing parts contain no whiskers. TI lead finishes and tin-plating controls
IEC 60068-2-82:2019, edition 2.0, published 14 May 2019, addresses testing the whiskering propensity of finished tin/tin-alloy components and mechanical parts used in electronic assemblies, including press-fit pins. Its scope and knowledge-based selection of test conditions should be matched to the actual material system. Identify the required test edition, specimen state, conditioning, inspected areas, and application acceptance criteria. Do not treat a generic “whisker resistant” claim as an assembly-level guarantee. IEC 60068-2-82:2019
Original example: plating a blade that enters a slot
The uniform-build model uses the stated nickel and tin thicknesses on both broad faces. It excludes pretreatment removal, substrate tolerances and local variation.
| Quantity | Calculation | Result |
|---|---|---|
| Buildup per face | 2 + (8–12) µm | 10–14 µm |
| Added blade thickness | Twice the face buildup | 0.020–0.028 mm |
| Finished blade thickness | 0.500 + buildup on both faces | 0.520–0.528 mm |
| Total thickness clearance | 0.530 − finished blade thickness | 0.002–0.010 mm |
Omitting nickel would overstate the minimum clearance by 0.004 mm. Centered clearance per side would be half the total, but the calculation does not establish centering, insertion force, contact pressure, or wear allowance. Include real machining tolerances, forming-induced thinning, local maximum buildup, and measurement uncertainty before release. If the slot is also plated, its narrowing consumes additional clearance.
Acceptance and useful handoff
B545's public abstract identifies appearance, purity, local and mean thickness, coating integrity, adhesion, solderability, and hydrogen-embrittlement relief among its requirements. Local thickness and average thickness answer different questions; a reported mean does not establish coverage at a specified contact region. ASTM B545-22 abstract
A useful finishing order records four groups of requirements:
- Part and coating: substrate/condition, drawing revision, underlayers, tin composition and appearance, preservative or post-treatment.
- Geometry: significant surfaces, masked areas, local thickness limits, finished dimensions, rack/contact-mark allowances, and inspection locations.
- Function and aging: solderability or contact tests, handling/storage conditions, mating materials and cycles, and the state tested after forming or assembly.
- Reliability and delivery: whisker qualification scope and acceptance, lot traceability, sampling, packaging, and approved rework procedures.
Diagnose peeling, poor wetting, out-of-tolerance buildup, and whisker findings separately. Increasing thickness or adding an anneal changes the process and can affect dimensions or other materials; qualify the changed stack and component rather than applying an isolated remedy.