Tin whiskers are microscopic, hair-like metallic filaments that spontaneously grow from tin surfaces, particularly electroplated or coated tin used in electronics.
They are single-crystal structures of β-tin, electrically conductive, and can grow without external stress—making them a serious long-term reliability risk in electrical and electronic systems.
Basic Characteristics
| Property | Description |
|---|---|
| Material | β-Tin (White Tin) |
| Length | Microns to several millimeters |
| Diameter | 0.5 – 10 μm |
| Growth Time | Days to years |
| Nature | Unintended metallurgical phenomenon |
Chemical Composition
| Element | Content |
|---|---|
| Tin (Sn) | ~100% |
| Impurities | Trace (Pb, Cu, Zn, Ni, Bi – coating dependent) |
| Oxides | Minimal |
Tin whiskers are nearly pure, single-crystal tin and are not alloys or coatings themselves.
Mechanical Properties (Micro-scale)
| Property | Typical Behavior |
|---|---|
| Tensile Strength | Very high (relative to bulk tin) |
| Elastic Modulus | ~50 – 70 GPa |
| Ductility | Limited |
| Brittleness | Moderate |
| Fracture Mode | Brittle snapping |
Physical Properties
| Property | Value |
|---|---|
| Electrical Conductivity | High (metallic) |
| Density | ~7.31 g/cm³ |
| Melting Point | 231.9 °C |
| Magnetic Behavior | Diamagnetic |
| Thermal Stability | Poor at elevated temperature |
Root Causes of Whisker Growth
| Cause | Explanation |
|---|---|
| Residual Stress | Introduced during electroplating |
| Intermetallic Growth | Cu₆Sn₅ formation under tin coatings |
| Oxide Stress | SnO₂ surface film |
| Compressive Stress | Coating thickness & constraint |
| Grain Structure | Columnar grains promote whiskers |
Key Risks of Tin Whiskers
✔ Electrically conductive
✔ Can bridge gaps under 1 mm
✔ Grow unpredictably
✔ Can detach and migrate
❌ No mechanical function
❌ Severe risk of electrical short circuits
❌ Latent system failure without warning
Mitigation Techniques
| Method | Effectiveness |
|---|---|
| Tin-Lead Alloy (SnPb) | Excellent (lead suppresses growth) |
| Annealing | Reduces residual stress |
| Nickel Underlayer | Blocks intermetallic formation |
| Conformal Coating | Partial protection |
| Alloyed Tin (Sn-Bi, Sn-Ag) | Reduces risk |
| Thick Tin Coating (>15 μm) | Slows growth |
Standards & Guidelines
JEDEC JESD201 – Tin whisker mitigation
IPC-A-610 – Electronics acceptability
NASA Tin Whisker Control Guidelines
IEC 60068-2-82 – Whisker testing
Summary
Tin whiskers are a dangerous, spontaneous metallurgical phenomenon associated with pure tin coatings, particularly in lead-free electronics. Their ability to grow silently, bridge conductive paths, and cause catastrophic failures makes them a critical concern in aerospace, medical, nuclear, and defense electronics.
Engineering rule:
You never choose tin whiskers — you design systems to prevent them.