Tin whiskers: what they are and how to reduce the risk
Tin whiskers are extremely thin metal filaments that can grow spontaneously on tin-plated surfaces, even after months or years. With the switch to lead-free they have become a reliability issue in electronics. This guide explains where they come from and which measures reduce the risk.
Dickmann Srl · updated
What they are
A whisker is a thread-shaped single crystal of tin, a few micrometres in diameter and up to millimetres in length. It grows outwards from the surface, without the need for current or humidity. The risk is electrical: a whisker can create a short circuit between adjacent conductors, for example between the leads of a fine-pitch component.
Where and why they grow
The phenomenon mainly affects electroplated pure tin finishes on components, connectors and terminals, rather than solder joints. The main cause is compressive stress in the tin layer, which the metal relieves by pushing out the filaments.
| Source of stress | Example |
|---|---|
| Growth of intermetallic compounds | Between tin and the copper substrate, forming slowly even at room temperature |
| Mechanical stress | Pressure on press-fit connectors, bending of leads |
| Thermal cycling | Different expansion of tin and substrate |
| Corrosion and oxidation | Humid or aggressive environments |
| Type of deposit | Bright tin finishes, with fine grain size and organic residues |
How to reduce the risk
To assess finishes, the industry uses specific test methods and acceptance criteria, such as those of the JEDEC whisker standards: critical applications must be qualified with the component supplier.
- Matte tin finishes instead of bright ones.
- A nickel underlayer between copper and tin, which slows the formation of intermetallic compounds.
- Heat treatment after deposition, according to the electroplating process specifications.
- Reflowing the finish, which changes its structure.
- Conformal coating on the boards, which hinders the growth and contact of the filaments.
- In applications exempted from RoHS, tin-lead finishes with at least 3 % lead.
What about solder joints?
Solder joints in alloys such as SAC305 or Sn99.3Cu0.7 are much less prone to whiskers than pure tin finishes, because they are reflowed and far thicker than an electroplated finish. As an electroplated finish, however, tin-copper is more prone to whiskers than matte tin. The risk should be assessed above all on component finishes. For electroplating, see our pure tin anodes; for solder alloys, our lead-free alloys.