Tin whiskers represent a serious threat to the reliability of modern electronics. These crystalline structures of tin grow from surfaces finished with pure or nearly pure tin plating. The phenomenon, first documented in the 1940s, became a major concern following the industry-wide shift toward lead-free materials mandated by regulations like the Restriction of Hazardous Substances (RoHS) directive. Their presence can compromise the functionality of devices ranging from consumer electronics to mission-critical aerospace systems.
The Physical Structure of Tin Whiskers
Tin whiskers are electrically conductive, hair-like filaments composed of single crystals of pure tin. They emerge from the tin layer on component leads and printed circuit boards. Their appearance is often straight, but they can also be kinked, hooked, or forked, sometimes with striations along their length.
These structures are exceptionally small in diameter, typically measuring only a few microns, though reports range from less than 100 nanometers up to 10 micrometers. In contrast to their thinness, whiskers can achieve surprising lengths, often reaching several millimeters and, in rare instances, exceeding 10 millimeters.
Understanding the Growth Mechanism
The fundamental reason tin whiskers form is the spontaneous relief of internal compressive stress within the tin plating layer. This stress acts as the driving force, causing tin atoms to migrate and extrude outward from the surface to an unstressed region. This growth continues until the local stress is relieved or the supply of tin atoms is exhausted.
A primary source of this internal stress is the formation and growth of intermetallic compounds (IMCs) at the interface between the tin plating and the underlying base metal, such as copper. As copper atoms diffuse into the tin layer, they react to form IMCs like Cu₆Sn₅, which occupy a greater volume than the original materials. This volumetric expansion creates a localized pressure that pushes against the surrounding tin layer.
The stress can also be generated by other factors, including the mechanical strain from component mounting, thermal expansion mismatches between the tin and the substrate, and the formation of a tin oxide layer on the surface. Tin atoms diffuse along grain boundaries to relieve this pressure, culminating in the outward growth of the whisker from the base. Whisker growth can begin anywhere from days to years after manufacturing, following an initial incubation period.
How Whiskers Lead to Electronic Failure
Since tin whiskers are electrically conductive, a whisker that bridges the gap between two adjacent conductors, like component leads or circuit traces, creates a short circuit. This risk is particularly high in the miniaturized geometry of modern electronics where conductive pathways are closely spaced.
In low-voltage, high-impedance circuits, the resulting short circuit can be stable and permanent, as there may be insufficient current to melt or fuse the whisker. Conversely, in high-current or high-voltage applications, the shorting event can be far more destructive. When the whisker fuses open due to excessive current, it can vaporize the tin, initiating a highly conductive plasma known as a metal vapor arc.
Whiskers can also break off due to vibration or mechanical stress, creating conductive debris that migrates across the circuit board. This debris causes intermittent failures or short circuits remote from the original growth site, making the cause of the system malfunction difficult to diagnose.
Controlling Whisker Formation
One highly effective method is to use tin alloys instead of pure tin, most notably by reintroducing a small percentage of lead, typically 3% or more by weight. The presence of lead disrupts the crystalline structure of tin and alters the stress-relieving diffusion process, significantly reducing whisker propensity.
For applications that must remain lead-free, manufacturers can apply a nickel underlayer between the copper substrate and the tin plating. This barrier layer slows the diffusion of copper atoms, thereby reducing the rate of IMC formation and the internal compressive stress that drives whisker growth.
Another technique involves annealing, a heat treatment process applied to the tin plating after deposition that relieves some of the residual stress and helps to control the initial formation of IMCs. The application of a conformal coating, a thin, non-conductive polymer layer over the component, is a physical mitigation strategy that encapsulates the whiskers, preventing them from bridging conductors.