Metal Whiskers: How Tin Mustaches Grow on Metal Surfaces

Metal whiskers are thin, needle-shaped single crystals that grow out of certain metal surfaces and possess strength approaching the theoretical maximum for their material. Before materials scientists ever studied them deliberately, electricians ran into them by accident.

What are metal whiskers?

Metal whiskers are microscopic, filament-like crystals that sprout perpendicular to a metal surface, each strand measuring roughly 1–10 micrometres across and typically a few millimetres long, though some reach up to a centimetre. Their defining characteristic is that they are single crystals rather than the many-grained polycrystalline structure of ordinary bulk metal, and this near-perfect internal order is what gives them their extraordinary mechanical properties.

Tin whiskers: how electricians first noticed them

Tin whiskers were first observed by electricians, who saw that tinned (tin-coated) metal surfaces would sometimes grow tiny brushes of fine tin filaments under the influence of an electric field. Nobody paid these brushes much attention at first, since they seemed harmless, but they occasionally caused real trouble by short-circuiting parts of a circuit and knocking instruments out of service.

Failures in electronics and communication lines

During wartime, a considerable number of failures in communication lines were traced directly to the growth of these tin filament brushes. Because the whiskers bridged conductors that were meant to stay separate, they created unwanted short circuits inside electronic assemblies. Since they were causing problems, engineers had to fight them — and to fight a phenomenon you must first understand it, so systematic study began.

Studying the phenomenon led materials scientists to a surprising conclusion: rather than eliminating metal whiskers, the goal should be to make them grow deliberately. Two observations drove this reversal of thinking.

• Each whisker turned out to be a needle-shaped single crystal — one continuous crystal, as opposed to a polycrystal, which is an aggregate of many tiny crystallites.
• When the thinnest whiskers were bent sharply into an arc and then released, they sprang back to their original shape. There was no trace of plastic deformation whatsoever — the bending was purely elastic.

The structure of metal whiskers

The single-crystal nature of whiskers was straightforward to confirm, because modern X-ray analysis methods can precisely determine the internal structure of a material. What was genuinely puzzling was the behaviour of the tin whiskers, since it flatly contradicted everything known about ordinary tin.

The single-crystal nature of whiskers

A whisker's status as a single crystal explains why it behaves so differently from bulk metal of the same composition. In an ordinary polycrystalline sample the boundaries between grains, together with countless internal defects, provide easy paths for deformation. A whisker, being one flawless crystal with essentially no dislocations, has no such weak points, which is precisely why it can carry loads that would permanently deform ordinary metal.

Elasticity and the absence of plastic deformation

Tin is normally a highly plastic metal, so under a sharp bend ordinary tin would never return to its starting state — it would stay deformed. Detecting the whiskers' contrary, purely elastic recovery required no elaborate apparatus: only a conventional microscope, a miniature pair of tweezers, attentiveness, a capacity for surprise, and the ability to apply what one already knows. Nature evidently granted those qualities generously to two American scientists, K. Herring and J. Galt, who were struck by something other specialists had overlooked.

The discovery of ultra-high strength

Metal whiskers were found to withstand stresses roughly a hundred times greater than solid bulk tin, putting their strength close to the theoretical limit. Engineers had no difficulty calculating the stresses developed inside a whisker as it bends, and when that simple calculation was carried out the result was astonishing.

The Herring and Galt research (1952)

The report of this ultra-high strength appeared in 1952, and it immediately triggered an intense assault on the subject of whiskers. Herring and Galt's finding — that a tin whisker could reach nearly theoretical strength — reframed whiskers from an electrical nuisance into a materials-science opportunity worth pursuing across many different substances.

Comparing the strength of whiskers of different materials

Whiskers of different materials proved dramatically stronger than solid articles made from those same materials, each by a characteristic factor. The measured advantages included:

• Chromium whiskers — nearly 20 times stronger than the bulk material.
• Iron and silicon carbide whiskers — more than 40 times stronger.
• Aluminium oxide whiskers — about 200 times stronger.
• Quartz whiskers — roughly 350 times stronger.
• Graphite whiskers — more than 1,000 times stronger than solid graphite products.

Growing whiskers of different materials

Electron-microscopy and X-ray studies confirmed that the very thinnest whiskers contain practically no dislocations — and not only tin whiskers. Scientists began deliberately growing whiskers of many materials — iron, copper, nickel, graphite, aluminium oxide, magnesium oxide, silicon carbide, silicon nitride, boron carbide and many others — and patents on methods for growing them poured out as if from a horn of plenty.

Methods and patents for growing whiskers

The rush of patents for whisker-growth methods reflected how many candidate materials responded to the same techniques, and how strong the commercial interest had become. Growth was pursued through vapour deposition, electrodeposition and controlled crystallisation, but at industrial scale whiskers tend to form as a tangled mass resembling cotton wool rather than as neatly separated individual filaments — a practical complication the patents could not fully resolve.

How strength depends on whisker thickness

Ultra-high strength is found most often in extremely thin whiskers, around 1–2 micrometres across, and it falls as the whisker grows thicker. By a thickness of 10–12 micrometres the strength of a whisker is already almost the same as that of ordinary polycrystalline material. Plotted against thickness, the relationship is close to a hyperbola, which means strength decreases in inverse proportion to diameter.

Analysing growth conditions and the role of dislocations

The reason for this behaviour is that increasing a whisker's thickness increases the number of defects inside it. Careful analysis of whisker growth conditions showed that a whisker begins life as a very thin, micrometre-scale filament with an ideally smooth surface and no dislocations at all.

The mechanism by which defects form during growth

As new atoms settle onto the surface and fresh layers build up, the whisker loses its smoothness and develops so-called growth steps, which arise because atoms deposit onto different layers at different rates. The surface ends up looking like a carelessly pulled-on stocking, wrinkled and uneven.

These wrinkles are sources of weakening: they act as stress concentrators within the whisker, their presence being equivalent to fine notches cut into the surface, which naturally lowers strength. In addition, as the crystal grows larger it develops dislocations that concentrate mainly near its surface. Together these effects cause the strength of whiskers to drop sharply as their dimensions increase.

Practical applications of metal whiskers

Achieving theoretical strength even in such tiny objects electrified the materials-science community and set scientists and engineers searching for ways to use whiskers in real structures. You obviously cannot build a turbine blade from whiskers alone, let alone a rocket or an aircraft, but the idea of using them as reinforcement to strengthen metals and alloys was compelling.

Whisker-reinforced composite materials

Whisker-reinforced composites embed oriented whiskers in a matrix so that even if each whisker's full strength is not realised, the overall system is far stronger than ordinary material. Intense efforts followed to introduce whiskers into aluminium, nickel, titanium and heat-resistant alloys, laying them, for example, along the direction of tensile loading. Laboratory composites based on aluminium, manganese, nickel and other metals have been made this way, but full success in industrial production has so far proved elusive because of several stubborn difficulties:

• The strongest crystals must be selected from the mass, and they are relatively few; thick whiskers are weak and irrational to use as reinforcement, yet at industrial scale whiskers grow as a cotton-wool-like tangle from which individual strands must somehow be pulled and tested.
• Methods must be developed to orient the whiskers within the metal so that they lie along the lines of maximum stress in the part.
• Above all, a whisker is only strong while its surface stays perfectly smooth, and embedding it in metal risks damaging that surface — through attack by the molten metal, through the external pressures needed to form a composite without melting, or simply through whiskers rubbing against one another. When the surface is damaged the strength collapses, and the whiskers' behaviour inside the composite falls far short of what was hoped.

Polymers are more forgiving hosts for whisker reinforcement than metals, since they damage the whisker surface far less. The trade-off is that polymer composites cannot exploit the whiskers' most valuable qualities, such as high heat resistance and scale resistance.

How to combat unwanted whisker growth

Combating unwanted whisker growth — the original electrical problem — remains as relevant as harnessing whiskers deliberately, because tin whiskers can still short-circuit modern electronics. Practical countermeasures used in the electronics industry include adding another element to tin coatings, applying conformal coatings over solder joints, providing adequate spacing between conductors, and annealing or reflowing tinned surfaces to relieve the internal stresses that drive filament growth. Understanding the growth mechanism — the accumulation of surface steps and near-surface dislocations — is what makes such preventive strategies possible.

The role of metal whiskers in the development of composites

Wide industrial use of metal whiskers is still hard to expect today, yet studying them played an important role in the development of composites. Research into whiskers acted as a catalyst that sharpened researchers' interest in creating materials in the form of fine threads and using them to strengthen polymers and metals.

In the course of this work scientists concluded that not only short whisker-like crystals but many materials shaped into thin threads possess very high strength. Such threads are less strong than metal whiskers, but if they are produced as continuous fibres rather than short segments they become far more manufacturable, easier to process, and better able to realise their strength — ultimately making it possible to obtain reinforced composites with strength far exceeding that of existing structural materials.