Unfortunately, both the electroplated pure tin now predominantly being coated on components and the immersion tin now being applied to Pb-free Printed Wiring Boards (PWBs) are susceptible to the electrically conductive tin whisker growths. Since the tin whiskers are documented to conduct several milliamps of electrical current without fusing (melting), they have caused short circuit failures of both military and commercial electronics (e.g. satellites, radars, heart pacemakers, etc) that utilize tin coated parts (Pb-free electronics). Tin whiskers can also vaporize at higher currents and voltages, such as in power supplies, creating a tin plasma that can conduct many tens or hundreds of amperes, thus causing catastrophic damage to the power supply.

Even though the military is not bound by the RoHS law, the military must use, in many cases, the same types of tin-plated components demanded by the vastly larger commercial electronics customer base. Commercial electronics now constitute approximately 95-to-98 per cent of the worldwide electronics market, while the RoHS exempt military applications represent the other 2 to 5%. Thus, tin-lead plated components are becoming less and less available from the component and PWB vendors as the vendors convert to pure tin plating. The military simply does not possess the economic leverage to influence the type of surface finishes offered by the vendors.

Tin whisker growth has become a prime source of risk with the switch to pure tin plating for lead-free (RoHS compliant) component terminals and circuit boards.

Tin whisker growth is not a new problem (figure 1). The Bell System documented the growth of tin, cadmium, and zinc whiskers and subsequent short circuit failures, on electroplated hardware in telephone switching equipment as early as 1946.(1) After extensive study, Bell Laboratories recommended alloying the tin with lead, and the result has been essentially whisker free electronic assemblies, using tin-lead alloy plating on component leads and circuit boards and tin-lead solder attachment in assembly, for the last 60 years.

Figure 1. Early photograph of spontaneously growing, tenth-inch long single crystal tin whiskers from Bell Laboratories investigation of electrical short circuit failures from tin plated details in telephone switching equipment in the 1940s.

From the 1940s until the last decade of the twentieth century, tin-lead electroplate and tin-lead solder were routinely used for electronic assembly, with virtually no chance of tin whisker failures because of the Pb content. The history since then begins to show the growing menace and attempts at mitigation.

In 1993, M. E. McDowell of the United States Air Force (USAF) (2) outlined the method used by the USAF in dispositioning tin (Sn) plated parts in inventory. No position was taken relative to the prohibition of pure Sn usage (as previously recommended by Dunn of the European Space Agency(3)). This would prove to be an unfortunate situation, as later events were to show, relative to reliability failures on USAF electronic equipment. Between 1990 and 2004, Brusse from NASA compiled a list of high reliability system failures caused by metal whisker growth. Tin, Zinc, and Cadmium are all highly prone to whisker growth. This list of failures in the time period from 1990 to 2004 counts scores of well-documented whisker failures in electronic assemblies, from heart pacemakers to NASA and commercial satellites, and many millions of dollars in monetary losses. A typical tin whisker shorting issue is depicted in Figure 2 (After Brusse/NASA ref. 4).

Figure 2. Tin whiskers causing electrical shorts between connector pins after 10 years in service. (Photo courtesy of NASA, 2003)

This increased incidence of whisker failures has not gone unnoticed by the scientific community.

In the first two years of the 21^st century, there were more presentations/papers on Sn whisker matters than in the prior 15 years. The advent of lead free electronics manufacturing (in response to environmental legislation) undoubtedly accounts for much of this renewed interest. Many commercial and military organizations have begun consortia and R&D programs to define mitigation techniques that might be able to stem the tide of whisker failures. As of this writing, no conclusive set of mitigation techniques is proven, and so the only accepted mitigation techniques for tin whiskers is to electroplate component terminals with a solderable metal other than pure tin. Immersion Silver, Electroless Gold, Nickel-Palladium, all can be theoretically substituted for tin, but at significantly higher cost than electroplated tin, so that electroplated tin is by far the most common RoHS-compliant replacement for the former SnPb electroplate.

Although the whisker prone, electroplated pure tin finish is commonly found in COTS electronic hardware, it is showing up in military electronic assemblies at an alarming rate. Because of this "inconvenient truth", as the tin whisker risk has been called, many attempts at mitigation of these risks have been made. An example of a recent tin whisker event, on the NASA Space Shuttle, is depicted in figure 3.

Figure 3. Tin whiskers growing on NASA Space Shuttle avionics hardware and documented in April 2006. (Photos courtesy of NASA, 2006)

It should be noted that these tin whiskers were not growing from tin plated electrical components, since the Space Shuttle avionics manufacturing pre-dated RoHS by a significant number of years, but from tin plated beryllium copper card guides, and may have been growing for years. Today's lead free electronics would be expected to have far more tin whisker susceptible components, since any tin plated (RoHS compliant) electronic component could be a source of whiskers, as well as any tin plated mechanical hardware.

Sixty years ago, the most commonly used surface finish for electronic components and printed wiring boards was tin-lead (SnPb) solder. The composition of this solder was most often "eutectic," or 63% tin (Sn) and 37% lead (Pb), both by weight. This composition was ideal for the soldering process, while the Pb content suppressed (virtually completely eliminated) tin whisker growth on the tin-lead (SnPb) coated electronic hardware.

White Sn (β tin), the allotropic phase of tin constituting the tin whisker, has a body centered tetragonal crystal structure. The other allotropic phase, grey tin (α tin), has a diamond cubic structure to the crystal unit cell. The c/a ratio for the white tin (β Tin), is less than one, meaning that the unit cell for the tetragonal white tin is longer on one side than the other (rectangular in cross section). This configuration of a crystalline material (i.e. not cubic) is usually an indication of anisotropic properties. For tin, the coefficient of thermal expansion (CTE) and the self diffusion coefficient are higher in the "a" direction (longer side of the unit cell) than in the "c" direction (shorter side of the unit cell). Other metals that have such an anisotropic crystal structure (c/a ratio less than one), such as Zinc and Cadmium with their hexagonal close packed (hcp) structures, also form whiskers readily (figure 4). In fact, the first well-documented occurrence of whiskers was on cadmium plated military hardware in 1948. Another common failure is for the zinc-chromate coated steel supporting structures for raised floor laminar flow clean rooms to grow spontaneous zinc whiskers over time, resulting in conductive particle contamination of the laminar air flow in the clean room.

Figure 4. Similarity between the tin and zinc spontaneous whisker growths. Both are conductive single crystals (From J. A. Brusse, NASA, 2003 reference 4)

Sn is known to have several active slip systems and direction along with an active twinning system, (Table 1). Multiple growth directions for Sn whiskers have been found during investigations of whisker growth.

Investigations of high strain deformation of Sn in a "Deformation Processed Metal-Metal Matrix Composite" (DMMC) shows Sn takes on a <001> texture. It’s assumed that this texture is brought about by the Sn deforming in a plane strain condition (6). Several authors investigating Sn whiskers have seen a similar texture in the Sn single crystals. Many researchers have cited the <001> direction as a predominant growth direction for tin whiskers.

Other sources of internal stress in tin electroplate are intermetallic compounds formed from the base metal onto which the tin is electroplated. For example, the tin is often plated onto a copper component terminal lead or printed circuit trace. An intermetallic compound, Sn₆Cu₅, or in some cases, Sn₃Cu are readily formed, and cause internal compressive stress in the tin layer as the intermetallic compound grows. This internal stress can accelerate whisker growth. A mitigation method for this effect is to electro- or electrolessly plate a nickel diffusion barrier between the tin and copper layers.

Hot-dipped or reflowed tin or zinc tends to grow fewer and shorter whiskers than electrodeposited tin as well, once again so long as precautions are taken to avoid compressive-stress-causing intermetallics (such as Cu₆Sn_5, common on the copper tin plated copper lead frames used in electronics).

There are two areas of tin surface finish that pose a much more severe risk for the long product life, high-reliability DoD (Department of Defense) applications than for the relatively short product lifetime commercial electronics world. While product lifetimes for military systems are routinely 30 to 40 years, there are few commercial products expected to last as long as 10 years, with average product life expectancies nearer to 3 years, and commercial product warrantees 90 days or less.

The first area of concern is the tendency of tin whiskers to grow for extremely long periods of time, long past the obsolescence (and replacement) timeframe of commercial electronics. Figure 5 is an electron microscopic picture of a circuit board surface having immersion tin plating (a common lead-free alternative finish) after 5 days of room temperature storage at ACI Technologies. Note the small, 10 to 20 micron tin whiskers that are present. These whiskers are too small to be a serious risk, even to fine pitch circuitry. Figure 6 shows the same sample after two and a half years of storage (long time for a commercial application, but a short time for a DoD application) showing some tin whiskers that have grown to over 90 microns (long enough to electrically short closely spaced modern electronic circuits). After 5 years, whiskers over 150 microns long (figure 7) were found on the same sample, implying continuous whisker growth over time.

Figure 5. Scanning Electron Micrograph of Tin Whiskers grown on immersion tin coating after only 5 days from the tin plating of storage at ACI in October of 2003.

Figure 6. Scanning Electron Micrograph of same sample (different field, equivalent area) stored at ambient at ACI, micrograph taken in 2006 (3 years after plating the immersion tin).

Figure 7. Scanning Electron Micrograph of same sample (different field, equivalent area) stored at ambient at ACI, micrograph taken in February 2009 (5.3 years after plating the immersion tin)

The literature contains abundant misinformation about tin whiskers. Many component and/or PWB vendors have claimed their pure tin (RoHS compliant) coatings are immune to whisker failures because of various "mitigation" techniques. Since none of these vendors have inspected product for tin whiskers for 20, 30 or 40 years, any such claims by vendors who have examined experimental product for less than two years are suspect. In fact, many of the commonly held (and often cited by vendors) "mitigation techniques," such as nickel under plating of the tin, annealing of the tin plate, use of "matte" rather than "bright" tin electroplate, have all been seen to allow tin whisker growth if observed for a couple of years.

The universally accepted driving force (perhaps not the only driving force) for tin whisker growth is internal compressive stress in the tin coating. Such stress might be due to the initial electroplating conditions used in the deposition of the tin (such as current density, tin concentration in the electrolyte, organic "brighteners" used in the electroplating process, mechanical scratches, or bending of the tin plated article, or even the growing of intermetallic compounds such as copper-tin (Cu₆Sn₅) within the tin coating. This intermetallic generation of internal stress is cited as potentially creating compressive stress over very long times after the plating as copper in the base metal substrate slowly diffuses into the tin coating.

It is commonly accepted that whisker growth follows a two part mechanism:

1. Diffusion of tin from anywhere in the tin plate to the site of the whisker, and
2. Incorporation of the tin atoms into the crystal lattice of the tin whisker.

Diffusion is encouraged by 1) temperature high or low enough to be a significant fraction of the melting temperature at the high end, and absolute zero at the low end, and 2) a stress gradient whereby the tin atom diffuses from a place of high free energy (compressively stressed tin plated surface finish) to low free energy (a perfect crystal of β tin) within the tin whisker.

Neither of the diffusion enhancing/impeding parameters can be readily mitigated. First, temperatures at which electronics operate are rarely high or low enough to affect the diffusion coefficient of tin (melting point 231 °C) appreciably.

Second, even though the internal compressive stress in the plated tin can be eliminated or caused to become tensile at the initial deposition of the tin coating film, the long term generation of intermetallic compounds in the tin coating on any of the electronic component lead frame alloys can eventually, often over very extended times, re-generate internal compressive stress in an initially stress-free tin finish coating. So, diffusion is not something that can be controlled on a practical, long term basis.

The second half of the mechanism for whisker growth—incorporation of the tin atom into the whisker crystal -- could possibly be prevented by coating the whisker, or the re-crystallized grain of the tin plate that provides the first few layers of perfect crystal in the whisker, with a conformal coating of a high elastic modulus material. If the elastic modulus of the conformal coat is higher than the modulus of tin, then the incorporation of the additional tin atoms may not be possible, because the elastic modulus of pure tin would have to be exceeded to add additional tin atoms (and therefore whisker crystal volume, to the whisker) thus "stretching the coating. Of course, the elastic modulus of the standard conformal coating materials (acrylics, silicones, epoxies, Parylene, and urethanes are all thousands of times too low, compared to the modulus of tin, to do this.

There is also a third approach possible to mitigate whisker risk, also the subject of an MDA Phase II SBIR, and this would be not to attack the growth mechanism of the whisker at all, but conformally coat the electronics with a tough viscoelastic conformal coating that tents the growing tin whisker and causes Euler buckling of the whisker rather than allowing the growing whisker to puncture the coating. Tin whiskers have been shown to puncture and grow through all standard conformal coatings in a matter of months or a few years.

The coating that is documented to exhibit higher elastic modulus than tin metal is called ALD Cap (applied to the completed CCA (Circuit Card Assembly) as a conformal coating by Atomic Layer Deposition). The ALD Cap material is a ceramic material with a much higher elastic modulus than tin metal.

The viscoelastic conformal coating shown to cause Euler buckling of growing whiskers is called Whisker Tough P1.

Several projects are underway and/or proposed by the EMPF and partner organizations to identify valid long term mitigation and/or complete prevention of tin whisker growth on RoHS compatible (Pb-free) electronic hardware, making use of COTS (Commercial Off the Shelf) electronic components by the military a less risky proposition than it is now.