Tin Whiskers

The term 'Tin Whiskers' refers to 'needle-like' crystalline structures of tin (Sn) that form and grow on surfaces that use pure or nearly-pure tin as final finish. Tin whiskers commonly (but not exclusively) appear as thin strands of tin, and can indeed look like whiskers, hence its name. Other metals such as zinc, cadmium, indium, and antimony also exhibit this whisker-growing phenomenon.

Tin whiskers have been observed to grow to several millimeters in length, with records showing them attaining lengths of up to 10 mm in rare instances. Whisker diameters, on the other hand, can go up to as high as 10 microns.

Tin whiskers are highly undesirable in the external pins or leads of semiconductor devices, since they can bridge two adjacent leads together and form an electrical short. The short will be transient if the resulting current flow is enough to 'fuse' open the whisker. Otherwise, the short will be stable and can result in real device failures.

The problem of tin whiskers is not a new phenomenon, having been documented as early as the 1940's. Its resurgence as a critical issue in the semiconductor industry, however, was heightened by recent efforts of the industry to move away from the use of lead (Pb) in its manufacturing processes. Early explorations revealed pure or nearly-pure tin systems to be viable alternative Pb-free lead finish materials. Their disadvantage, of course, is their tendency to exhibit tin whiskers.

Not all tin whiskers look like whiskers, and even those that do also vary in form - they can be straight, kinked, hooked, or forked. Those that do not look like whiskers at all can appear as nodules or in pyramidal structure. A word of caution though - many people confuse tin whiskers with a more commonly-encountered attribute, i.e., dendrites, so novice engineers must be trained to distinguish between the two.

Figure 1. Photo of a straight filament whisker;

source: http://nepp.nasa.gov

Dendrites exhibit fern-like or snowflake-like patterns that propagate along the surface, whereas whiskers protrude out of the surface. Dendritic formation involves the dissolution of the metal atoms in moisture and their redistribution on the surface under the influence of an electric field, such as when the device is biased.

The amount of time needed for whiskers to grow varies as well from just a few days to a few years, with reported growth rates ranging from 0.03 mm to 0.9 mm per year. This is one reason why whiskers are a major reliability concern - they can not be screened out at t=0 and can appear when least expected.

There is still a lack of thorough understanding as to why whiskers form and grow. In fact, many independent studies on the whisker phenomenon have yielded contradictory results, underscoring the fact that whisker formation mechanisms are complex phenomena.

A common and widely-held explanation for the formation of whiskers states that the phenomenon is a stress-relief mechanism. According to this theory, the tin layer or deposit becomes subject to internal residual stresses once the tin plating process is completed. These residual stresses are reduced by whisker formation. The origin of these internal stresses is discussed in the next paragraphs.

As soon as Sn is deposited over the copper leadframe, an oxide layer starts to form over the deposited Sn layer. At the same time, Cu atoms from the substrate start to diffuse into the Sn layer, forming Sn-Cu intermetallics. Since the oxide layer over the Sn coating impedes outward movement of the Sn atoms, this process of Sn-Cu intermetallic formation builds up internal compressive stresses within the Sn layer as more Cu atoms diffuse into the same volume of the Sn coating.

Eventually the stress becomes too large that excess Sn material begins to extrude at the weakest points of the oxide layer. This protrusion originates as an Sn nodule over the oxide, which eventually grows into an Sn filament or 'whisker', as it is more commonly referred to.

Figure 2. Photo of a 'bend' whisker (left) and a 'nodule' whisker (right);

source: http://nepp.nasa.gov

This process has been known to be driven by the following factors: 1) application of internal and external (compressive) stresses, e.g., trimming and forming of the leads ; 2) thickness of the coating; 3) structure of the crystal; 4) substrate used; 5) temperature; and 6) humidity. Thus, the presence of contaminants does not directly induce the formation of whiskers, unless it results in a change to any of the factors above.

The risks posed by whisker formation generally fall under four categories: 1) stable short circuits in low-current circuits (low voltage, high impedance); 2) transient short circuits; 3) metal vapor arcing, wherein the whisker is turned into a highly-conductive plasma for several seconds (if the conditions for sustaining the metal vapor arc are met), consuming adjacent materials as it conducts hundreds of amperes; and 4) debris or contamination.

Tin whisker formation may be avoided by not using pure tin in the lead finish process. Adding about 3% of Pb by weight to Sn greatly reduces the occurrence of whiskers, while using 5% Pb virtually eliminates it. Then again, the industry is moving away from the use of Pb, so other Pb-free lead finish alternatives must be explored. Please see the article "A Pb-free Semiconductor Industry" for more on this.

The industry has yet to come up with a single standard for acceptance testing of lots in relation to whisker formation. Recently, however, the National Electronics Manufacturing Initiative (NEMI) has offered the electronics industry a revised set of proposed recommendations for "tin whisker acceptance test requirements and acceptance criteria for evaluating devices with tin finishes."

The revised acceptance test requirements have already been submitted to both IPC and JEDEC for approval and subsequent release as a formal standard or guideline for tin whisker acceptance testing for the industry.

See also: Lead Finish; Pb-free Manufacturing

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