Tin Whiskers: What’s the Risk?

The adoption of lead-free technology, driven by legislative requirements and market forces, has involved significant changes in materials, processes, and supply chain in the electronics industry. The changeover has raised a number of concerns for the electronics industry in ensuring product reliability while maintaining reasonable costs.

As the EU’s Restriction of Hazardous Substances (RoHS) deadline of July 2006 approaches, the industry has predominantly settled on tin-silver-copper as the replacement for conventional tin-lead solder. To comply with RoHS, part manufacturers have sought lead-free finishes to replace the traditionally used tin-lead finishes. The finish selection is important in providing corrosion resistance, good solderability, and durable solder joints. Currently, selected lead-free finishes include pure tin, tin-bismuth, tin-silver, and tin copper, and to a lesser extent nickel-palladium-gold and nickel-gold. Due to its low cost and compatibility with existing solders, the pure tin and tin-rich lead-free alloys have been adopted by a significant portion of electronic part manufacturers.

Particularly among the high-reliability product community (space, military, and high-end computer servers and storage), the adoption of pure tin and tin-rich lead-free finishes has raised a reliability issue pertaining to the formation of conductive whiskers. In 2002, CALCE in collaboration with individuals from NASA, Boeing, Raytheon, and the U.S. Navy, issued an alert outlining the potential risks imposed by the selection of pure tin and tin-rich lead-free alloys. The concern prompted several industrial groups, including CALCE, to examine the phenomenon of tin whisker formation, the effectiveness of various risk mitigation strategies, and methods for quantifying the risk presented by the use of these finishes in electronic products. As a gauge to the level of concern, iNEMI has three working groups on tin-whisker-related issues, and CALCE maintains a mailing list of individuals (engineers, scientists, and managers) from more than 60 organizations who have participated in a weekly forum on whisker-related issues that has been ongoing for almost 3 years.

While the phenomenon of tin whiskers has regained considerable attention during the past 3 years, key questions related to the tin whisker growth mechanisms remain. Major issues include:

1. Wide proliferation of pure tin and tin-rich lead-free finished electronic components in the market.
2. No consensus on tin whisker growth mechanism.
3. No recognized method of accelerating whisker formation.
4. No guaranteed method of avoiding tin whisker growth.

As a result, the industry continues to be challenged in answering the basic question of quantifying the risk presented by this wide adoption of pure tin and tin-rich lead-free alloys.

What Are Tin Whiskers?

For the uninitiated, tin whiskers are spontaneous growths of tin from tin-finished surfaces such as those shown in Figure 1. These growths can be in the form of nodules, needle-like filaments, or odd-shaped eruptions. The needle-like, straight whiskers are a chief concern, because there is a higher chance for bridging adjacent electrical conductors. Such bridging may result in excessive current leakage and/or electrical shorts in the electronic system.

The phenomenon of tin whisker formation was first reported in the 1950s by the Bell Telephone Co.1 To date, a wide variation in the growth characteristics, including length (0.3 to 10 mm), growth rate, and incubation time (days to years), has been reported. While leadframe parts have received significant attention, other parts including connectors, cover plates, and fasteners must also be considered at risk if finished with whisker-prone materials such as tin, zinc, and cadmium

For tin whiskers to form, tin must migrate to the whisker site. However, it is unclear if the tin moves along the surface or from under the surface. It may very well come from both routes. Although the exact cause of whisker growth is not completely understood, it is widely believed that compressive stress within the tin finish influences tin whisker growth.2,3 Stresses within the finish can arise from:

• Intermetallic compound formation between the plating material and substrate, resulting in compressive stress within the plating;
• Mismatches in coefficient of thermal expansion (CTE) of the tin-based plating material and substrate, or underlayer;
• Presence of residual stress from electroplating process itself;
• Extrinsic compressive stress, such mechanical bending and forming.

Regardless of the surface finishing technology, namely electroplating, hot air leveling, or immersion tin, tin whiskers have been observed to form. However, electroplated surfaces are considered to be a higher risk, since the electroplating process involves various parameters affecting the level of stress in the plating deposit. Key parameters include plating chemistry, organic additives, current density of the plating bath, and bath contamination. While the effect of those plating parameters on tin whisker growth is not fully understood, some consider mechanical handling, which the finished surface may be subjected to after the plating, to be even more critical.

Intermetallic formation between the tin plating and substrate material appears to be a contributing factor. In the case of tin-plated copper substrates a dominant diffusion species, tin, migrates relatively quickly through grain boundaries and dislocations, or more slowly through the bulk, resulting in Cu6Sn5 intermetallic (IMC) formation at room temperature. The scallop-like Cu6Sn5 IMC with density lower than that of copper is known to give rise to compressive stress within the finish. The usage of underlayer, such as nickel, is a possible means to reduce the effect of intermetallic formation on copper leadframes, because tin-nickel IMC grows slower than tin-copper IMC. In fact, some part manufacturers have adopted a nickel underlayer and its beneficial influence for retarding whisker growth has been reported in various studies.4-7 In the case of tin-plated alloy 42 substrate, compressive stress within the finish may arise under thermal cycling, due to a larger CTE mismatch with pure tin.

Review of the Electronic Component Market

For the past 3 years, CALCE has tracked the selection of finishes by more than 100 part suppliers and has observed a continued increase in selection of pure tin and tin-rich lead-free alloys for lead finishes (Figure 3). Tin-rich finishes have been a preferred choice, mainly due to the low cost, good processability, good corrosion resistance, and compatibility both with conventional tin-lead and lead-free solders.8 The finishes offered by a variety of part manufacturers include pure tin, tin-bismuth, tin-copper, and tin-silver.

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