Circuits Assembly Online Magazine - Tin Whiskers and Conversion to Pb-Free

A low-lead-content SnPb finish reduces whiskers while cutting lead volume by 85%.

Current SnPb finishes, although virtually whisker-free, do not meet the lead limits imposed by the RoHS directive, forcing component manufacturers to search for alternatives. Many have elected pure tin or alloys of tin containing bismuth, silver or copper as a component lead termination. (NiAu or PdNiAu finishes are whisker-free, but are cost prohibitive for most manufacturers.)

FIGURE 1: Sn-plated connector pins after 10 years (courtesy NASA GSFC).

Electroplated tin on component leads and other hardware introduces a finite, long-term reliability risk through its propensity to grow whiskers. The tin electroplated finish has been proven unreliable, and is sited in many well documented failures as a root cause.¹ Tin, zinc, cadmium and silver are known to exhibit an odd metallurgical surface phenomenon of whisker formation.

Despite intense technical research, the exact whisker growth mechanism is not fully understood and acceleration factors for whisker growth not fully defined. Absent a clear-cut solution, manufacturers have devised a variety of finishes without fully addressing the whisker risk. Tin plating is being phased in, and traditional SnPb finishes discontinued or shelved. In many instances, the vendor is not changing the component part number, making it much more difficult to determine if the component is plated with pure tin (whisker risk) or SnPb (virtually whisker immune).

The consumer industry, with shorter product lifecycles than military, automotive or telecom, does not view whiskers as a big issue. Component users that demand the highest degrees of reliability – military, aviation and medical electronics; products used in critical missions or life/death situations – are concerned, however. Without a solution, telecom, high-end computing and industrial machinery manufacturers will be also be assuming risks.

A number of investigations are trying to find the fundamental growth mechanism of whiskers. The data from these scientific studies point out several different factors and causes of the whisker, but there is no scientific consensus on whisker formation and growth fundamentals.² The most commonly cited driving force for whisker formation is a buildup of compressive stresses in the plated tin layer. Possible origins of the compressive stresses include: plating process, intermetallic growth, grain size, surface damage, environmental stresses and CTE mismatch. Based on this understanding, several mitigation strategies are being implemented:

Underplate copper leads with a nickel layer as barrier layer to diffusion.
Low stress tin finish.
Annealing heat treatment.
Tin surface reflow or fusing to relive stresses.
Solder dipping.
Conformal coating.

Solder dipping and conformal coating offer reasonable protection against whiskers, but cannot be implemented in all types of hardware due to cost and process concerns. Remaining mitigation strategies are fundamentally sound, yet lack experimental proof of effectiveness over time. With no verified acceleration factors, no standard set of tests can accelerate whisker formation and growth with any reasonable degree of certainty. Past data show that room temperature conditions are best to grow tin whiskers. This means one cannot accelerate the growth of whiskers in the laboratory, and testing parts for 25-year life means waiting 25 years. Recently, iNemi proposed a test to accelerate whisker growth,³ but the validity of the test to promote and accelerate whisker growth remains unproven (Ed.: see related iNEMI article, p. 42). Current mitigation strategies do not permit whisker growth in the first two to three years. How effective these strategies are over 25 years and whether they eliminate whisker growth over the long-term remains to be seen.

The brunt of this uncertainty is borne by high-end users. To mitigate whisker-related risks, the technical community in the defense industries has proposed draft standard/guidelines. iNEMI and the AIA LAEWG/GEIA documents recommend restrictions on Sn-plated components for whisker risk level IV and V electronics. Dependence upon COTS suppliers and the phaseout of SnPb finish parts will force users into the cumbersome task of identifying, screening or refinishing Sn-plated parts to meet acceptable standards. Replacement will not only raise costs but create a handling, tracking and documentation nightmare for DoD suppliers.

Alternative options which will meet historical reliability standards and achieve environmentally significant reductions in lead content must be evaluated. Many promising mitigation strategies have been proposed, but hard experimental data or tests to recommend long-term use do not exist.

As a last resort, consider a low-lead-content SnPb finish. There is evidence that a tin alloy containing greater than 4% lead greatly reduces the tendency to form whiskers, and whiskers that do form are very small. If this type of plating is accepted universally, it will reduce the amount of lead by ~85% compared to the current SnPb finish. The low-Pb finish is low cost, can be easily implemented and promises to lower whisker risks significantly (without the need for extensive verification).

This has been suggested before, without much traction. In 1999-2000, when this idea was initially floated, it was not known what the EU would allow, or how exemptions would be handled. Researchers viewed whiskers as a problem that could be resolved, given time and effort. The most affected industries are not required to convert to Pb-free, and did not recognize that conversion would encompass most components anyway.

Reintroduction of low-lead tin plating seems like a step backward, but it is a step in a safer direction while meeting the push for reducing lead. This approach needs to be pursued on both legislative and technological fronts. It is not unusual for the EU Commission to grant an exemption for the technological problems that do not have a solution. Last year the EU exempted the use of high (more than 85%) lead solders, which melt around 300°C, since no suitable technological alternative exists. In our technical opinion, a similar case can be made of proving technological and reliability issues surrounding the use of whisker-prone pure tin plating.

Lack of a reliable and provenreplacement finish, and the technological quagmire associated with testing, can be presented to the EU Commission and a case can be made to request a temporary exemption (until 2010) for use of reduced Pb finishes. In the next five years, a lot of needed data regarding the validity of accelerated tests and the effectiveness of many mitigation techniques will come available. This will allow us to choose the optimum mitigation technique. Simultaneously, we can work on determining the lowest lead content necessary to minimize whiskers. If in five years no solution to tin whiskers emerges, the tin with low-Pb finish could remain as a reliable and environmentally sound solution.

References

NASA Goddard Space Flight Center Whisker Failures, http://nepp.nasa.gov/whisker/failures.
iNEMI Tin Whisker User Group, “Interim Recommendations on Lead-Free Finishes for Components Use in High Reliability Products,” March 2004.
iNEMI Tin Whisker Accelerated Test Project, http://thor.inemi.org/webdownload/projects/ese/Tin_whisker_test_methods.pdf.

Bibliography

NASA Web reference on whiskers, http://nepp.nasa.gov./whisker.
iNEMI Web reference, inemi.org/projects/ese/.
Ja y Brusse et al, “Tin Whiskers: Attributes and Mitigation,” CARTS 2002: 22^nd Capacitor and Resistor Technology Symposium, March 2002.
M . Osterman, “Tin Mitigation Strategies for Tin Whiskers,” calce.umd.edu/lead-free/tin-whiskers/TINWHISKERMITIGATION.pdf.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.