A review of available assessment tools and their pros and cons.

When mitigating and managing tin whisker risk, consider two distinct points in the product cycle. One is a new system with zero-storage life. Another (often overlooked) point is the time spent in storage before active service begins. There can be a loss of service life and an increase in system failures caused by tin whiskers that grow while a system is in storage prior to activation. This affects spare parts, backup systems and repair parts.

The risk of failure with tin whiskers is usually associated with Sn-coated parts, Pb-free solders and components. If the assembly does not have pure tin component finishes and uses eutectic SnPb solder, tin whiskers are not usually a concern.

A tin whisker is a filament of single-crystal tin that can grow directly from tin or a tin coating. It is a conductive filament and can cause electrical shorts in the assembly. Since tin whiskers can grow when the assembly is inactive or in storage, as well as during actual use,1 the impact is that the total operational life of an assembly now needs to be recalculated to include this storage time and not solely the time in service.

Before discussing a mitigation and management strategy, determine the expected operational service life of the product (Figure 1). If product life is short and consequence of failure insignificant, electrical failure due to tin whisker shorting is not a concern.

There are reasonable assessment tools to calculate the risk from tin whiskers with new products and assemblies. This assessment is best done in the design stage, so any modifications can be made before the assembly is built. The Pinsky Algorithm2 can be used to develop a risk assessment (Figure 2), and mitigation strategies can be executed to circuit boards and systems. The Pinsky Algorithm identifies 12 different risk factors and assigns each factor a numerical value. The 12 numerical values can then be combined to provide a composite risk number.

GEIA-STD-0005-2, “Standard for Mitigating the Effects of Tin Whiskers in Aerospace, Defense, and High Performance Electronic Systems” provides guidance into the steps to take to mitigate tin whisker growth. Applying conformal coating is a common and effective strategy to mitigate risk, and removal of any tin-coated parts is another. If conformal coating is used to contain tin whisker growth, it is assumed that no significant time has elapsed from time of assembly to time of conformal coating. There was no chance for the whisker to grow.

Conformal coatings are a practical consideration to contain tin whisker hazards. Two coatings with proven experimental records of performance are Parylene C and Arathane 5750. NASA has conducted studies of several conformal coatings applied to Sn-plated samples to determine the ability of these coatings to prevent penetration by the tin whisker. A 0.5 mil coating thickness of Parylene C saw no penetration by tin whiskers after five years. A 2 mil coating of Arathane 5750 had no penetrating whisker growth after 11 years. The full study is available on the NASA website. (http://nepp.nasa.gov/whisker/).

Acceleration tests. There is no accepted mechanism to accelerate the growth of a tin whisker. The goal of such an acceleration test would be to produce the expected types and lengths of whiskers, but in less time than in normal storage or use. If it is necessary to prove a coating will last for 15 years, it must be observed for 15 years. This differs from structures like solder joints that often fail due to accumulated stress-related damage. This damage can be accelerated by increasing the time and temperature for the lifetime number of thermal cycles. A complete 15- or 20-year lifetime of a solder joint might be accelerated to the point where its lifetime could be validated in just a few months. Tin whisker growth cannot be so reliably accelerated.

Most reports and studies have offered mitigation actions when the system is freshly built and with a storage time of zero, not a system that has been in storage for five or 10 years. Systems, assemblies and repair parts with tin plating and Pb-free components are likely to have tin whiskers of significant length if they have been in storage. Conformal coating can be applied, but after tin whiskers are removed.

NASA experienced a similar problem with a space shuttle system. After examination, it was discovered that the line replacement units (LRUs) had a significant whisker problem. The assemblies were almost impossible to replace; therefore, they were disassembled and the whiskers blown off with jets of ionized air. The work was performed under a vacuum hood to prevent scatter of the whisker fragments. The enclosures themselves were vacuumed out. The point is that if it is recognized that stored systems are likely to have tin whisker problems, there are solutions.

For commercial products with tin whiskers, an inline cleaning system has the capability of removing the whiskers. Inline cleaners have powerful wash and rinse jets that could reasonably “power wash” the whisker off the assembly. After inspection, conformal coating could then be applied.

To summarize, reliable data exist to deal with tin whisker issues, and the NASA website is an excellent source for guidance in this area. Systems that have been in storage for years should be appraised for tin whiskers and risks that these whiskers present. If the assembly does contain tin whiskers, there are cases where tin whiskers have been removed and both the storage and service clocks set to zero.

References

1. CALCE-EPSC Lead Free Forum, GlobalTransition to Pb-free/Green Electronics, 2004.
2. D. Pinsky, “An Updated Application-Specific Tin Whisker Assessment Algorithm Within A Process Compliant to GEIA-STD-0005-2,” Raytheon, March 2007.

ACI Technologies Inc. (aciusa.org) is the National Center of Excellence in Electronics Manufacturing, specializing in manufacturing services, IPC standards and manufacturing training, failure analysis and other analytical services. This column appears monthly.