Tin whisker management guidelines: although full understanding of whisker growth is lacking, these specifications and practices will reduce risks.

Ed.: For the full article, please see circuitsassembly.com/cms/content/view/2444/

Despite more than five decades of research on tin whiskers, there is no consensus on the mechanisms that control whisker growth, although much progress has been made. We do know that tin-based plating--the preferred surface coating for leads on electronic components--can be susceptible to the formation of needle-like protrusions, or whiskers, especially when pure tin or high tin content alloys are used. Tin whiskers have been measured at maximum lengths of up to several hundred microns. These whiskers could seriously impact product reliability. If they grow to critical lengths in service, whiskers could cause electrical shorts, disrupt moving parts and degrade RF/high-speed performance. For years, manufacturers controlled tin whiskers by adding small amounts of lead, but as we quickly move toward Pb-free electronics, we must again address the risks associated with the use of pure tin.

Although there is no scientific consensus on whisker formation and growth fundamentals, many agree that compressive stress in the tin film is the fundamental driving force behind whisker growth. Certain factors have been identified that promote or accelerate their growth. High humidity conditions result in increased oxygen penetration in tin finishes with associated increases in tin oxides and resulting stress. High humidity may also increase the rate of tin grain boundary or surface diffusion, which affects tin whisker growth. Temperature may either increase or inhibit tin whisker growth. Excessive temperature results in stress relief by mechanisms other than whisker growth. At temperatures between 20[degrees] and 50[degrees]C, the [Cu.sub.6][Sn.sub.5] intermetallic forms quite rapidly (within a few days), adding to stress in the plated film. Mismatch of the coefficients of thermal expansion between tin and base materials results in stress during thermal cycle conditions, which drives whisker growth. Although whiskers can grow rapidly under high compressive stress conditions, incubation times before whiskers begin to grow can be very long, extending into many years. The mechanism(s) for the necessary movement of tin atoms within the structure is still under considerable debate.

In 2001, iNEMI embarked on a series of experiments to find accelerated tests for tin whiskers. It quickly became apparent that standard accelerated test conditions (e.g., higher temperature, humidity and thermal cycling) were not going to provide a nice, clean set of tests that would predict whisker growth. However, industry was racing to introduce Pb-free electronics and needed some methodology for ensuring reliability of tin coatings. After much discussion, iNEMI members embarked on a serial methodology to minimize reliability exposure. The first step was to define a set of test conditions that would promote tin whisker growth and to recommend a protocol for inspecting whisker growth and data recording. The second step was to gain fundamental understanding of whisker formation. The third step was to develop acceptance test criteria and mitigation practices that would provide an interim way to minimize reliability exposure of long life, high-reliability electronics systems.

The factors that promote or accelerate tin whisker growth and the observed incubation periods under test conditions provided the foundation for JEDEC standard JESD22A121, Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes. (1) The iNEMI project team recognized that, without knowing acceleration factors, it would not be possible to write a qualification specification; but using what is known along with engineering judgment, it should be possible to write an acceptance specification. Extending these test methods into an acceptance test standard has been a challenging proposition. Although JESD22A121 specifies a standard set of tests for measuring tin whisker growth, there is no way at this time to relate these tests to field conditions with any reasonable degree of certainty.

Joe Smetana is principal engineer, advanced technology, Alcatel (alcatel.com), a distinguished member of the Alcatel Technical Academy, chair of the iNEMI Tin Whisker User Group and a member of the JEDEC and IPC tin whisker standards committees; [e-mail]. Ron Gedney is a consultant to iNEMI's Pb-free and tin whisker projects; [e-mail].

Table 1. Maximum Recorded Whisker Lengths

                                  Maximum Historical Data
Mitigation Practice               on Tin Whisker Length

Reflowed tin                      <100 [micro]m
Hot-dipped tin (or tin alloy)     300-400 [micro]m (1)
Matte tin over nickel             up to 200 [micro]m (2)
Annealed matte tin over copper    300-400 [micro]m (1)
None (matte tin over copper) (3)  800 [micro]m

Notes:
1. Results > 100 [micro]m are believed to be process-dependent,
geometry-dependent and possibly the result of corrosion effects.
2. Whiskers of this length occurred where the nickel underlayer was
cracked, which further demonstrates the need for testing in addition to
mitigation practices.
3. Matte tin over copper is not considered a viable mitigation process
and is included here to illustrate how long whiskers can grow when no
steps are taken to control their occurrence.