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Open Forum Standards for Tin Whisker Test Methods on Lead-Free Components Tadahiro Shibutani, Michael Osterman, Member, IEEE, and Michael Pecht, Fellow, IEEE
Abstract—Tin whiskers are widely recognized as being a reliability threat to lead-free electronic components. The current standards to assess the tin whisker susceptibility of lead-free finishes in electronic components are problematic. Key problems include lack of data collection, lack of agreement on proper techniques for measuring whisker length, lack of established acceleration transforms for prescribed tests, and disagreement over acceptance criteria. This paper overviews the tin whisker standards and then describes the problems listed above. Recommendations for improvements are then presented and discussed. Index Terms—Environmental test, lead-free electronics, standard, tin whisker.
I. INTRODUCTION
Current standards for tin whisker growth propensity identify various test conditions based on reviews of known data from around the globe. However, the current standards contain several problems. In this paper, we review industry standards related to tin whisker formation. We examine test conditions and inspection procedures described in the standards. Problems with the existing tin whisker standards are then identified and recommendations for improvement are proposed. II. OVERVIEW OF STANDARDS RELATED TO TIN WHISKERS Industry standards are created to provide common approaches to and procedures for addressing specific issues of concern. With regard to tin whiskers, two standards have been issued by JEDEC. The Japan Electronics and Information Technology Industries Association (JEITA) has issued test standards. The International Electrotechnical Commission (IEC) has also issued an environmental testing standard. The Government Electronics Information Technology Association (GEIA) of the USA has issued its whisker mitigation standard. A. JEDEC Standards
T
IN–LEAD alloys have been commonly used as surface finishes in the electronics industry due to their low cost and ease of manufacture. However, legislative requirements and market forces have resulted in the selection of lead-free tin-based alloys as a replacement for tin–lead alloys. The removal of lead, a known tin whisker mitigator, has raised concern over the potential failure risk presented by the formation of tin whiskers on lead-free finished surfaces. Tin whiskers were first reported and recognized as a threat to the electronics industry in a laboratory study by Bell Telephone Labs in 1951 [1]. Tin whiskers form on pure tin or tin-alloy finished surfaces. Whiskers can grow up to several millimeters in length with a diameter of a few micrometers. Since the smallest distance between components can be less than a few hundred micrometers, whiskers can create a mechanical bridge between adjacent conductors. Due to the conductive nature of tin whiskers, such a mechanical bridge can result in an unintended electrical short circuit. It is commonly believed that compressive stress in a tin finish is a necessary but insufficient factor in the formation of tin whiskers. While tin whisker growth has received considerable attention, the fundamental mechanisms of tin whisker formation and growth are still not clear. Under pressure from equipment manufacturers, standard tests for assessing the propensity of tin or tin-based lead-free finishes to grow tin whiskers have been established. With the growing use of pure tin and tin-based lead-free finishes, industry groups have initiated research projects to help quantify tin whisker reliability risk. Manuscript received December 03, 2008. Current version published February 27, 2009. T. Shibutani is with Yokohama National University, Yokohama 240-8501, Japan (e-mail:
[email protected]). M. Pecht is with the City University of Hong Kong, on leave from the Center of Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, MD 20742, USA (e-mail:
[email protected]). M. Osterman is with the Center of Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, MD 20742 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/TCAPT.2009.2013311
JEDEC has published two tin whisker standards, JESD22A121A and JESD201, based on the work of companies participating under the International Electronics Manufacturing Initiative (iNEMI) tin whisker working groups. JESD22A121A provides a test method for measuring whisker growth on tin and tin alloy surface finishes [2]. Based on testing conducted within the iNEMI working group, three tin whisker environmental test conditions were demonstrated to produce whisker growth. The three test conditions include two isothermal conditions with controlled humidity and temperature cycling. In addition to defining environmental test conditions, JESD22A121A describes sample requirements, preconditioning and whisker inspection, and recording procedures. JESD201 provides acceptance criteria for the tin whisker susceptibility of tin and tin-alloy surface finishes [3]. This standard was generated under the auspices of the JEDEC JC14.3 Subcommittee on Silicon Device Reliability Qualification and Monitoring and the iNEMI Tin Whisker User Group. JESD201 defines product classes, test duration periods, and acceptance criteria based on measured whisker length precipitated from the three test conditions. Class 3 products, which include military, aerospace, and medical products, are identified as not being allowed to use tin or high tin content alloys. B. JEITA Standards JEITA has established two standards, ET-7410 and RC-5241, based on experimental works. ET-7410 defines environmental tests for tin whiskers and is based on experimental data and the hypothesis of tin whisker growth mechanisms [4]. This standard focuses on tin whisker tests for electronic components with tin and tin-based lead-free terminal finishes. As with the JEDEC standards, three environmental test conditions are defined. Here, the environmental test conditions are related to assumed growth mechanisms. Ambient storage addresses tin whisker growth as being due to internal stresses, specifically irregular intermetallic compound (IMC) growth. The high-temperature/humidity test addresses whisker growth arising from oxide layer formation. The temperature cycling test addresses whisker growth associated
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TABLE I STANDARDS FOR ENVIRONMENTAL TESTS TO ASSESS TIN WHISKER GROWTH FOR ELECTRONIC COMPONENTS
with the accumulation of internal compressive stress caused by a mismatch between the coefficients of thermal expansion (CTEs) of the base material and the finish. RC-5241 provides a test method for measuring pressure-induced tin whisker growth on connectors [5]. Ambient storage is used with the application of mating pressure. According to experimental works by JEITA, temperature does not accelerate pressure-induced tin whisker growth. C. IEC Standard IEC60068-2-82 is the international standard for environmental testing to evaluate tin whiskers growth on electric or electronic components [6]. This standard was largely based on ET-7410. The standard does not specify tests for pressure-induced tin whisker formation. Three environmental tests—an ambient storage test, a damp heat test (high temperature/humidity), and a temperature cycling test—are described. The test conditions and test procedures are similar to those in the previously mentioned standards. D. GEIA-STD-0005-2 GEIA established a standard for classifying electronic equipment relevant to mitigating the effects of tin whiskers in aerospace and high-performance electronic systems [7]. Five categories are identified, ranging from no restrictions on the use of tin-based lead-free finishes to complete restriction on the use of lead-free tin-based finishes. This standard sets forth requirements for equipment manufacturers to categorize their products and follow whisker mitigation approaches based on a defined category.
E. Summary of Environmental Test Standards The JEDEC, JEITA, and IEC standards are similar in that they define environmental tests to assess tin whisker growth propensity. These defined conditions and requirements are summarized in Table I. III. KEY PROBLEMS WITH CURRENT TIN WHISKER STANDARDS AND RECOMMENDATIONS Since the growth mechanism for tin whiskers has not been fully understood, several problems remain in the standards described in Section II. In this section, key problems with current tin whisker standards are described and our recommendations are presented. A. Measurement of Tin Whisker Length Whiskers tend to grow in various directions, sometimes exhibiting complicated shapes. It is often the case that the initial viewing of a whisker does not give a correct estimate of its length. In fact, it is more likely that a projection of a whisker will be measured rather than the entire length of the whisker, thereby providing an underestimation of the length. 1) Standard Method for Measuring Tin Whisker Length: JESD201 defines the estimation of whisker length as its maximum shorting distance: the distance from the root to the furthest point away on a whisker (Fig. 1). The same definition was used in JESD22-A121A, the JEITA standard, and the IEC standard. 2) Problem: The process for measuring whisker length is not clear. Either the sample or the microscope has to be adjusted until the whisker is positioned perpendicular to the viewing direction for measurement. Such adjustments are time consuming and may be impractical depending on the sample and microscope. It has been
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Fig. 1. Whisker length definition according to JESD201.
noted that whisker identification and length measurement can vary from observer to observer [8]. The measurements of whisker length may vary up to 20 or 30 m, which can be a significant error. 3) Recommendation: A measuring procedure for getting the exact length of a whisker is required. CALCE proposed measuring whisker length by using two different views of the same whisker at the Second International Symposium on Tin Whiskers [8]. The exact length of the whisker can be calculated based on measurements taken from the two views and geometric theory. B. No Acceleration Transform The major issue with the environmental tests defined in the various standards lies in the fact that no acceleration transform has been established to relate growth under test to growth under field conditions. As a result, and as stated in JESD201, “the testing described in this document does not guarantee that whiskers will or will not grow under field life conditions.” 1) Ambient Storage: The ambient test addresses the risk of whisker growth associated with internal stresses due to the plating process and interfacial diffusion between the finish and the underlayer or base material. The IEC standard specifies two severities, 30 C/60% RH and 25 C/50% RH, which can be selected by the substrate and plating material. The duration time is 4000 h based on observed preliminary results. JESD201 defines the test conditions for temperature/humidity storage to be 30 C/60% RH. The JEITA standard also specifies the same conditions. The total duration time should be 4000 h for business critical applications (Class 2) and industrial/consumer products with a medium lifetime (Class 1), and 1000 h for consumer products with short lifetimes (Class 1A) as defined in JESD201. The inspection interval should be 1000 h. 2) Damp Heat Test: The high-temperature/humidity test (damp heat test) was first proposed in order to accelerate the ambient test, but another growth mechanism based on oxidation of the finish has been suggested. Thus, this test addresses the risk of whisker growth associated with the oxidation of the finish surface. The 85% RH in the standards was chosen in order to avoid condensation in the test chambers. 3) Temperature Cycling Test: The temperature cycling test addresses the tin whisker growth associated with stress in the finish arising due to a mismatch in thermal expansion between the base material and the finish. The minimum and maximum temperatures used in the temperature cycling test are defined in the standards. The minimum temperature is either 055 C or 040 C. The maximum temperature is 85 C in JESD201 and ET-7410. IEC60068-2-82 specifies two maximum temperatures, 85 C and 125 C.
4) Problems: The temperature of the ambient test is equal or near to real application environmental conditions. Therefore, the ambient test does not provide any acceleration. The damp heat test may address tin whisker risk from oxidation. However, the oxide layer thickness has not been related to tin whisker growth and no correlation has been defined between time under damp heat and time in use. Temperature cycling can provide for the acceleration of a test by a change in temperature. However, this test does not correspond with the other two tests since there is another mechanism for tin whisker growth. Dwell times of maximum and minimum temperatures are also defined in the standards, but the effect of dwell time in thermal cycling on tin whisker growth has not yet been fully understood. 5) Recommendations: The fundamental growth mechanism has to be clarified in order to establish the acceleration transform. Another test condition should be developed in order to accelerate tin whisker growth. The effect of multiple environments may be a key to accelerating tin whisker growth. GEIA-STD-0005-2 describes this effect as a factor in accelerating tin whisker growth. Recently, the effect of a multiple environments test was reported by CALCE [8]. Sequential temperature cycling and elevated temperature and humidity tests were effective at producing whisker growth on tin finish copper with and without a nickel underlayer.
C. Acceptance Criteria A major risk related to tin whiskers is electrical shorting. When a whisker reaches the length of the minimum spacing between components, it can bridge to an adjacent conductor. To assess the bridging risk, both whisker length and whisker density must be considered. 1) Standards of Acceptance Criteria: JESD201 provides detailed guidelines for acceptance criteria. The maximum allowable tin whisker length for Class 2 components is 40 m for isothermal storage or 45 m for temperature cycling. For Class 1 components, the criteria depend on component type. The maximum allowable length is 50 m for highfrequency components, 67 m for lead components, and 100 m for components with large spacing. JESD22A121A provides a whisker density range. The whisker density range can be classified into three levels—low, medium, and high—by the total number of whiskers per lead, termination, or coupon area. 2) Problems: Only maximum whisker length is used as an acceptance criterion. Whisker length is known to obey lognormal distributions [9]. Distribution of whisker length is a key for qualifying maximum tin whisker length as well as assessing product failure risk. Tin whisker density is also important in assessing tin whisker failure. With the lack of a growth model or acceleration transform, only reporting the maximum length or that no whisker greater than a prescribe length was observed does little to assist in assessing the failure risk presented by tin whiskers. Electrical shorting is a main tin whisker failure mode. However, the shorting properties of tin whiskers are not well characterized and these properties have not been considered in any acceptance criteria. 3) Recommendations: More acceptance criteria should be added. With lack of a growth model or acceleration transform, the statistical distribution of whisker lengths and whisker density should be documented. The shorting properties of tin whiskers should also be part of the acceptance criteria. Limited research results associated with the shorting properties of tin whiskers are available. The voltage potential and contact parameters required for whiskers to overcome a surface oxide layer to make an electrical connection need to be characterized.
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IV. SUMMARY This paper presents an overview of standards related to tin whiskers. The JEDEC, JEITA, and IEC standards are similar in their defined environmental tests to assess tin whisker growth propensity. However, since the growth mechanism or mechanisms are still not completely understood, the current standards are problematic. The variation in length measurements required in the three test standards can result in a significant error. A consistent length measurement procedure is not defined in the standards. As a result, length measurement may vary from observer to observer. These variations may allow some products to pass defined acceptance criteria that may otherwise failure. A more consistent approach for whisker length measurement like the one identified in this article should be adopted. The most glaring issue with the current standards is the lack of accepted acceleration transforms. With no acceleration transform, the usefulness of conducting the environmental tests defined in the existing standards is highly questionable. More studies on tin whisker growth are required to establish acceleration transforms. Finally, without an acceleration transform, the use of maximum whisker length as the sole acceptance criteria is problematic. The probability of a whisker-induced failure is a function of the number of whiskers with sufficient length to bridge isolated conductors. Thus, density and whisker length distribution are important. With the lack of a growth model and acceleration transform, standards should require documentation of both whisker density and whisker length distribution. Another problem is that there is no mention of the shorting property of tin whiskers in the standards. Even though electrical shorting is a main tin whisker failure mode, there is no shorting criterion for electronic products.
REFERENCES [1] K. G. Campton, A. Mendizza, and S. M. Arnold, “Filamentary Growth on Metal Surfaces—‘Whiskers’,” Corrosion, vol. 7, pp. 327–334, 1951. [2] Test Method for Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes, JEDEC Standard JESD22A121A, 2008. [3] Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes, JEDEC Standard JESD201 , 2006. [4] Whisker Test Methods on Components for Use in Electrical and Electronic Equipment, JEITA Standard ET-7410, (in Japanese), 2005. [5] Whisker Test Methods on Connectors for Use in Electrical and Electronic Equipment, JEITA Standard RC-5241, (in Japanese), 2007. [6] Whisker Test Methods for Electronic and Electric Components, IEC Standard IEC60068-2-82, Edition 1.0, 2007. [7] Standard for Mitigating the Effects of Tin Whiskers in Aerospace and High Performance Electronic Systems, GEIA Standard GEIA-STD0005-2, 2006. [8] M. Osterman, “Tin Whisker Growth Measurements and Observations,” in Proc. 2nd Int. Symp. Tin Whiskers, Tokyo, Japan, 2008, 1–1–1.
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[9] T. Fang, M. Osterman, and M. Pecht, “Statistical analysis of tin whisker growth,” Microelectron. Rel., vol. 46, no. 5–6, pp. 846–849, 2006. T. Shibutani received the Ph.D. degree in mechanical engineering from Kyoto University, Kyoto, Japan, in 2000. He was a Visiting Scholar at the Center of Advanced Life Cycle Engineering (CALCE), University of Maryland, in 2007. He is currently a Research Associate at Yokohama National University, Yokohama, Japan. His research interests include fracture of small components, reliability for electronic products, and failure analysis for microelectronics.
Michael Osterman (M’91) received the Ph.D. degree in mechanical engineering from the University of Maryland, College Park. He is the Director of Center for Advanced Life Cycle Engineering (CALCE) and the Electronic Products and Systems (EPS) Consortium at the University of Maryland. He manages the information systems and oversees the development of software for CALCE EPSC. His research interests include virtual qualification techniques for electronic products, failure analysis for electronic systems, and information systems for electronics design. He has written various book chapters and numerous articles in the area of electronic packaging. Dr. Osterman is a member of the ASME and SMTA.
Michael Pecht (F’92) received the B.S. degree in acoustics, the M.S. degree in electrical engineering, and the M.S. and Ph.D. degrees in engineering mechanics from the University of Wisconsin, Madison, in 1976, 1978, 1979, and 1982, respectively. He is currently a visiting Professor at City University of Hong Kong. He is the founder of the Center for Advanced Life Cycle Engineering (CALCE) and the Electronic Products and Systems Consortium at the University of Maryland, College Park. He is also a Chair Professor. He has been leading a research team in the area of prognostics for the past ten years and has now formed a new Electronics Prognostics and Health Management Consortium at the University of Maryland. He has consulted for over 50 major international electronics companies, providing expertise in strategic planning, design, test, prognostics, IP, and risk assessment of electronic products and systems. He has written 26 books on electronic product development, use, and supply chain management. He is Chief Editor for Microelectronics Reliability. Dr. Pecht is a Professional Engineer and an ASME Fellow. He received the IEEE Reliability Society Lifetime Achievement Award, 3M Research Award for electronics packaging, the IEEE Undergraduate Teaching Award, and the IMAPS William D. Ashman Memorial Achievement Award for his contributions in electronics reliability analysis. He served as Chief Editor of the IEEE TRANSACTIONS ON RELIABILITY for eight years and on the advisory board of IEEE Spectrum. He is an Associate Editor for the IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY.
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