1、GEIA ENGINEERING BULLETIN Reducing the Risk of Tin Whisker- Induced Failures in Electronic Equipment GEIA-GEB-0002 NOVEMBER 2003 GOVERNMENT ELECTRONICS AND INFORMATION TECHNOLOGY ASSOCIATION A Sector of the Electronic Industries Allianc Copyright Government Electronics & Information Technology Assoc
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9、 under the cognizance of the GEIA G-12 Solid State Devices Committee) Published by O 2003 Government Electronics and Information Technology Association Standards & Technology Department 2500 Wilson Boulevard Arlington, VA 2220 1 All rights reserved Printed in U.S.A. Copyright Government Electronics
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12、 Reproduced by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-GEB-0002 ACKNOWLEDGMENT The GEIA G-12 Solid State Devices Committee acknowledges the significant contributions of the authors and organizations whose published works, ci
13、ted in the bibliography, provided the foundation for this document. Members of Task Group GO202 of the GEIA G-12 Solid State Devices Committee developed this document. The Task Group and Committee would like to recognize the principle contributors shown below and to extend gratitude to the many othe
14、rs who assisted in the evolution of this Bulletin. Mr. Gary Ewell Mr. Henry Livingston Mr. William Dieffenbacher Ms. Anduin Touw Mr. Jan Bunting Mr. Michael Cooper Ms. Dottie Fields Mr. Kent Walters Mr. James Moffett Mr. Jay Brusse Mr. Michael Sampson Mr. David Hillman Mr. John Nirschl Mr. Jeff Jarv
15、is Aerospace Corporation BAE SYSTEMS BAE SYSTEMS Boeing Satellite Systems General Dynamics General Dynamics Microsemi Corp. Microsemi Corp. Northrop Grumman Mission Systems QSS Group, Inc. NASA Goddard Space Flight Center Rockwell Collins Rockwell Collins US Army Aviation and Missile Command We wish
16、 to express our sincere appreciation to the NASA Goddard Space Flight Center Tin Whisker Investigation Team for their significant contributions to this Bulletin. I Copyright Government Electronics & Information Technology Association Reproduced by IHS under license with GEIA Not for ResaleNo reprodu
17、ction or networking permitted without license from IHS-,-,-GEIA-GEB-0002 Introduction As a result of world-wide consumer electronics demand for lead-free products, component manufacturers are increasingly converting to lead-free materials. A popular choice for these finishes is tin. Tin finishes can
18、 be susceptible to the spontaneous growth of single crystal structures known as “tin whiskers”which can cause electrical failures, ranging from parametric deviations to catastrophic short circuits, and may interfere with sensitive optical surfaces or the movement of micro-electromechanical systems (
19、MEMS). Though studied and reported for decades, tin whiskers remain a potential reliability hazard, particularly for space applications and for equipment subjected to long term dormant storage and use (e.9. missiles and expendables). There is no pending US legislation mandating lead-free electronic
20、products, and should such legislation arise, military, aerospace and medical equipment manufacturers would likely be exempt. Nevertheless, Department of Defense and NASA believe that the use, and therefore the risk, of tin finish on electronic components will increase because: 1) commercial industry
21、 have stated initiatives to eliminate lead (Pb) from electronics, 2) defense and aerospace industry trends show increasing usage of commercial components, and 3) continuing reductions in circuit geometry and power means that even small whiskers may cause catastrophic failures. Many factors can contr
22、ibute to whisker formation, and their relative importance have not been determined. The quantitative risks of whiskers under various sets of material, manufacturing, and application conditions have not been determined. Historically, whiskers have been of greatest risk to space and missile applicatio
23、ns. This may be explained in part by: the longer lifetimes, the more destructive effects in low-pressure environments, and higher reliability requirements. However, it is felt by this committee that a wider community may have concerns about whiskers as pure tin plating becomes more common and circui
24、t geometries become smaller. This Bulletin includes various findings of government and industry experts regarding characteristics of tin whiskers and methods to mitigate future risks associated with them. It is meant to summarize the literature on tin whisker growths, not mandate a particular risk m
25、itigation strategy. The appropriate actions will depend on the specific application, lifetime, and reliability requirements. II Copyright Government Electronics & Information Technology Association Reproduced by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted withou
26、t license from IHS-,-,-GEIA-GEB-0002 Reducing the Risk of Tin Whisker-Induced Failures in Electronic Equipment CONTENTS . 1 2 Tin Whisker Formation . 1 2.1 2.2 Tin Whisker Growth Mechanisms 2.3 Environmental Factors . 4 Common Tin Whisker Attributes . 3 Methods to Reduce the Risk of Tin Whisker-Indu
27、ced Failures 5 . 5 3.1 3.2 Parts and Material Selection Application Specific Risk Assessment 3.2.1 Avoid Using Components with Pure Tin Finishes If Possible 5 3.2.2 Physical Barriers 6 Choice of Underplating or Substrate Material Select a Matte or Low Stress Tin Finish . . 7 3.2.3 3.2.4 3.2.5 Platin
28、g Process Considerations. . 7 3.3 Material and Assembly Processing . 8 . 8 3.3.1 3.3.2 Avoid Applying Compressive Loads on Plated Surfaces Conformal Coat or Foam Encapsulation Over Whisker Prone Surfaces 3.3.3 Heat treatments 9 3.3.4 Solder Dip Tin-finished Surfaces . 9 3.3.5 Replate Whisker Prone A
29、reas . . 9 Bibliography 11 4 Conclusions iii Copyright Government Electronics & Information Technology Association Reproduced by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-GEB-0002 iv Copyright Government Electronics & Informat
30、ion Technology Association Reproduced by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-GEB-0002 1 Scope This Bulletin provides a brief description of tin whisker formation and describes various methods recommended by government an
31、d industry to reduce the risk of tin whisker-induced failures in electronic hardware. It is not a mandate nor does it contain any requirements. A tin whisker is a single crystal that emerges from tin-finished surfaces. Tin whiskers can pose a serious reliability risk to electronic assemblies that ha
32、ve pure tin finish. The general risks fall into several categories: I, 2, 3, 8, 161 Short Circuits: The whisker can create a short circuit, either by 1) growing from an area at one potential to an area at another or 2) breaking free and later bridging these areas. In some cases, these shorts may be
33、permanent and cause catastrophic system failures. A transient short may result if the available current exceeds the fusing current of the whisker, and the whisker can fuse open. The amount of current needed to fuse open the whisker depends on the atmospheric pressure and the diameter of the whisker.
34、 Low-pressure-lnduced Mefa/ Vapor Arcing (Plasma): In low-pressure environments, even a transient short can result in a catastrophic failure. Under certain current and voltage conditions (current more than a few amps and supply voltage over 12 V), when a tin whisker fuses open, the vaporized tin may
35、 initiate arcing or a plasma. The plasma can conduct over 200 A and may continue until all the available exposed tin is consumed or the supply current is interrupted. I, 81 DebrislConfaminafion: The tin whiskers small diameter may allow it break free under handling or other vibration. A free floatin
36、g whisker may cause the same problems typically associated with free floating particles, .e., interfering with the movement of mechanical parts or contaminating optical surfaces. I, 81 Tin Whisker Formation 2.1 Common Tin Whisker Attributes The following paragraphs provide an overview of some of the
37、 observed characteristics of tin whiskers. These features are discussed in more detail in the following references:l, 2, 3, 8, 9, 16, 19, 20, 23, 25, 27, 28, 30, 33, 351 Shapes & Surface Features: True whiskers are cylindrical, needle-like crystals that can grow either straight or kinked. The surfac
38、e is usually striated longitudinally. Whiskers may grow directly out of the surface or from pyramid-shaped nodules on the surface. Nodules, which may grow tens of microns in length, may also appear without whiskers. Because of their shorter length and larger diameter, they do not usually pose a reli
39、ability risk in and of themselves. (See Figure 1 for examples) Incubation (Dormancy) Period: Experimenters report an incubation period ranging from days to years before whiskers appear. This period is likely related to the amount of compressive stress. 27 This period is of particular concern because
40、 experiments to determine the propensity for a particular process to form whiskers may need to span very long periods of time. This property also complicates decisions about whether applications with short storage and usage lives are at risk for tin whiskers. Growth Rate: Growth rate of tin whiskers
41、 is also variable: rates from 0.03 to 9 mm/yr have been reported. Some experiments also document non-linear growth rates and times when the growth has stopped all together. 27, 30, 331 Interrelated factors such as substrate materials, grain structure, plating chemistry, and plating thickness may inf
42、luence growth rate. 1 Copyright Government Electronics & Information Technology Association Reproduced by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-GEB-0002 Classic “needle-like“ whisker Kinks and striations Pyramidal shaped w
43、hiskers Nodules Nodules originating along a Whisker growing from a nodule (note how it appears to originate from amechanically induced surface scratch very fine surface scratch) Irregular shaped whisker tip Focused Ion Beam “cross section“ of a tin whisker conformal coat (note the solid, not hollow,
44、 structure and stnations) Whisker nodules beneath (growing preferentially along the line of a surface defect) Figure 1 -Whisker Shapes SI (Photographic images courtesy of NASA Goddard and The Aerospace Corporation) 2 Copyright Government Electronics & Information Technology Association Reproduced by
45、 IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-GEB-0002 Whisker Length: Tin whisker length obviously depends on growth rate and sustained periods of growth. However, in experimental results, most measure between 0.5 mm and 5.0 mm.
46、 The longest reported length is 10 mm. 8, 231 Whisker Diameter: Whiskers are typically very thin with diameters between 1 and 5 microns 8, 301, yet diameters between 0.006 pm and 7 pm have been recorded. 33 Whiskers are not always the same diameter throughout their entire length. 33 Despite some ear
47、ly hypotheses to the contrary, some whiskers have developed cross-sectional areas greater than either the grain size of the original tin or the thickness of the plating. 8 Density of Growths: Although whisker densities up to I 04/cm2 have been observed, this measurement also varies greatly in the li
48、terature. 8 Variation may be due to an inconsistent definition of what length should be counted 30 or to differences in the compressive stresses in the tin or substrate layers 8. Experiments also have recorded changes in whisker density from both radiation 34 and plating thickness 29. Current-Carryi
49、ng Capacity: Under normal atmospheric conditions, the capacity typically measures between 10 and 32 mA. 23,35 However, capacities as high as 75 mA have been observed. 8 The capacity depends on the thickness of the whisker and on the environment. Because air can provide cooling which might lead to higher current capacity, lower current capacity might be
