Tin whiskers growth: An open problem for a material scientist

 

Image of one long tin whisker acquired by Dimas J Morilla using an Scanning electron microscope at Alter Technology TÜV NORD S.A.U. – Sevilla, Spain. Alter is a company dedicated to increase the reliabiity of electronic components.

I post here some information about the growth tin whiskers, an spontaneous phenomenon that occurs in the tin-finishes of leads of electronic components. The phenomenon has been studied for more tan 50 years but a complete understanding and control of it does not exist.

 

With the elimination of lead (Pb) in commercial electronic components in 2006 as a result of two European Union directives, RoHS and WEEE, the industry moved toward Pb-free components and assembly processes for non-critical, usually commercial, applications. The predominant terminations in commercial components are Cu leads with pure Sn and Sn-based alloy electrodeposits, and solder-dipped finishes. The main drawback of this technology is that Sn finishes may grow tin whiskers spontaneously. These are conductive hairy-like crystalline structures that may be long enough to cause an electrical short circuit what is a big problem particularly for those applications demanding high level of reliability. For this reason, the use of SnPb is not banned in military, space and medical systems.

A large amount of research has been made with the aim of understanding the phenomenon of tin whisker growth. No single, widely accepted explanation of this mechanism has been established but there are some experimental facts that can be mentioned about the phenomena of tin whiskers and their growth [1-6]:

  1.  Sn whiskers are very thin conductive filaments that grow, most usually from tin-plated copper and tin-plated brass. They do not grow when Sn-Pb alloys are used as solder. Tin whiskers grow from the bottom in the form of single crystals of pure tin with very high length to width ratios. Lengths can exceed 100 μm, some reach 2 mm, but their diameters are typically 1 – 4 μm.
  2. Whiskers may be straight, kinked or hooked and their outer surfaces are usually striated.
  3. The spontaneous growth of whiskers is an irreversible process that starts after an incubation period that varies from seconds to years. Whiskers have been proven to have caused the failure of satellites, heart pacemakers and caused the shutdown of nuclear power stations.
  4. The most likely driving force for whisker formation is a possitive stress gradient in the Sn layer and tin whisker growth is a stress relief phenomenon [7-8,10-11]. Stress may come from the as-plated film, with its associated intermetallic formation or corrosion. Interfacial reactions occur as a result of chemical potential differences when two dissimilar materials are brought into contact, such as in the metallization and soldering processes. It is commonly observed that local equilibrium is developed at the solid\solid interfaces, and in most of the systems intermetallic compounds grow at the interface as well. It is believed that one of the prime causes of stress that may result in whisker formation, as seen in testing, is the irregular growth of the Cu6Sn5 intermetallic, when Sn is plated on Cu based substrates, which occurs rather quickly under room ambient conditions. Stress on Sn-finishes may be induced mechanically, therefore, bending, scratches or nicks in the Sn may favor tin whisker growth.
  5. The presence of grain boundaries either by the spontaneous growth of by the use of eutectic solder seem to increase the rate of growth of whiskers. Therefore, the texture of the solder finish is important (e.g. matte versus bright Sn) [12-14]. Also the role of surface oxide seem to play an important role [15].
  6. Whiskers grow spontaneously without an applied electric field. However,  electromigration may have also a role [16-20].
  7. The use of different alloys for Pb-free finishes, such as Nickel, Palladium and Gold will mitigate against growth, but they are more expensive than pure tin and they may need to be soldered at a higher temperature and with more active fluxes. And some of the alternatives to Pb-free finishes have been found to be brittle, and therefore are not suitable for environments with high vibration loading, like satellites in the launch stage of their lives.

The unpredictable nature of their subsequent growth is of particular concern to electronic, electric and electromechanical systems requiring long term, reliable operation such as in medical, military or space applications.  A set of guidelines have been standarised by the electronic industry in the document JESD22-A121A (“Measuring Whisker Growth on Tin and tin Alloy Surface Finishes”) of JEDEC with the objective of providing a series of accelerated tests for the measurement and comparison of whisker propensity for different platings, surface chemistries and processes [4-5].

At present, and because the phenomenon is not yet fully understood, the growth of tin whiskers can only be mitigated but not eliminated. And it is a very important reliability problem especially in medical, military or space applications.

References:

[1] Dunn BD. A laboratory study of tin whisker growth. European Space Research and Technology Centre, Noordwijk, The Netherlands; September 1987.

[2] Galyon GT. A history of tin whisker growth: 1964 to 2004. IBM eSG Group, SMTAI International Conference, Chicago, Illinois; September 2004. p. 26-30.

[3] Crandall, EK. Factors governing tin whisker growth. PhD thesis, University of Alabama, USA; 2012.

[4] JEDEC/IPC, JP002: Current tin whiskers theory and mitigation practices guideline; 2006.

[5] JEDEC, JESD22-A121: Test method for measuring whisker growth on tin and tin alloy; 2005.

[6] Osterman M. Mitigation strategies for tin whiskers. The Center for Advanced Life Cycle Engineering, CALCE-EPSC; 2002.

[7] Fisher RM, Darken LS, Carroll KG. Accelerated growth of tin whiskers. Acta Metallurgica 1954;2: 368-373.

[8] Lee CC, Wang PJ, Kim, JS. Are intermetallics in solder joints really brittle?. Electron Compon Technol Conf, IEEE; 2007. p. 648-652.

[9] Choi WJ, Lee TY, Tu KN, Tamura N, Celestre RS, MacDowell AA, Bong YY, Nguyen L. Tin whiskers studied by synchrotron radiation scanning X-ray micro-diffraction. Acta Materialia 2003;5:6253-6261.

[10] Osenbach JW, Shook RL, Vaccaro BT, Amin A, Potteiger BD, Hooghan KN, Suratkar P, Ruengsinsub P. Tin whisker mitigation: Application of post mold nickel underplate on copper based lead frames and effects of board assembly reflow. Proceedings Surface Mount Technology Association; 2004:724.

[11] Tu K-N, Suh JO, Wu AT-C, Tamura N, Tung C-H. Mechanism and prevention of spontaneous tin whisker growth. Materials Transactions 2005; 46(11): 2300.

[12] Sheng GTT, Hu CF, Choi WJ, Tu KN, Bong YY, Nguyen L. Tin whiskers studied by focused ion beam imaging and transmission electron microscopy. Journal of Applied Physics 2002;92:64.

[13] Baated A, Kim K-S, Suganuma K. J Mater Sci: Mater Electron 2011;22:1685.

[14] Ashworth MA, Wilcox GD, Higgison RL, Heath RJ, Liu C, Mortimer RJ. The effect of electroplating parameters and substrate material on tin whisker formation. Microelectronics Reliability 2015;55:180.

[15] Jiang B and Xian A-P. Discontinuous growth of tin whiskers. Philosophical Magazine Letters 2006;86(8):521-527.

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