Avoiding tin whisker reliability problems: fighting tin whiskers involves knowing the available mitigation strategies and risk analysis based on lead spacing.[TEXT NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] The electronics industry is just beginning a conversion to lead-free processes and products to comply with the European Union's Reduction of Hazardous Substances (RoHS) Directive. RoHS requires some segments of the electronics industry to convert to lead-free soldering soldering Process that uses metal alloys with low melting points to join metallic surfaces without melting them. Tin-lead solders, once widely used in the electrical and plumbing industries, are now replaced by lead-free alloys. , and all segments must develop lead-free replacements for the tin-lead (Sn-Pb) coatings currently used on most component lead-frames and printed circuit boards (PCBs). A simple manufacturing solution for many lead-frame suppliers is to utilize pure tin coatings. However, pure tin is known to be susceptible to formation of needle-like protrusions, or whiskers See metal whiskers. (Figure 1), capable of causing electrical shorting in tightly spaced electronic circuitry. (1) Whiskers of tin, zinc, cadmium cadmium (kăd`mēəm) [from cadmia, Lat. for calamine, with which cadmium is found associated], metallic chemical element; symbol Cd; at. no. 48; at. wt. 112.41; m.p. 321°C;; b.p. 765°C;; sp. gr. 8. and silver have caused serious service failures that have been both life threatening and financially disastrous to the people and companies involved. (2,3) [FIGURE 1 OMITTED] System developers face two key problems: (1) no consensus exists on a reliability test that can accelerate whisker growth, so qualifying tin-plated terminations is virtually impossible, and (2) no universal consensus exists about the fundamental aspects of whisker formation and growth. As a result, developers, particularly those concerned with high-reliability and/or long design life (greater than five years), do not have sufficient information to safely specify tin coatings for their products. System designers are bombarded with information about matte tin, alloyed tin, rack plated tin, fused fuse 1 also fuze n. 1. A cord of readily combustible material that is lighted at one end to carry a flame along its length to detonate an explosive at the other end. 2. tin, barrel plated tin, mechanically plated tin, tin with an underlay, tin on copper alloy substrates, tin on alloy 42 (i.e. Fe-40% Ni), tin on iron, annealed tin and others. In most cases, a supplier will provide data indicating that its particular offering is whisker free. The authors have not found any solution that will guarantee whisker-free products over long periods of time, but some solutions appear to be better than others. This article examines the literature on tin whiskers See metal whiskers. and provides some potential solutions for system designers. Military and aerospace systems, which often have unique requirements, are not covered not covered Health care adjective Referring to a procedure, test or other health service to which a policy holder or insurance beneficiary is not entitled under the terms of the policy or payment system–eg, Medicare. Cf Covered. here. Assessing Risk Whiskers take a finite time to form and grow. The literature, along with experiments conducted by the National Electronics Manufacturing This article presents a typical manufacturing process of an electronic assembly. Component manufacturing Components such as resistors, capacitors and integrated circuits are generally made by specialized contractors. Initiative (NEMI NEMI National Electronics Manufacturing Initiative NEMI National Environmental Methods Index , Herndon, VA), show that whiskers, once started, tend to grow quickly but not indefinitely in·def·i·nite adj. Not definite, especially: a. Unclear; vague. b. Lacking precise limits: an indefinite leave of absence. c. . In many cases, whiskers never appear; in others, whiskers appear several years after the component has been fabricated fab·ri·cate tr.v. fab·ri·cat·ed, fab·ri·cat·ing, fab·ri·cates 1. To make; create. 2. To construct by combining or assembling diverse, typically standardized parts: . These inconsistencies leave system designers in a difficult position with respect to accepting pure tin (Sn) lead-frame finishes on componentry. Prevailing theory is that whiskers are caused by compressive stress Compressive stress is the stress applied to materials resulting in their compaction (decrease of volume). When a material is subjected to compressive stress, then this material is under compression. Usually, compressive stress applied to bars, columns, etc. leads to shortening. buildup build·up also build-up n. 1. The act or process of amassing or increasing: a military buildup; a buildup of tension during the strike. 2. in the tin plating. This stress can increase with time, perhaps due to the growth of intermetallic layers at the film/substrate interface. Possible mitigation practices to relieve this compressive stress include use of a nickel nickel, metallic chemical element; symbol Ni; at. no. 28; at. wt. 58.69; m.p. about 1,453°C;; b.p. about 2,732°C;; sp. gr. 8.902 at 25°C;; valence 0, +1, +2, +3, or +4. underlay, annealing annealing (ənēl`ĭng), process in which glass, metals, and other materials are treated to render them less brittle and more workable. or reflow (1) The process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. Surface mount chips (SMT) use the reflow method. Contrast with wave soldering. See also reflowable text. of the plating, thicker tin (greater than 15 microns) and/or use of an additive additive In foods, any of various chemical substances added to produce desirable effects. Additives include such substances as artificial or natural colourings and flavourings; stabilizers, emulsifiers, and thickeners; preservatives and humectants (moisture-retainers); and such as bismuth bismuth (bĭz`məth) [Ger. Weisse Masse=white mass], metallic chemical element; symbol Bi; at. no. 83; at. wt. 208.9804; m.p. 271.3°C;; b.p. about 1,560°C;; sp. gr. 9.75 at 20°C;; valence +3 or +5. . Designers for applications that have considerable temperature cycling may decide not to allow use of any tin-based finish because whiskers readily form under conditions of temperature cycling. But the bottom line is that all system designers should clearly understand that no pure or high tin content electroplated e·lec·tro·plate tr.v. e·lec·tro·plat·ed, e·lec·tro·plat·ing, e·lec·tro·plates To coat or cover with a thin layer of metal by electrodeposition. film is risk free with respect to whisker formation, despite any claims to the contrary. Of course, a risk-free decision is to not use any high tin content finish. Nickel-palladium-gold (NiPdAu) lead-frames are not prone to whiskers and have been in common use for over 10 years. They currently satisfy a relatively small percentage--about 10%--of commercially available electronic components. These NiPdAu films do not have the innate corrosion resistance of high tin content films, but system designers can easily assess whether a particular application needs significant corrosion resistance capability. Mission and life critical applications should make every effort to utilize NiPdAu finishes. [FIGURE 2 OMITTED] System designers must be aware of accepted mitigation practices and their limitations. Table 1 summarizes the commonly accepted mitigation practices, based on a study of the available literature in this field, as well as data from the experimental matrices carried out by NEMI projects. Unfortunately, very few electrical component manufacturers utilize any of the mitigation practices listed in Table 1. Many current commercial lead-free finishes involve matte tin, and test data usually indicates that matte tins are preferable to bright tins with respect to whisker formation. Matte tin is an electroplated tin with a relatively large (1 to 10 micron micron: see micrometer. One micrometer, which is one millionth of a meter or approximately 1/25,000 of an inch. The tiny elements that make up a transistor on a chip are measured in micrometers and nanometers. See process technology. ) grain size and hopefully low built-in internal stress. Unfortunately, industry data show matte tin is not whisker free. Nonetheless, suppliers have rushed to implement pure matte tin finishes, and only a few of the larger firms have implemented any of the mitigation practices shown. A component user often has to make a risk evaluation on a pure tin finish because no alternative is available. [FIGURE 3 OMITTED] [FIGURE 4 OMITTED] [FIGURE 5 OMITTED] The authors describe below a personal risk decision roadmap. Risks are assessed as minimal, marginal and unacceptable based on the following criteria: * For a mission critical or life threatening application, any high tin content film should be defined as a marginal, possibly even unacceptable, risk. * For mechanically agitated ag·i·tate v. ag·i·tat·ed, ag·i·tat·ing, ag·i·tates v.tr. 1. To cause to move with violence or sudden force. 2. applications such as automobiles, fan and blower assemblies, any component that does not utilize one of the Table 1 practices should be defined as a marginal risk. * For applications where the temperature will be cycled between zero/sub-zero and high temperatures (85 to 140[degrees]C), any tin-rich films should be defined as a marginal risk. These applications include products such as cell phones, laptops, personal digital assistants (PDAs) and cameras. * For all other applications, the risk decision should be based on the impact of failures. Special attention must be paid to metal can packages. Whiskers growing from internal or external surfaces can short out to a voltage plane within the device structure and cause a failure. Lead-Frame Spacing Finding whiskers more than 300 to 400 microns in length is unusual, and the majority of matte tin film whiskers are 50 microns or less. There are examples of tin whiskers several hundred microns long, (3) but these seem to be in the minority. Exploiting this fact in system design may be possible by using component lead spacing to develop mitigation practices. We examined several typical lead-frame configurations used for electrical component packages to determine how lead spacing could be used for mitigating whisker effects. These package types are shown in Figures 2 to 6. Figure 2 is a quad flat pack (QFP (Quad FlatPack) A square, surface mount chip package that has leads on all four sides and comes in several varieties. PQFP (Plastic QFP) may refer to all of the following QFP types. All quad flatpacks use gull-wing leads, except for the CQFP, which stick straight out. ) with a 250-micron gap between adjacent lead-frames. Similar component packages have lead-frame spacings as small as 50 microns. Figure 3 is a TO-220 package with lead spacings of 1,000 microns. [FIGURE 6 OMITTED] The D2 surface-mount package shown in Figure 4 has a center stub A small software routine placed into a program that provides a common function. Stubs are used for a variety of purposes. For example, a stub might be installed in a client machine, and a counterpart installed in a server, where both are required to resolve some protocol, remote procedure lead that should have been cut flush to the molded plastic housing. However, in this case, the center stub contact protrudes from the molded plastic housing. As a result of this protruding pro·trude v. pro·trud·ed, pro·trud·ing, pro·trudes v.tr. To push or thrust outward. v.intr. To jut out; project. See Synonyms at bulge. stub, the lead-frame spacing is 1,130 microns rather than 3,000 to 5,000 microns. Axial axial /ax·i·al/ (ak´se-al) of or pertaining to the axis of a structure or part. ax·i·al adj. 1. Relating to or characterized by an axis; axile. 2. leads (Figure 5) are commonly used with resistors, optics packages and certain types of capacitors. Figure 6 shows a surface-mount multi-layer-ceramic (MLC (MultiLevel Cell) A flash memory technology that stores more than one bit per cell. Traditional flash memory defines a 0 or 1 bit based on a single voltage threshold. ) capacitor capacitor or condenser, device for the storage of electric charge. Simple capacitors consist of two plates made of an electrically conducting material (e.g., a metal) and separated by a nonconducting material or dielectric (e.g. with end cap terminations. Both of these package types have large lead spacings. Risk assessments based on lead-frame spacings are difficult, and the type of device and lead-frame substrate The base layer of a structure such as a chip, multichip module (MCM), printed circuit board or disk platter. Silicon is the most widely used substrate for chips. Fiberglass (FR4) is mostly used for printed circuit boards, and ceramic is used for MCMs. material must be taken into account. Surface-mount and pin-through-hole devices (Figures 2 to 5) typically utilize copper or iron-nickel alloys. Axial-leaded devices use iron or copper-plated iron wire for the leads. Surface-mount bricks (Figure 6) generally use tin over nickel end caps, although some bricks utilize pure tin end caps. Some components use tin-plated brass (CuZn) alloys, which are the worst substrates with respect to tin whisker formation. Table 2 summarizes our risk decision matrix. Problems at High Frequency Whisker protrusions affect circuit performance even if shorting of adjacent lead-frames does not occur. Whiskers act as antennas in high-frequency circuits and become an issue at 6 GHz (RF) and above. For a digital circuit, analysis shows that the effect on circuitry is a function of rise time. The rule of thumb for the effective operating frequency as a function of rise time is: f(freq in GHz) = .35/[t.sub.r](rise time in nsec) Working backward, a [t.sub.r] of 58 psecs would be the equivalent rise time for the effect to be significant. This number is becoming more common as device geometries get smaller, integrated circuits Integrated circuits Miniature electronic circuits produced within and upon a single semiconductor crystal, usually silicon. Integrated circuits range in complexity from simple logic circuits and amplifiers, about 1/20 in. (1. (ICs) get faster and buss speeds increase. The total effect is a function of whisker length, whisker density and frequency. To simplify the issue, analysis shows that the tin whisker needs to stay below 75[micro]m in length to avoid affecting a high-speed circuit. Conclusion A methodology for protecting long life, high-reliability systems against failures due to tin whiskers has been suggested. Given the complexity of this topic, any system designer who does not have relevant technical experience and know-how should seek expert advice. NEMI has active projects working to develop accelerated testing techniques as well as a greater understanding of basic whisker fundamentals. The organization is also working with groups in Japan and Europe to attempt to develop acceptable solutions worldwide. The reader is cautioned that any high tin content finish has some associated risk. Caveat emptor [Latin, Let the buyer beware.] A warning that notifies a buyer that the goods he or she is buying are "as is," or subject to all defects. When a sale is subject to this warning the purchaser assumes the risk that the product might be either defective or (and good luck)!
Mitigation Practice Description
Nickel Underlay Use of nickel plate (0.5-5.0 microns)
between the electroplated tin film and
the substrate.
Fused (Reflowed) Tin Fused tin is a process commonly
available in plating shops.
Hot-Dipped Tin Galvanized sheet steel, etc.
Immersion Tin Chemical displacement process; very
thin (0.5 micron) films.
Annealed Tin 150-200[degrees]C for 2-8 hours
Tin Alloys Relatively small amounts (2-10%) of
some alloying elements are recognized
as a mitigation practice. Bi is one such
alloy; Ag may be another. Cu is generally
recognized as a whisker enhancing
additive to Sn films, although some
investigators (4) report that they have
specific SnCu plating formulations that
mitigate whiskers.
Mitigation Practice Comments
Nickel Underlay * Over 50 years of history on Cu
substrates.
* Not effective with iron-based substrates.
* Rarely available on surface-mount devices.
* Common on connectors/bus bars/heat
sinks and can packages.
Fused (Reflowed) Tin * Over 50 years of history.
* Rarely utilized today.
Hot-Dipped Tin * Rarely used on lead-frames.
* Commonly used on sheet steel.
Immersion Tin * Not used in lead-frame industry.
* Commonly found on PCBs.
* Not a historical mitigation practice,
but specific industries have long-term
experience without whisker formation.
Annealed Tin * A historical mitigation practice.
* Commonly used in the 1960s.
* Gradually supplanted by Pb-Sn alloys.
* Recently resurrected for lead-frames.
Tin Alloys * Sn-Bi is not a historical mitigation
practice. It has been offered recently
as a lead-frame finish by some
suppliers. There is some supporting
data in the technical literature.
TABLE 1: Common mitigation techniques.
Lead-Frame
Spacing (microns) Comments
50-100 * Ultra fine spacings not commonly used.
* Non-tin finishes strongly recommended.
* Mitigation practices strongly recommended for tin
finishes.
100-500 * Common fine-pitch spacings.
* Non-tin finishes strongly recommended for critical
applications: military, medical, automotive,
mission critical hardware, aerospace, etc.
* Mitigation practices strongly recommended for tin
finishes.
500-1000 * A fairly long gap for a tin whisker on matte tin
finishes.
* Long-term reliability (> 5 yrs.) requires
mitigation.
* Short-term (< 3 yrs.) may use pure tin without
mitigation.
* Special care is recommended to not use pure tin on
iron substrates without either an underlay or an
anneal.
1000-2500 * A very long Sn whisker, longer than any matte tin
whisker to date.
* Common spacings for pin-through-hole (PTH) devices.
* Pure matte tin over alloy 42 or Cu a minimal risk.
* Mitigation is recommended for critical
applications.
2500-5000 * Whiskers this long have been reported, but they are
extremely rare, and all known to these authors have
been on bright tin deposited onto an iron
substrate.
* Common spacings for pin-through-hole (PTH) devices.
* Mitigation recommended for critical applications
where there is either mechanical shock or
temperature cycling.
>5000 * There are no matte tin whiskers of this length in
the known technical literature.
* There is at least one recorded bright tin whisker
>10.0 mm in length known to these authors. (5)
* Mitigation is recommended for critical applications
where there is either mechanical shock or
temperature cycling.
TABLE 2: Risk analysis based on lead spacing.
References 1. K.G. Compton, A. Mendizza and S.M. Arnold, "Filamentary fil·a·ment n. 1. A fine or thinly spun thread, fiber, or wire. 2. Botany a. The stalk that bears the anther in a stamen. b. A chainlike series of cells, as in many algae. 3. Growths on Metal Surfaces-Whiskers," Corrosion, 7: pp. 327-334, 1951. 2. J.R. Downs, "The Phenomenon of Zinc Whisker Growth and the Rotary Switch (or How the Switch Industry Captured the Abominable Snowman abominable snowman or yeti (yĕt`ē), humanlike creature so named because it is associated with the perpetual snow region of the Himalayas. ," Metal Finishing, pp. 23-25, August 1994. 3. J. Brusse, G.J. Ewell and J.P. Siplon, "Tin Whiskers: Attributes and Mitigation," Proc. Of the 22nd Capacitor and Resistor resistor, two-terminal electric circuit component that offers opposition to an electric current. Resistors are normally designed and operated so that, with varying levels of current, variations of their resistance values are negligible (see resistance). Technology Symposium (CARTS), pp. 67-80, March 2002. 4. J.C. Bing-Lee, Y.L. Yao, F.Y. Chiang, P.J. Zheng, C.C. Liao and Y.S. Chou, "Characterization Study of Lead-Free Sn-Cu Plated Packages," Proc. Of the 2002 Electronic Components and Technology Conference (IEEE (Institute of Electrical and Electronics Engineers, New York, www.ieee.org) A membership organization that includes engineers, scientists and students in electronics and allied fields. ECTC ECTC Electronic Components and Technology Conference ECTC Erosion Control Technology Council ECTC Earth Commission for Thermostatic Control (from environmentalist book The Weather Makers) ECTC Expected Cost to Company ), pp. 1238-1245, 2002. 5. Private communication, J. Smetana, Alcatel Corporation, 2003. George Galyon is a senior technical staff member for IBM (International Business Machines Corporation, Armonk, NY, www.ibm.com) The world's largest computer company. IBM's product lines include the S/390 mainframes (zSeries), AS/400 midrange business systems (iSeries), RS/6000 workstations and servers (pSeries), Intel-based servers (xSeries) , Poughkeepsie, NY; email: Galyon@us.ibm.com. He chairs the NEMI Tin Whisker Modeling Project and organized the NEMI Tin Whisker User Group. Ron Gedney, email: rgedney@nemi.org, is retired from NEMI and continues to consult on several of the consortium's lead-free projects. For additional information about NEMI's tin whisker and lead-free activities, visit www.nemi.org/projects/ese/index.html. |
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