Whisker formation potential in Pb-free electroplated connector finishes: pure tin over nickel performed best in these environmental aging tests.Although no generalized gen·er·al·ized adj. 1. Involving an entire organ, as when an epileptic seizure involves all parts of the brain. 2. Not specifically adapted to a particular environment or function; not specialized. 3. mechanism for whisker initiation and growth has been adopted, some factors are commonly believed to encourage the 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. of mechanical stresses in tin electrodeposits and, thus, whisker formation (1): [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. ] 1. Residual stress Residual stresses are stresses that remain after the original cause of the stresses (external forces, heat gradient) has been removed. They remain along a cross section of the component, even without the external cause. within the tin deposit--caused by factors such as the plating chemistry and process. 2. Intermetallic formation at the substrate/deposit interface causing the buildup of stresses. 3. Forming/application stresses, such as bending of component leads or pressures at contact interfaces. 4. Surface blemishes--inadvertent handling defects such as scratches or dents that induce stress. 5. Thermal expansion thermal expansion Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change. mismatches between plating deposits and the underlying 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. [FIGURE 1 OMITTED] [FIGURE 2 OMITTED] The whisker testing methodologies developed to date for electrodeposited finishes have largely relied upon accelerating the factors listed above using traditional environmental stresses such as elevated temperature, elevated humidity humidity, moisture content of the atmosphere, a primary element of climate. Humidity measurements include absolute humidity, the mass of water vapor per unit volume of natural air; relative humidity (usually meant when the term humidity and thermal cycling between temperature extremes. Interestingly, past observations of whiskering have shown that temperature can serve to either enhance or reduce whiskering depending on the precise value of the temperature. Excessively high temperatures may introduce competing effects by increasing the rate of diffusion diffusion, in chemistry, the spontaneous migration of substances from regions where their concentration is high to regions where their concentration is low. Diffusion is important in many life processes. while simultaneously relieving stress by annealing annealing (ənēl`ĭng), process in which glass, metals, and other materials are treated to render them less brittle and more workable. the tin grain structure. Because of complexities such as these, the industry has migrated from the use of a single stress to encourage whiskering to the use of multiple stresses carried out either in series or in parallel. A "preconditioning preconditioning preparation of 6 to 8 months old range-reared, recently weaned beef calves for entry into a feedlot and an intensive fattening program. Includes castration, dehorning and branding 3 weeks before and all vaccinations 2 weeks before weaning, and weaning 3 to 4 weeks " treatment of the sample is generally accomplished by either mechanical (bending) or thermal (shock) means to encourage the initiation of whiskers See metal whiskers. . Although a truly meaningful whisker test will only arise once the actual mechanism for whisker initiation/growth is understood, appropriate acceleration factors are selected, and confirmation is achieved through extensive field correlation work, the pressing need to ensure the selection of the most suitable lead-free electrodeposited finishes has necessitated the use of tests containing various combinations of the environmental stresses described above. Thus, to determine which lead-free 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. connector finishes may be least susceptible to whiskering, a two-phase evaluation was carried out. To force distinctions in whiskering behavior, the first phase consisted of exposing various pure Sn, Sn-Cu and Sn-Bi deposits to intentionally in·ten·tion·al adj. 1. Done deliberately; intended: an intentional slight. See Synonyms at voluntary. 2. Having to do with intention. over-stressed dry heat, elevated temperature/humidity and thermal cycling aging conditions. The second phase of testing exposed the most promising finishes from the first phase to one of the whisker test variants that had circulated within a consortium of connector companies (Molex, Tyco, FCI (Flux Changes per Inch) The measurement of polarity reversals on a magnetic surface. In MFM, each flux change is equal to one bit. In RLL, a flux change generates more than one bit. and Amphenol). Although test methodologies continue to be refined, the present evaluation has implications for whisker testing and mitigation strategies. [FIGURE 3 OMITTED] Pb-Free Finish Screening Samples for whisker testing were created from square phosphor bronze phosphor bronze n. A hard, strong, corrosion-resistant bronze containing tin and a small amount of phosphorus, used in machine parts, springs, and fine tubing. Noun 1. pins (C510 alloy, 0.635 X 0.635 mm) plated with the finishes of interest in a single high-speed, reel-to-reel plating line. The barrier layer thicknesses were 2.0-2.5 [micro]m, and the lead-free and tin-lead deposits were 3.0-3.5 [micro]m thick. The following plating deposits were included in the first phase of the evaluation: All plating chemistries were sourced from a single vendor. Any finishes noted above as "modified" simply mean that the chemistry had been altered from its normal formulation to facilitate various independent evaluations by the chemistry vendor, not necessarily that the chemistry had been intentionally modified to suppress whiskering. On the other hand, the "whiskerless" notation notation: see arithmetic and musical notation. How a system of numbers, phrases, words or quantities is written or expressed. Positional notation is the location and value of digits in a numbering system, such as the decimal or binary system. indicates early versions of chemistries that were created specifically to suppress whisker formation (these variants may or may not have subsequently seen commercialization). [FIGURE 4 OMITTED] Samples were preconditioned pre·con·di·tion n. A condition that must exist or be established before something can occur or be considered; a prerequisite. tr.v. to introduce deposit stresses by bending around a mandrel mandrel /man·drel/ (man´dril) the shaft on which a dental tool is held in the dental handpiece, for rotation by the dental engine. man·drel or man·dril n. 1. , producing a 1.6 mm diameter curvature curvature Measure of the rate of change of direction of a curved line or surface at any point. In general, it is the reciprocal of the radius of the circle or sphere of best fit to the curve or surface at that point. . This procedure induced compressive stresses 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. on the inner part of the curved section (Figure 1). Samples were then exposed to dry-heat aging conditions (50[degrees]C with uncontrolled/low relative humidity relative humidity n. The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage. ). Samples were inspected in the compressed portion of the bend via scanning electron microscopy electron microscopy Technique that allows examination of samples too small to be seen with a light microscope. Electron beams have much smaller wavelengths than visible light and hence higher resolving power. (SEM) for whiskers both before and after aging by viewing the inside surface of the curved section of the pin. Higher magnifications were used to document any whiskers or plating surface alterations that were observed. The aging was carried out for a total of 16 months with SEM documentation at the end of every month of exposure. After the 16 months of dry-heat exposure were completed, the samples were split into two groups and subjected to either six months of elevated temperature/humidity (50[degrees]C/90% RH) or two months of thermal cycling (-40[degrees] to 125[degrees]C, 90 min. cycles with 30 min. dwell at each temperature extreme). Phase 1 results. The formation of whiskers occurred readily for the pure tin over copper barrier layer samples. In fact, whiskering was observed after only a short period of exposure to dry heat (50[degrees]C and uncontrolled/low humidity). The SEM images (Figures 2 and 3) highlight some of the whisker morphologies. In contrast, the pure tin over nickel barrier layer samples did not exhibit whiskering in the dry heat even up to 16 months of total exposure time. Only with exposure for a six-month period to an elevated temperature/humidity environment did one of the pure tin over nickel barrier layer samples, the reflowed matte finish, exhibit the formation of some whiskers (Figure 4). The as-plated matte tin over nickel barrier layer samples and the bright tin over nickel barrier layer samples remained whisker-free in this environment. Two months of thermal cycling also caused the reflowed matte tin over nickel to whisker. The thermal cycling caused some surface reconstruction In surface physics, surface reconstruction is the name given to the process by which the atoms at the surface of a crystal rearrange themselves to form a structure with a different periodicity and/or symmetry than that of the bulk crystal. for the matte tin over nickel, but no whiskering was observed (Figure 5). Both of the whiskerless tin deposits were able to survive exposure to the dry heat and thermal cycling conditions without the formation of whiskers. When subjected to the elevated temperature/humidity environment, differences were noted in the behavior of the two finishes. The "whiskerless 1" samples displayed some extensive surface reconstruction that, although not necessarily definable as "whiskering," could be a precursor precursor /pre·cur·sor/ (pre´kur-ser) something that precedes. In biological processes, a substance from which another, usually more active or mature, substance is formed. In clinical medicine, a sign or symptom that heralds another. to the formation of such structures (Figure 6). The "whiskerless 2" samples did not exhibit whiskering or this surface reconstruction phenomenon. (A summary of results for all the tin finishes can be found in the online version.) Significant differences were noted in the whisker susceptibility susceptibility the state of being susceptible. Refers usually to infectious disease but may be to physical factors such as wetting or to psychological factors such as harassment. for tin-copper, depending upon whether the finish was bright or matte. In the case of a matte tin-copper finish, no whiskering was observed following exposure to 16 months of dry heat, regardless of whether the composition was 99/1 or 96/4 tin-copper. With the introduction of the samples to an elevated temperature/humidity environment, whiskering occurred readily for both compositions (Figures 7 and 8). Two months of thermal cycling only served to cause surface reconstruction/nodular type whisker formation for one of the four types of matte tin-copper finishes (Figure 9). Unlike the matte finishes, the bright tin-copper finishes did experience some susceptibility to the dry-heat environment with a reconstruction of the surface of the electrodeposit e·lec·tro·de·pos·it tr.v. e·lec·tro·de·pos·it·ed, e·lec·tro·de·pos·it·ing, e·lec·tro·de·pos·its To deposit (a dissolved or suspended substance) on an electrode by electrolysis. n. The substance so deposited. readily observable ob·serv·a·ble adj. 1. Possible to observe: observable phenomena; an observable change in demeanor. See Synonyms at noticeable. 2. (Figure 10). Although the modified bright (96/4) tin-copper deposit experienced a slight amount of whisker formation, generally the bright tin-copper finishes (Figure 11) were unaltered by six months of exposure to the elevated temperature/humidity environment. The exposure to thermal cycling was significantly more detrimental for these bright finishes. Only one month of exposure was required to induce the formation of significant whiskering on both the 99/1 tin-copper composition (Figure 12) and the 96/4 tin-copper composition (Figure 13). [FIGURE 5 OMITTED] The whiskerless tin-copper plating variants exhibited some similar trends to those noted for the matte tin-copper finishes. No problems were experienced in the dry heat and thermal cycling environments; however, exposure to elevated temperature/humidity did lead to the formation of significant whiskering. Both "whiskerless 1" and "whiskerless 2" variants produced numerous fine whiskers on the surface. (A summary of results for all tin-copper finishes can be found online.) The tin-bismuth finishes were unaltered by exposure to dry heat for up to 16 months. With the introduction of an elevated temperature/humidity environment, whisker formation was observed; however, some differences in morphology morphology In biology, the study of the size, shape, and structure of organisms in relation to some principle or generalization. Whereas anatomy describes the structure of organisms, morphology explains the shapes and arrangement of parts of organisms in terms of such were noted relative to the pure tin and tin copper deposits (Figure 14). In this case, the whisker-like structures appeared to have a growth direction close/parallel to the surface of the electrodeposit. No significant differences were noted between the finishes from the normal and the modified tin-bismuth plating chemistries. Exposure to thermal cycling induced the growth of a similar type of whisker (Figure 15). [FIGURE 6 OMITTED] Matte and Bright Tin Evaluations Based on the results from the first phase of the study, pure tin over nickel electrode-posits were selected for further evaluation. One type of connector terminal (Type 1) was plated with tin-lead (90/10), satin satin, lustrous silk in which the filling is so arranged as to bind the warp as seldom as possible and so spaced that practically nothing shows but the warp. Satin was first woven by the ancient silk weavers of China and was greatly desired by early Greeks and Romans. tin or bright tin. A second type of connector terminal (Type 2) was plated with tin-lead (90/10) or matte tin. All the tin and tin-lead platings were 3.0-3.5 [micro]m thick and were deposited on top of a nickel barrier layer 2.0-2.5 [micro]m thick. The connector type 1 and connector type 2 finishes represented the product offerings of a single commercial plating chemistry vendor. [FIGURE 7 OMITTED] The preconditioning and aging conditions used to test for tin whisker susceptibility were based upon one of the test variants that had circulated within a consortium of global connector companies. Thus, the terminals were preconditioned to introduce deposit stresses by thermal cycling 500 times from -55[degrees] to 85[degrees]C with cycles of 20 min. duration and dwell times The time cargo remains in a terminal's in-transit storage area while awaiting shipment by clearance transportation. See also storage. of 7 min. at either temperature extreme. Samples representing each type of plating finish were then separated into three groups for testing in parallel according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. one of the following aging conditions: 1. Dry heat at a temperature of 50[+ or -]5[degrees]C (uncontrolled humidity) 2. Heat/humidity storage at a temperature of 52[+ or -]5[degrees]C and 90[+ or -]5% relative humidity. 3. Room temperature storage at a temperature of 23[+ or -]5[degrees]C. Samples were inspected via SEM for whiskers both before and after aging by viewing an area at least 0.5 X 0.5 mm at a magnification Magnification A measure of the effectiveness of an optical system in enlarging or reducing an image. For an optical system that forms a real image, such a measure is the lateral magnification m of 70-80X. Higher magnifications were used to document any whiskers or plating surface alterations that were observed. The aging was carried out for a total of 6 months with SEM documentation at the end of every month of exposure (only the results through the end of 5 months of exposure will be reported here). Phase 2 results. As expected, the samples plated with tin-lead did not experience the formation of any tin whiskers See metal whiskers. . Similarly, regardless of the particular aging conditions utilized (dry heat, temperature/humidity or room temperature), the type 1 terminals plated with satin tin and bright tin did not exhibit the formation of tin whiskers. The results were comparable for the finishes on the type 2 terminals. The whiskering of tin was observed to be highly dependent upon the composition of the barrier layer, copper or nickel, presumably pre·sum·a·ble adj. That can be presumed or taken for granted; reasonable as a supposition: presumable causes of the disaster. due to the more rapid formation of tin-copper intermetallics than tin-nickel intermetallics. The amount of tin-copper intermetallic formed, and therefore stress induced, would simply have been dependent upon the amount of time spent at elevated temperature, such as occurred during the 16 months of dry-heat testing. Other researchers have hypothesized that the formation of the intermetallic tin-copper layer produces an accumulation of significant stresses within the electrodeposit that are then relieved via the formation of whiskers. The selection of a matte or bright tin finish did not appear to be a significant factor, although the surface roughness, grain structure and (presumably) deposit stress were significantly different between the two deposit types. Interestingly, even with a nickel underplate, once reflowed, the matte tin samples exhibited some whiskering in elevated temperature/humidity or thermal cycling environments. This was somewhat surprising because it is generally believed that the stress relief offered to the plated coating via the 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. process would be beneficial in suppressing whisker formation. This could be an indication that the reflow process actually induced some stresses during the solidification so·lid·i·fy v. so·lid·i·fied, so·lid·i·fy·ing, so·lid·i·fies v.tr. 1. To make solid, compact, or hard. 2. To make strong or united. v.intr. process that were greater than those inherent in the as-plated electrodeposit. Note also that one of the two whiskerless plating chemistry formulations was found to exhibit significant surface reconstruction, potentially a precursor to whiskering, after aging in the elevated temperature/humidity environment. The whiskerless plating chemistries have continued to be refined in the years that have elapsed e·lapse intr.v. e·lapsed, e·laps·ing, e·laps·es To slip by; pass: Weeks elapsed before we could start renovating. n. since these test samples were first plated; thus, whisker susceptibility for these sorts of samples may now be different. At a minimum, the results for the whiskerless tin chemistry and the reflowed matte tin finish would suggest that electrodeposit properties and characteristics could be manipulated such that an otherwise whisker resistant type of finish whiskers quite readily. [FIGURE 8 OMITTED] [FIGURE 9 OMITTED] The tin-copper finishes appeared to whisker more readily than the pure tin finishes over nickel. Although the composition of the tin-copper deposits generally did not alter the likelihood of whiskering, the selection of a matte or bright finish did have an influence. Enhancement of stress in the bright deposit through thermal soaking and cycling appeared to encourage surface reconstruction and whisker formation, respectively. On the other hand, a surface chemistry factor seemed to be a limitation for the matte tin-copper finishes, as they readily whiskered once humidity was introduced into the aging environment. Further study would be required to understand this phenomenon, but this could indicate the important role of moisture in chemically altering the inherent oxide structure on tin-copper. The tin-bismuth deposits required either thermal cycling or humidity to induce whisker formation. It was interesting to note that the whisker morphology for these deposits seemed to be significantly different from the morphologies noted for the pure tin and tin-copper deposits. The size, growth direction and proximity of these whiskers to the surface of the deposit may suggest a different level of risk to device performance for these whiskers than those from the other two types of electrodeposit. [FIGURE 10 OMITTED] [FIGURE 11 OMITTED] Within the present testing, the use of environmental exposure conditions in both a parallel and serial manner helped to highlight an important aspect of whisker formation. A given electrodeposit could robustly withstand many months in a particular environment without whiskering, yet in a relatively short span of time with a one-factor modification of the aging environment the same deposit could suddenly experience significant whisker initiation and growth. In addition to providing some clues about the factors/mechanisms contributing to whisker formation, this observation indicates that the selection of an appropriate test environment and linkage linkage In mechanical engineering, a system of solid, usually metallic, links (bars) connected to two or more other links by pin joints (hinges), sliding joints, or ball-and-socket joints to form a closed chain or a series of closed chains. of the accelerated test to the field environment will be critical. It will also be important to make sure that the samples under test are not "stressed" beyond the limits that they will experience in the field. Otherwise, various failure thresholds may be exceeded, resulting in the activation of a failure mode that otherwise would not be relevant in the field. Although it is not possible to predict the performance of the candidate electrodeposits in all environments, from all plating chemistries, and under all conditions, based upon the results presented in this work, it is suggested that the lead-free electroplated finish for electronic connectors that would appear to be least susceptible to whiskering is either a matte or bright pure tin finish over a nickel barrier layer. The test conditions used in this study were based upon some of the evolutionary trends Noun 1. evolutionary trend - a general direction of evolutionary change trend, drift, movement - a general tendency to change (as of opinion); "not openly liberal but that is the trend of the book"; "a broad movement of the electorate to the right" in the industry; however, a truly meaningful whisker test will only come about once the actual mechanism for whisker initiation/growth is understood, appropriate acceleration factors are selected and confirmation is achieved through extensive field correlation work. [FIGURE 12 OMITTED] Conclusions The whiskering performance of a particular finish is known to depend strongly on a variety of factors, including surface oxide condition, electrodeposit thicknesses, composition of the barrier layer, level of residual stress, temperature and time. This study has examined 20 specific electrode-posited connector finishes within controlled experiments "Controlled Experiment" is an episode of the original The Outer Limits television show. It first aired on 13 January, 1964, during the first season. Introduction A martian controller is assigned to investigate the phenomenon of murder on Earth. using mechanically/thermally induced residual stresses, elevated temperature, thermal cycling and elevated humidity. Within the constraints of the aging conditions, plating chemistries and sample types utilized for this study, several conclusions can be reached: 1. For pure tin electrodeposits, nickel barrier layers were significantly more effective than copper barrier layers in the suppression of whiskering. [FIGURE 13 OMITTED] 2. Matte tin and bright tin over nickel barrier layers did not whisker when subjected to the various environmental aging conditions used in this study. 3. Some whiskering of reflowed matte tin over a nickel barrier layer occurred when samples were subjected to extended times in either dry heat followed by elevated temperature/humidity or dry heat followed by thermal cycling. [FIGURE 14 OMITTED] 4. The various tin-copper and tin-bismuth finishes in this study experienced whiskering when exposed to dry heat followed by elevated temperature/humidity or dry heat followed by thermal cycling. 5. The whiskerless plating chemistries produced deposits that differed greatly in whisker susceptibility and that actually exhibited whiskering when subjected to certain environmental aging conditions. 6. The lead-free electroplated finish for electronic connectors that would appear to be least susceptible to whiskering was either a matte or bright pure tin finish over a nickel barrier layer. [FIGURE 15 OMITTED] Sample Description Barrier SnPb (90/10) Nickel SnCu (99/1) matte Nickel SnCu (99/1) matte modified Nickel SnCu (96/4) matte Nickel SnCu (96/4) matte modified Nickel Sn (Ni barrier) matte Nickel Sn (Ni barrier) matte reflowed Nickel Sn (Cu barrier) matte Copper Sn (Ni barrier) bright Nickel Sn (Cu barrier) bright Copper SnCu (99/1) bright Nickel SnCu (99/1) bright modified Nickel SnCu (96/4) bright Nickel SnCu (96/4) bright modified Nickel Sn "Whiskerless" 1 Nickel SnCu alloy "Whiskerless" 1 Nickel Sn "Whiskerless" 2 Nickel SnCu alloy "Whiskerless" 2 Nickel SnBi Nickel SnBi modified Nickel Ed.: This paper was originally published at SMTA SMTA Surface Mount Technology Association SMTA Standard Material Transfer Agreement SMTA Subordinate Message Transfer Agent SMTA Sewing Machine Trade Association (UK) SMTA Sekolah Menengah Tingkat Atas International (smta.org) in September 2004 and is reprinted here with permission of the authors. References (1.) J.A. Brusse, G.J. Ewell and J.P. Siplon, "Tin Whiskers: Attributes and Mitigation," CARTS 2002: 22nd 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. 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, 2002. Dr. Brett Rickett, director of reliability, and Pete Elmgren are with Molex Inc. (molex.com); brickett@molex.com. George Flowers, S. Gale and Jeff Suhling are on the mechanical engineering faculty at Auburn University Auburn University, main campus at Auburn, Ala.; land-grant and state supported; opened 1859 as East Alabama Male College, reorganized 1872 as the Agricultural and Mechanical College of Alabama; became coeducational 1892; renamed Alabama Polytechnic Institute 1899, (eng.auburn Auburn (ô`bərn). 1 City (1990 pop. 33,830), Lee co., E Ala.; inc. 1839. The city's economy centers around Auburn Univ.; there is some manufacturing. 2 City (1990 pop. 24,309), seat of Androscoggin co. .edu). |
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