Printer Friendly
The Free Library
22,728,043 articles and books

Surface oxidation as a tin whisker growth mechanism: while water vapor accelerates whiskers, they also grow in ambient air.



A popular alternative to SnPb solders used for surface finish applications is pure tin. Pure tin plating can develop "whiskers See metal whiskers. ," outgrowths of small, wire-like protrusions from tin (Figure 1). These whiskers are electrically conductive conductive

having the quality of readily conducting electric current.


conductive flooring
flooring or floor covering made specially conductive to electrical current, usually by the inclusion of copper wiring that is earthed
 and therefore can create the potential for short circuits in electronics. They can also break off and form conductive debris that could also cause shorts. (1,2)

The mechanisms of whisker formation are not understood, although several theories have been forwarded. (2) Of these mechanisms, stress of the plated tin surface is widely believed to be a contributing factor. Oxide formation is associated with volumetric volumetric /vol·u·met·ric/ (vol?u-met´rik) pertaining to or accompanied by measurement in volumes.

vol·u·met·ric
adj.
Of or relating to measurement by volume.
 expansion of the surface layers of metal. This could conceivably cause a change in the mechanical stress in the surface. This possibility prompted the investigation of Sn-plated brass coupons exposed to oxidizing atmospheres, particularly humidity. Preliminary results are striking; moderate to rapid whisker growth occurred with most of the oxidizing atmospheres when compared to the control group.

[FIGURE 1 OMITTED]

Our first experiments involved placing Sn-coated brass coupons over cups containing deionized de·i·on·ize  
tr.v. de·i·on·ized, de·i·on·iz·ing, de·i·on·iz·es
To remove ions from (a solution) using an ion-exchange process.



de·i
 and tap water (McKinney, TX, municipal supply). After several weeks, whisker growth was noted on these coupons. Initially, we believed that whisker growth was more pronounced on the coupon suspended over tap water than DI water.

This preliminary observation led to an investigation of the tap water to identify possible contributing agents. There are several differences between tap and DI water--not the least of which is the presence of ionic i·on·ic
adj.
Of, containing, or involving an ion or ions.



ionic

pertaining to an ion or ions.


ionic medication
iontophoresis.
 salts. However, the presence of such materials seemed unreasonable as a contributing factor as they have limited volatility. Another, more likely compound present in tap water that may account for accelerated whisker growth is chlorine dioxide chlorine dioxide,
n an oxidizing agent used in oral care to decrease amounts of volatile sulfur compounds that may cause halitosis.
, a strong oxidant oxidant /ox·i·dant/ (ok´si-dant) the electron acceptor in an oxidation-reduction (redox) reaction.

ox·i·dant
n.
See oxidizer.
 used in water purification It has been suggested that , , and be merged into this article or section. . At this early stage, it was theorized that the tap water might present a more oxidizing environment than DI water, thus explaining our initial observations. As a result, an investigation was initiated to determine the role of water, both tap and DI, on whisker growth. Because oxidation was a suspect mechanism, experiments were also designed with the role of the ambient environment in mind (i.e., air, inert gas inert gas or noble gas, any of the elements in Group 18 of the periodic table. In order of increasing atomic number they are: helium, neon, argon, krypton, xenon, and radon. , etc.). Chemical pretreatment pretreatment,
n the protocols required before beginning therapy, usually of a diagnostic nature; before treatment.

pretreatment estimate,
n See predetermination.
 of coupons with common cleaners, which could contribute to whisker growth, was also investigated. As results of the following experiments show, moisture plays a significant role on whisker formation; however, it is not clear if DI or tap water preferentially accelerate whisker growth.

Methodology

Coupons used for the experiments were 1 X 3" brass strips with bright tin electroplating electroplating: see plating.
electroplating

Process of coating with metal by means of an electric current. Plating metal may be transferred to conductive surfaces (e.g., metals) or to nonconductive surfaces (e.g.
. One set of chemically treated coupons was rinsed in dichloromethane and another with a 10% aqueous aqueous /aque·ous/ (a´kwe-us)
1. watery; prepared with water.

2. see under humor.


a·que·ous
adj.
 solution of 60:20, ethanolamine ethanolamine /eth·a·nol·amine/ (eth?ah-nol´ah-men) monoethanolamine.

ethanolamine oleate
:butoxyethanol, which was then rinsed with DI water to imitate a common inline board cleaner application. These two sets of coupons represent the extent of the chemical pretreatment investigation that was pursued. All other investigations were performed with "as received" coupons which were not manipulated in any way. Experiments were performed in most cases by placing two coupons over 12 oz. Styrofoam cups, filled to about 1/8" from the top of the cup with either DI or tap water (control cups were left empty). An additional experiment was carried out by vertically hanging single coupons over tap and DI water. In all cases, water was added biweekly to replace losses by evaporation evaporation, change of a liquid into vapor at any temperature below its boiling point. For example, water, when placed in a shallow open container exposed to air, gradually disappears, evaporating at a rate that depends on the amount of surface exposed, the humidity .

To test environmental influence, one set of experiments was carried out in a nitrogen cabinet to create an "inert" atmosphere, another in room air and others in environments containing gaseous gas·e·ous
adj.
1. Of, relating to, or existing as a gas.

2. Full of or containing gas; gassy.
 oxidizing materials.

More aggressive oxidizing environments were created by chemical reactions This is the 18th episode of television drama Men in Trees. It originally aired on June 25, 2007 on the TV2 network in New Zealand as a continuation of season 1. Recap
Marin and Cash have a stew cook off, she admits his is better than hers.
 that liberated oxidizing gases. These were carried out in sealed desiccant desiccant /des·ic·cant/ (des´i-kant)
1. promoting dryness.

2. an agent that promotes dryness.


des·ic·cant
n.
 chambers in which "as received" coupons were placed on top of the Styrofoam cups. Of these environments, only the results of an experiment regarding exposure of a coupon to nitrogen oxides formed upon reaction of nitric acid nitric acid, chemical compound, HNO3, colorless, highly corrosive, poisonous liquid that gives off choking red or yellow fumes in moist air. It is miscible with water in all proportions.  with metallic copper will be discussed. This experiment was accomplished by placing several pieces of copper at the bottom of one of the coffee cups, placing a coupon on the top of the cup and adding three drops of nitric acid to generate the gasses. The desiccant chamber was then sealed. Additional applications of nitric acid were performed at 24- and 48-hr. intervals, after which the coupon was not disturbed until examination one month later.

Mild Atmospheres

The first coupons observed growing whiskers (after six weeks) were those contained in the nitrogen cabinet suspended over water. Initially this seemed unlikely, as a nitrogen environment is more inert than room air. The control coupons suspended over an empty cup in the nitrogen cabinet did not grow similar whiskers. The accelerated growth of the coupons suspended over water was rationalized by the fact that the nitrogen chamber, as used, was actually venting nitrogen at a substantial rate leading to greatly accelerated water evaporation (these cups lost water to evaporation about four times faster than air samples). In essence, instead of creating an inert atmosphere experiment, we created an increased water evaporation experiment. These findings, as well as the initial experiments we carried out using only water, led us to conclude that oxidation of the tin surface as a result of exposure to water accelerates whisker formation. In fact, only the sides of the coupons facing the water showed accelerated growth; the top sides were not different than control room air samples.

[FIGURE 2 OMITTED]

For the experiments carried out in room air, similar results were noted, except whisker formation took longer before appreciable growth was noted (about three months in most cases). In these cases, water-facing sides of coupons grew whiskers at an accelerated rate while the air-facing sides and control samples did not. Also, these water-facing sides grew significantly more whiskers in areas directly over the water compared with the coupon areas that were outside the rim of the cups and not directly over the water. Additionally, the coupons vertically hung over water showed similar growth on both sides of the coupons (overall growth was more limited in these cases). The differences between DI and tap water, which were initially suspected, could not be proven by these experiments. The growth rate seemed somewhat random with some coupons suspended over DI water and others over tap water seemingly growing whiskers more rapidly. While some evidence suggests that chemical pretreatment may accelerate growth rates Growth Rates

The compounded annualized rate of growth of a company's revenues, earnings, dividends, or other figures.

Notes:
Remember, historically high growth rates don't always mean a high rate of growth looking into the future.
, it is impossible to verify from the data collected from these experiments. The following images are typical whiskers resulting from these experiments.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Figure 2 shows that while most whiskers are relatively thin, some variation in diameter is noted. Also, in most cases the whiskers grow unidirectional The transfer or transmission of data in a channel in one direction only. ; but this is not always the rule, as noted in Figures 3 and 4. In fact, the whisker in Figure 4 is observed growing in five different directions along its length, forming a half-circle loop with an extension.

Extreme Atmospheres

The sample exposed to nitrogen oxides provided the most surprising observations. After one month, the coupon was removed and several dozen massively thick whiskers were noted. The number of whiskers was much lower than more typical examples, however, the diameters of these whiskers dwarfed those previously observed (Figure 5).

In this case rapid surface oxidation, resulting from exposure to the extreme environment, resulted in rapid stress formation that was relieved to a degree upon formation of these large whiskers. The rates of all processes occur more rapidly than the relatively slow water oxidation described in the previous section that leads to typical thin whisker formations.

Interestingly, after this coupon was exposed to room air for one day, extreme corrosion beyond that induced by the nitrogen oxides environment was noted. For example, oxidation of the whisker in Figure 5 was observed by SEM using backscatter backscatter

in radiology, radiation deflected by scattering processes at angles greater than 90 degrees to the original direction of the beam of radiation. Important in radiotherapy when estimating surface exposure dose.
 mode (low atomic weight atomic weight, mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope.  species, such as oxygen, appear dark in this mode, as in Figure 6).

These surprising results seemed to verify our assumptions that surface oxidation leading to stresses of the tin plate can accelerate whisker formation. More surprising and less easily rationalized is the fact that five days after the initial investigation revealed large whiskers, many more traditional sized whiskers were noticed on this same coupon--small whiskers grew less than one week after this coupon was exposed to ambient air. The morphology of the whiskers, however, is atypical. These were also observed to oxidize oxidize /ox·i·dize/ (ok´si-diz) to cause to combine with oxygen or to remove hydrogen.

ox·i·dize
v.
1. To combine with oxygen; change into an oxide.

2.
 rapidly (Figure 7).

Note that the ends of the whiskers in Figure 8 are rounded, leading to the appearance of tiny matchsticks. This is the first time we observed such morphology.

Experiments carried out under aggressive oxidizing environments have been difficult to reproduce in a consistent fashion. Additional work is needed to characterize the conditions under which these features develop.

In conclusion, we believe that the examples presented reveal that oxidation of tin coatings accelerates whisker formation. This may be related to a change in the state of stress in the surface. The experiments performed with increased water evaporation grew whiskers more rapidly than samples exposed to water at normal evaporation rates. Moreover, both grew whiskers faster than coupons exposed simply to room air. Finally, in dramatic fashion, by exposing a coupon to an artificial oxidizing environment rapid and unusual whisker formation was observed. Further experimentation concerning oxidizing environments is ongoing.

References

1. J.A. Brusse, G.J. Ewell, J.P. Siplon, "Tin Whiskers See metal whiskers. : Attributes and Mitigation", 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 Proceedings, March 2002, pp. 1-21.

2. http://nepp.nasa.gov/whisker/

W. John Wolfgong, Ph.D. is chemist, Bob Ogden is reliability engineer, Robert Champaign is failure analysis engineer and Barbara Waller is manager, Raytheon Failure Analysis Lab (raytheon.com); wolfgong@raytheon.com.

W. John Wolfgong, Ph.D., Bob Ogden, Robert Champaign and Barbara Waller
COPYRIGHT 2005 UP Media Group, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2005, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

 Reader Opinion

Title:

Comment:



 

Article Details
Printer friendly Cite/link Email Feedback
Author:Waller, Barbara
Publication:Circuits Assembly
Article Type:Cover Story
Date:Dec 1, 2005
Words:1622
Previous Article:Lead-free new problems--new solutions.
Next Article:Gauging true speed and accuracy: the authors propose new standards for measuring throughput based on production floor environments.



Related Articles
ELECTRODEPOSITED Pb-FREE SOLDER AND WHISKER PREVENTION.
Close shave: consortia to standardize on Tin Whisker test. (Around The World / Manufacturing).
Consortia hold Tin Whisker meeting to develop test method.
Avoiding tin whisker reliability problems: fighting tin whiskers involves knowing the available mitigation strategies and risk analysis based on lead...
Tin Whiskers: an old problem reappears: the answers to today's problems may lie in yesterday's research.
Whisker formation potential in Pb-free electroplated connector finishes: pure tin over nickel performed best in these environmental aging tests.
Tin Whisker management guidelines: although full understanding of whisker growth is lacking, these specifications and practices will reduce risks.
Tin whiskers and conversion to Pb-free: a low-lead-content SnPb finish reduces whiskers while cutting lead volume by 85%.
Tin whisker management guidelines: although full understanding of whisker growth is lacking, these specifications and practices will reduce risks.
Tin whisker management guidelines, part 2: how 40 [micro]m was found to be the maximum acceptable whisker length for high-rel product, and...

Terms of use | Copyright © 2014 Farlex, Inc. | Feedback | For webmasters