0108DB0602
02/2007
Cedar Rapids, IA, USA
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Abstract —This paper presents a thermal study of aluminum with tin plating deposited on Al by means of three different types of tin plating using various cyanide and noncyanide processes. We determined the quality of plating before and after it was exposed to the temperature of 150°C in air for up to eight weeks using multiple testing techniques, such as an adhesion test, thermal shock test, SEM techniques,
EDS X-ray analysis, and thickness measurements. We determined that after aging, test tin plating shows aging related cracks, voids, and separation appearing at the plating interfaces; although in different concentration and progression rates depending on the plating technique used. All types of plating demonstrated deteriorated plating adhesion.
Accelerated thermal aging tests at 150°C discriminate plating types. Aged samples exhibit medium to poor adhesion characteristics, and these results are likely related to the plating process itself. At the origin of this behavior, we might suggest thermal-induced mechanisms, including formation of intermetallics and diffusion, presence of impurities at the plating-bar interface, defect propagation and stress relaxation at the interface.
Tin is one of the most common commercial plating applications for aluminum to improve the stability and decrease the galvanic corrosion of aluminum-to-copper connection
1
. Tin-plated aluminum bus bars are also often used
in electrical application as a cost-effective replacement of more expensive tin-plated copper.
In any electrical application where tin is used as protective plating for aluminum, it is important to determine what kinds of changes of the plating quality can be produced by the aging process and how it may affect the properties of the plating. The possible mechanisms of plating aging are not always fully understood and explained, though numerous processes, such as oxidation, stress relaxation, differential thermal expansion, and galvanic and fretting corrosion are attributed to irreversible changes of contacting interface and corresponding change in
contact resistance
2, 3
.
In other cases, for example for tin plating over copper, highresistant layers of tin-copper intermetallics are usually formed; particularly if the electrical contact is exposed to high temperatures. Degradation of tin and tin alloy plating on copper was studied in many aspects due to the formation of
CuSn intermetallic compounds
4, 5
.
The goal of the following study is to determine the possible aging mechanisms of tin plating on aluminum due to exposure to elevated temperatures for an extended period of time. This study is focused solely on the quality of tin plating on Al in as-plated conditions, and after exposure to elevated temperatures. The choice of ambient temperature (150
°
C) in an accelerated aging study is based on International Standard
ISO 2819:1980 “Metallic coatings on metallic substrates.
Electrodeposited and chemically deposited coatings. Review of methods available for testing adhesion” that recommends
150
°
C as the temperature for the testing of tin plating on Al,
Cu, and Zn alloys and steel substrates. The quality and aging behaviors have been studied for three different traditional tin plating techniques on aluminum.
To provide a good adherence of the plating layer to aluminum, various techniques are developed and are currently used by multiple manufacturers of aluminum bus bars to protect aluminum from oxidation. One of the techniques is using a cyanide-based solution for activation of an aluminum surface.
Another activation technique is using non-cyanide chemicals, which are considered more environmentallyfriendly. Either with or without cyanide-containing solutions, these processes prepare the surface of aluminum bus bars for tin plating through multiple-step procedures, including underplating of a thin layer of bronze or copper.
To study the quality of tin aluminum plating for electrical contact applications, three sets of aluminum bus bars have been plated with tin, using three different techniques of plating most often used in the electrical industry.
Two of them are based on two different versions of the process with proprietary solutions, including cyanidecontaining chemicals, Type 1 (Alstan
®
80) and Type 2
(Alstan
®
88) . Both techniques are supposed to provide tin plating on aluminum with low surface-to-surface contact resistance, and to minimize surface corrosion. The first set of aluminum bus bars was plated with tin using the process consisting of an immersion or electrolytic activation step in the proprietary solutions followed immediately by a tin bronze strike without an intermediate water rinse. Tin plating is following this processing sequence and a subsequent appropriate acid dip and water rinse.
1

Thermal Aging Study of Tin Plating on Aluminum
Data Bulletin
0108DB0602
02/2007
The primary advantage of the Type 1 process is that it is supposed to improve corrosion resistance of the plated part.
The second set of the Al bus bars was plated using the Type 2 process, which is similar to the Type 1 process , but uses different proprietary solutions. Aluminum surface is activated and pre-plated with copper bronze strike, followed by tin plate.
Non-cyanide plating technique Type 3 (Bondal
®
) , applied to the third set of samples, uses a surface activation procedure with cyanide-free zincate solution and copper strike prior to tin plate.
Sample Preparation.
The plating process may affect the quality of the plating, depending on the age of the plating bath or position of the aluminum bar in the bath. To determine if this influence is significant, the bus bars were plated in three different periods in the life cycle of the bath solution. The samples were prepared during two different life cycles. SEM testing of the samples in these batches could define how well the quality of plating is reproduced in regards to bath age. To investigate how the position in the bath influences the quality of the plating, each batch consisted of two bars plated in the same bath but in different position on the rack, the total number of bars equals being 12. Each of the 12 bars was cut in three parts, 50 cm long, one part in the middle of the bar, and two at each edge of the bar. Total number of the samples for each type of plating was 36.
Non-aged (as-received) samples.
Plating adhesion of as received tin-plated aluminum was tested for samples plated using all three plating techniques. Indeed, if the plating fails the adhesion test at a non-aged status, further tests are no longer necessary and the plating technique is considered not meeting the specifications. In this study, all of the plating techniques succeeded scribing tests for non-aged samples. Original plating quality is good for both Alstan
®
techniques and cyanide-free zincate techniques.
Aged samples.
Adhesion testing was performed on the samples of Al bus bars plated using all three different tin plating techniques after aging tests. Type 1 samples failed adhesion tests after one week of thermal aging (Figure 1), and
Type 2 samples failed adhesion tests after four weeks of thermal aging (Figure 2). Type 3 samples succeeded adhesion tests after four weeks of thermal aging.
Figure 1: Adhesion Test on Type 1 Aged Sample
Accelerated Aging Test.
To determine the original quality of tin plating on aluminum the bus bar, and compare it with the plating quality of aged samples, the samples have been exposed to elevated temperatures. The accelerated aging test consisted of the exposure of plated bus bars in a classical oven at the temperature of ambient air stabilized at 150
°
C for the period from one to eight weeks. After the specified period, a randomly selected bar was removed from the oven and the samples were cut and prepared for further testing. Aging test for Type 1 plating lasted for 8 weeks and the bars have been removed consequently after 1, 4 and 8 weeks of thermal exposure. For two other types of plating (Type 2 and 3) a more detailed aging process was studied, with the bars removed from the chamber after each week of exposure during a total four weeks.
1. Adhesion tests
Adhesion tests were conducted following an ISO 2819:1980
“Metallic coatings on metallic substrates. Electrodeposited and chemically deposited coatings. Review of methods available for testing adhesion”. The adhesion test involves cross-cut and tape tests. It is first required to scribe down to the metal, then, an adhesive tape with peeling force >180g/cm is applied on the surface and pulled off quickly. If there is any coating peeled away with the tape, the adhesion is considered as nonsatisfactory.
2. Thermal shock tests
The tests were conducted according to International
Standard ISO 2819, which determines the technique of the test. According to the standard the samples must be heated up at the recommended temperature. The samples are then quickly thrown in room temperature water. 150
°
C is the recommended temperature for tin plating over Al, Cu, and Zn alloys and steel substrates. Samples are then examined. If coating is blistering, peeling or exfoliating, then the adhesion is considered not satisfactory.
According to the thermal shock test, the adhesion of tin plating on aluminum is not significantly affected by short exposure to temperature up to 200
°
C. For samples exposed to an elevated temperature for a longer period of time, the adhesion of the plating begins to deteriorate. According to results of the scribing test, the Type 3 plating survives longer exposure to high temperature than Type 1 and Type 2 platings.
2 © 2007 Schneider Electric All Rights Reserved

0108DB0602
02/2007
Figure 2: Adhesion Test on Type 2 Aged Sample
Thermal Aging Study of Tin Plating on Aluminum
Data Bulletin
Plating thickness measurements
Tin and bronze thickness measurements have been done on each SEM capsule (Figure 3), containing three parts of each bar: one part from the middle of the bar and two parts from the edges of the bar. Considering the differences between coating on curved side and linear side, measurements were taken on each side in three different places.
Average values of tin plating and bronze underplating for
Type 1 samples are given in Table 1. For samples of Type 2
(Alstan
®
88) and Type 3 (Bondal
®
), the SEM study shows that tin plating and underplating thickness are homogenous.
Average values of the thickness are given in Table 1.
Figure 3: SEM Sample for Thickness Measurements
Scribing was applied to every sample before and after thermal shock. According to results of scribing after thermal shock at 150
°
C, all samples plated using all three techniques, passed the test and the coating can be considered properly adhesive. In order to study the quality of the plating beyond the limit of 150
°
C, thermal shock test was performed also at
200
°
C. After thermal shock at 200
°
C the surface had an iridescent purple tint. No change in the adhesiveness of the coating was observed.
The quality of the tin-plated aluminum bus bar was investigated using SEM/EDS metallographic analysis to determine:
A. the thickness of tin plating and pre-plating layer (copper or bronze);
B. the presence of pores, voids, and gaps along each interface
(Al-Cu/bronze, Cu/bronze-Sn);
C. morphology of tin plating across the layer.
D. chemical composition of copper/bronze pre-plating layer.
All measurements have been performed using XL Series
Scanning Electron Microscope from Phillips Electron Optic with an EDAX column for EDS analysis. For SEM study, the samples were encapsulated with an epoxy resin polymerizing at room temperature. It is to be noted that polymerization process is heating samples at 70
°
C within a few minutes.
A slight difference in thickness is observed between curved and linear sides. This well-known phenomenon is the result of differences in the electrical field. For all samples, including different plating bath life cycles, different positions in the bath, and different sides of the samples (linear or curved), tin plating thickness varies in the range between 7.1 and 13.9 µm.
Bronze under-plating thickness varies ranges between 0.7 and
1.5 µm. Thickness of both plating and underplating are in agreement with OEM specifications.
Table 1: Thickness of tin plating and underplating layers on aluminum bus bars
Plating
Technique
Type 1 (Alstan 80)
Type 2 (Alstan 88)
Type 3 (Bondal
®
)
Number of
Measurements
Tin Plating
Thickness (µm)
Underplating
Thickness (µm)
280
40
40
13
11
5
1.1 (bronze)
1.5 (bronze)
1.0 (copper)
© 2007 Schneider Electric All Rights Reserved 3

Thermal Aging Study of Tin Plating on Aluminum
Data Bulletin
0108DB0602
02/2007
SEM/EDS analysis of composition and quality of non-aged tin plating on aluminum
SEM microphotograph of cross section and chemical composition of Type 1 plating is shown in Figure 4.
Figure 4: SEM microphotograph and EDS data of randomly selected Type 1 sample #5: cross section (a), composition of base metal - aluminum (b), composition of underplating - tin bronze (c) and the upper layer - tin (d).
The upper layer of the plating consists of pure tin, and the underlayer is composed of Sn and Cu with the proportion corresponding to 70Sn30Cu type of bronze (70% Sn and 30%
Cu). Results of the SEM and EDS studies of freshly plated Al using the Type 1 technique show that the quality of non-aged plating is good: no separation, voids, or cracks have been found.
X-ray analysis confirmed the composition of plating as pure tin, and the underplating as tin-bronze for Type 1 samples.
SEM analysis shows that non-aged Type 2 samples plated have no pores, voids, or gaps detected along each interface
(Al-Bronze, Bronze-Sn) (Figure 4, a).
SEM analysis has shown that non-aged samples plated using the Alstan
®
88 process have no pores, voids or gaps detected along each interface (Al-bronze, bronze-Sn) (Figure 5). The quality of Type 3 samples is good as well: no pores, voids or gaps detected along each interface (Al-Cu, Cu-Sn).
Figure 5: Cross section of Type 2 non-aged tin plating on aluminum a
4 b c d
SEM Analysis of Type 1 samples was performed after 1, 4 and 8 weeks of thermal aging. For two other types of plating,
SEM analysis was done after 1, 2, 3 and 4 weeks of exposure.
All types of tin plating on aluminum show serious deterioration after exposure to elevated temperature (Figures 6 – 8, and
Table 2). The details and rate of deterioration may differ slightly for different plating procedures, but none of the plating withstands the aging test.
It was found that Type 1 and Type 2 platings survived the first week of exposure to high temperature, and Type 3 plating survived 2 weeks, but then the deterioration process proceeded rapidly.
© 2007 Schneider Electric All Rights Reserved

0108DB0602
02/2007
Figure 6: SEM of the Type 1 plating after thermal aging:
1 week (a), 4 weeks (b), and 8 weeks (c)
Figure 7:
Thermal Aging Study of Tin Plating on Aluminum
Data Bulletin
SEM of the Type 2 plating after thermal aging:
2 weeks (a), 3 weeks (b), and 4 weeks (c) a a b b c c
We suggest that at least three different mechanisms could cause deterioration of tin plating on aluminum during exposure to a high temperature for an extended period of time: different ability of three materials (Sn, Cu or bronze and Al) to expand at elevated temperatures, possible pre-existing mechanical stress in the plating, and the quality of tin plating on aluminum.
1. Difference in thermal expansion of three layers.
Base metal is Al, middle layer is Cu or bronze and upper layer is Sn. If the difference in thermal expansion is large enough, then thermal shock or temperature elevation can produce mechanical stress for coating different than for substrate, which may cause delamination of the plating or form cracks through the plating.
However, thermal expansion coefficients for Sn (27 x10-6 /oC) and Al (24 x10-6 /oC) are quite close, it is not likely that bronze with Sn as a major component will be significantly different.
© 2007 Schneider Electric All Rights Reserved 5

Thermal Aging Study of Tin Plating on Aluminum
Data Bulletin
0108DB0602
02/2007
Figure 8: SEM of the Type 3 plating after thermal aging:
2 weeks (a), 3 weeks (b), and 4 weeks (c) a deterioration. Most evident proof of this hypothesis is SEM observation that more cracks, voids and coating peeling are seen on stressed sides than of linear sides of the aged samples.
3. Quality of plating procedure.
In fact, during the different steps of the plating process, including surface activation and cleaning, plating interfaces are exposed to several chemicals, some of them with proprietary composition. If, in the course of the plating process, any of the activation and rinsing phases are not properly done, residues of chemicals may pollute the plating interfaces. Test results show that these impurities, if present, do not produce a visible damage to the plating before they are exposed to long-term thermal stress. But as soon as samples have been heated for a period of just one week, the plating has been weakened enough to fail adhesion test. Over a time (longer than 1 week) the chemicals' residues on interfaces could produce bubbles of pressurized gases that tear the plating from inside. Detail SEM/EDS study may reveal small amounts of such elements as K, N and C on interfaces.
There could also be other light elements included into composition of non-disclosed chemicals, and in the materials used in the bath rubber lining, such as S and F. Even Cl may be found on the surfaces rinsed with chlorinated water.
6 b c
2. Mechanical stress.
It is known that mechanical stress in the plating is higher on curved and angular sides of a bus bar than on linear sides. In processing, mechanical stress is absorbed and contained in existing defaults. Heating helps to release this stress, leading to defaults’ propagation, creation and multiplication. Therefore heating and mechanical stresses are supporting each other in the process of material
1. Aging studies of various types of tin plating on aluminum have been performed to determine an influence of thermal exposure on the quality of tin plating.
2. Three different types of tin plating over Al, including various cyanide and non-cyanide processes, have been studied using multiple testing techniques, such as: adhesion test, thermal shock test, SEM metallography, EDS X-ray analysis, and thickness measurements.
3. The quality of plating has been studied before and after it was exposed to the temperature of 150
°
C in air for up to eight weeks. All types of plating in as-received samples demonstrated good adhesion properties according to the adhesion tests and thermal shock tests. SEM study did not discover any void, gaps, or cracks in as-received plating of any type. Strong adhesion quality was determined for Type
1 plating after 200
°
C (below Sn melting temp) with a thermal shock test that would reveal any adhesion issues.
These results are consistent with a strong adhesion of both the plating and the underlayer over the bar.
4. According to the adhesion test for aged samples, the adhesion of the plating begins to deteriorate for the samples exposed to the elevated temperature for one week. According to the results of scribing test, Type 3 plating survives longer exposure to high temperature than
Type 1 or Type 2 plating.
5. All three types of tin plating displayed poor plating adhesion after aging test. SEM study determined that tin plating shows aging-related cracks, voids, and separation appearing at the plating interfaces; although in different concentration and progression rates depending on plating technique.
© 2007 Schneider Electric All Rights Reserved

Thermal Aging Study of Tin Plating on Aluminum
Data Bulletin
0108DB0602
02/2007
Table 2: Results of SEM Study of the Aged Samples
Thermal Aging, Weeks Type 1 Plating, Alstan
®
80 (Figure 6) Type 2Plating, Alstan 88 (Figure 7) Type 3 Plating, Bondal
®
(Figure 8)
1
2
3
4
8
No pores, voids, or gaps
—
—
Multiple pores, voids and gaps along each interface Al-bronze, bronze-Sn
No pores, voids, or gaps
Some voids and gaps are detected along interfaces Al-bronze, bronze-Sn, coating is peeling in few locations
No pores, voids, or gaps
No pores, voids or gaps
Multiple pores, voids and gaps along interfaces, coating is peeling in more locations
Multiple pores, voids and gaps along interfaces, multiple locations with coating peeling
Some voids and gaps along interfaces Al-
Cu, Cu-Sn. coating is peeling in few locations
Multiple pores, voids and gaps along interfaces, multiple locations with coating peeling
Cracks along each interface, separation and fractures of long parts of both plating and underplating
— —
6. Accelerated aging tests at 150
°
C discriminate plating types. Aged samples exhibit medium to poor adhesion characteristics, and these results are likely related to the plating process itself. At the origin of this behavior, we might suggest thermal-induced mechanisms including intermetallics formation and diffusion, presence of impurities at the plating-bar interface, defect propagation and stress relaxation at the interface.
1. M Braunovic. “Effect of Fretting on the Contact Resistance of Aluminum with Different Contact Materials” IEEE
Transactions on Components, Hybrids, and Manufacturing
Technology, V. 2, Issue 1, p. 25–31, March 1979.
2. M Braunovic. “Fretting Damage in Tin-Plated Aluminum and Copper Connectors”, IEEE Transactions on
Components, Hybrids, and Manufacturing Technology,
V. 12, Issue 2, p. 215–223, June 1989.
3. M Braunovic. “Evaluation of Different Platings for
Aluminum-to-Copper Connections”, IEEE Transactions on
Components, Hybrids, and Manufacturing Technology,
V. 15, Issue 2, p. 204-215, April 1992.
4. R.J. Geckle. “Metallurgical Changes in Tin-Lead Platings
Due to Heat Aging”, IEEE Transactions on Components,
Hybrids, and Manufacturing Technology, V. 14, Issue 4, p. 691–697, December 1991.
5. M. Braunovic. “Effect of Intermetallic Phases on the
Performance of Tin-Plated Copper Connections and
Conductors”, Proceedings of the Forty-Ninth IEEE Holm
Conference on Electrical Contacts, p. 124–131, September
2003.
Bella H. Chudnovsky, Ph.D.
Schneider Electric/Square D
9870 Crescent Park Drive West
Chester, OH 45069
(513) 755-4249 chudnovb@squared.com
Dr. Chudnovsky received her
MS and Ph.D. degrees in Physics from the Rostov Institute of
Physics in Russia. At Schneider
Electric/Square D, she conducts research in various application fields. Bella has authored multiple scientific and technical papers in the fields of material science, corrosion protective plating, lubrication, and maintenance. She is a member of
IEEE and APS.
Vincent Pavageau
Schneider Electric SA
Material Research Laboratory
38050 Grenoble, France
+33 04 76576736 vincent.pavageau@mail.schneider.fr
Mr. Pavageau graduated as a process agreement and failure analysis. materials engineer from the Ecole
Nationale de Chimie at Clermont
Ferrand in France. In Schneider
Electric material research center, he conducts analysis and evaluations in various metallurgic R&D projects, supplier and
Schneider Electric USA
3700 Sixth St SW
Cedar Rapids, IA 52404 USA
1-888-SquareD (1-888-778-2733) www.us.SquareD.com
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