Effect of Electrolyte pH on Sn-plating
Morphology and Whisker Growth
Kirstyn Sudhoff¹, Uttara Sahaym²
¹Purdue University, West Lafayette, IN 47906 ²Washington State University, Pullman WA 99164
Introduction
Results
Discussion
A tin whisker is a formation that grows on the surface of tin which has been used
as a coating for some other type of metal. When tin is electroplated to another
metal, very commonly copper, an intermetallic compound is produced between
the two metals. The intermetallic compound generates a biaxial compressive
stress within the tin layer. This stress causes single tin crystals to grow between
grain boundaries and normal to the surface.
These whiskers were discovered in the 1940s and were later found to cause
major malfunctions in electronic devices when tin was used as a coating. The
issue occurs when the tin whiskers grow to such a length that they are able to
reach nearby components on the device and cause short circuiting. This type of
behavior was the reason for the failure of several satellites, pacemakers, relays
on military airplanes, and military missiles. Whisker growth can be prevented
by alloying tin with lead; however, in recent years restrictions have been put on
the use of lead, including alloying with tin for electrical devices. Without lead
for an alloy there has been an enormous drive to discover ways in which tin can
be manufactured and altered to form a coating that does not generate whiskers.
Set 1:
• After 30 days of aging, K4 had a vastly different morphology, a much higher
whisker density and whiskers of the longest length and thinnest diameter
compared to the other three samples electroplated at lower pH values.
K1 (pH 7.5)
Day 30
200 µm
K2 (pH 10.7)
Day 29
200 µm
• With a decrease in electrolyte’s pH there is a decrease in whisker density and
an increase in surface porosity.
Similarities of Set 1, Set 2 & Set 3:
K3 (pH 12.1)
Day 30
200 µm
K4 (pH 13.6)
Day 30
200 µm
Figure 2) The above image is a comparison of all samples in set 1, each electroplated
at a different pH, viewed via SEM. These SEM micrographs show the variation of
whisker growth depending on the pH of the electrolyte.
Method
Three sets of four copper plates, each at a different pH value, were electroplated
with tin. Four solutions were mixed to be used throughout the experiment. Each
solution had 28.4 grams of sodium tin, 200 milliliters of de-ionized water and
varying amounts of sodium hydroxide depending on desired pH.
K5 (pH 7.5)
Day 21
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• While K3 & K2 have very similar whisker densities, whiskers from K3 are
characterized by longer, thinner and straighter whiskers, whereas, K2 has
whiskers which are shorter and stubbier.
300 µm
K6 (pH 10.7)
Day 21
• After 30 days of aging, K4 (set 1), K8 (set 2) and K12 (set 3) had a much
different morphology than the other three samples in their specific set. K4,
K8 and K12 had much smaller grains and more surface uniformity in
comparison to the other three samples of lower pH values.
• Samples K4, K8 & K12 had a much smoother and more reflective surface
finish after electroplating than the other samples.
K3 (pH of 12.1)
K7 (pH of 12.1)
K11 (pH of 12.1)
K4 (pH of 13.6)
K8 (pH of 13.6)
K12 (pH of 13.6)
300 µm
+10A MAX
Voltmeter
-COM
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Sn4+
Sn/Fe
Anode
Cu
Cathode
K7 (pH 12.1)
Day 21
Sn4+
300 µm
K8 (pH 13.6)
Day 21
300 µm
Sn4+
Sn4+
Sn4+
Figure 3) This collection of SEM micrographs are of set 2 and show the difference in
whisker accumulation on the tin surface.
Sn4+
Sn4+
Electrolyte
Figure 6) The above image is a compilation of high resolution photographs of the
tin plated copper, which illustrate a clear difference between those electroplated at
a pH of 12.1 (top) and those at a pH of 13.6 (bottom).
Differences between Set 1 & Set 2/Set 3:
• In set 1 there is a more obvious variation in whisker density and whisker
formation throughout the set than in sets 2 & 3. The reason for this is that
set 1 was only electroplated to a thickness of 1µm and sets 2 & 3 were
electroplated to a thickness of 5µm. Since set 1 has a thinner tin layer there
is a much greater concentration of stress within the material, making any
behavioral differences as a result of pH, more noticeable.
Figure 1) The image above is a schematic of the experimental setup.
Prior to electroplating, the copper plates (1 inch x 1 inch) were cleaned with a
sodium hydroxide solution of 15 grams of NaOH and 30 milliliters of de-ionized
water and then washed with pure sulfuric acid. Once the solutions were
prepared and the copper plates were polished and cleaned they were
electroplated with a current density of approximately 50 mA/cm².
K9 (pH 7.5)
Day 30
K10 (pH 10.7)
Day 30
Conclusions
Table I) Conditions and solutions used for each specimen in the experiment
Sample
K1
K2
K3
K4
K5
K6
K7
K8
K9
K10
K11
K12
Solution
D
C
B
A
D
C
B
A
D
C
B
A
pH
7.5
10.7
12.1
13.6
7.5
10.7
12.1
13.6
7.5
10.7
12.1
13.6
Thickness
1µm
5µm
5µm
Anode
Tin
Tin
Steel
Set #
1
2
K11 (pH 12.1)
Day 30
K12 (pH 13.6)
Day 31
Figure 4) The image above is comprised of the four samples from set 3 viewed under
SEM. Sample K12 clearly shows the highest density of whiskers and tin crystals on
the surface.
K3
1µm
K7
K4
1µm
K8
5µm
K11
5µm
K12
5µm
3
After the copper plates were coated with tin via electroplating, each specimen
was individually stored at room temperature and later viewed with scanning
electron microscope (SEM) after aging.
5µm
Figure 5) The images above are high-magnification SEM micrographs which capture
the tin’s surface morphology. Those plated at the highest pH, 13.6, had much smaller
grains and a more uniform surface than those plated at a pH of 12.1 or less.
In conclusion to this experiment, it was observed that when tin is plated at a
higher pH value there is also a greater whisker density produced on the surface
after aging. The morphology of the tin plated at a pH of 13.6 had much smaller
grains and a more evenly plated surface than the other three samples that were
plated at lower pH values (12.1, 10.7 & 7.5). As the pH was decreased through
the four samples they gradually became less reflective. The tin samples
electroplated to a thickness of 1µm had more variation in regards to whisker
growth depending on pH value than the samples plated to a thickness of 5µm. As
the pH was decreased, the first change appeared in the whisker density and the
second was in the whisker morphology. Further studies should be conducted in
relation to how variation in an electrolyte affects whisker growth to gain a better
understanding of whisker behavior.
Acknowledgements
This work was supported by the National Science Foundation’s REU program
under grant number DMR-1062898.