The Effects of pH Variation on Whisker Growth on Tin
Plated Copper
Jeffrey Wu
Advisors: Uttara Sahaym, M. Grant Norton
Materials Science and Engineering REU, Department of Mechanical and Materials
Engineering
Introduction:
1
2
The grain structure of the base layer of sample 1 is the
roughest. It contains the highest number of pores among the
three samples. The grain mutation smoothes out for sample
2. For sample 3, the base layer becomes a little more porous
but remains less severe compared to sample 1.
3
Whisker Density:
Increasing NaOH concentration
From left to right, samples 1, 2, and 3 showing the
Same matte surface finish
Field Failure Caused by Tin Whisker Short 20 YEARS
after System Fielded
Whiskers Growing Inside of Hermetically Sealed
Relays
(Photos from http://nepp.nasa.gov/whisker/photos/pom/index.htm)
Tin is an element commonly desired to plate on electrical
components because of it’s corrosion resistance, conductivity,
and its ease in soldering applications. Tin whiskers are single
crystals that spontaneously grow from plated surface due to
compressive stresses within the material, which can cause
short circuits and failures in electronics. Traditionally lead has
been used as an alloying element to suppress the growth of
tin whiskers. However, the recent move to eliminate lead from
consumer electronics has renewed interest in developing
methods for minimizing whisker growths in tin. This research
investigated the effects of changing the pH levels in the
electroplating solution on tin whisker growths.
Diagram of the electroplating apparatus
Sample 1
Sample 2
Sample 3
Ideal Conditions
Total NaOH added 0.97g
1.99g
3.03g
1.00g/2.00g/3.00g
Bath Temperature
55°C
55°C
56°C
55°C
Electrolyte
Temperature
55°C
56°C
56°C
55°C
Voltage
1.16V
1.14V
1.09V
-
Current
0.05A
0.05A
0.05A
0.05A
Area
~1.2cm2
~1.1cm2
~1.05cm2
1cm2
Current Density
0.042 A/cm2
0.045 A/cm2
0.048 A/cm2
0.05 A/cm2
Weight Before
Plating
1.625g
1.194g
1.215g
-
Weight After
Plating
1.629g
1.196g
1.216g
-
Time Prior to
Plating
~8s
~6s
~8s
0s
Table of experimental conditions versus intended conditions
Results:
Surface Feature Density:
Copper sample preparation:
Plating solution setup:
28.5g of sodium tin(IV) oxide hydrate and 1.00g of sodium
hydroxide were added to 200mL of deionized water for the
first plating solution. It was then placed in a 1.5L water bath
and stirred at a constant rate of 320rpm. Both, the water bath
and plating solution, were heated and maintained at 55°C. For
each subsequent sample, 1.00g of sodium was added to the
plating solution.
Plating process:
The copper sample was attached to the power source as the
cathode while a Sn anode of similar size was attached to the
other end. Once the cathode and anode was dipped into the
plating solution, the power source was kept at 0.05A for 3
minutes and 56 seconds.
2 mm
Sample 1
Sample 2
2 mm
Sample 3
The density of the crystals is highest in sample 1. The density
decreases significantly for sample 2 but rises again for sample
3.
Surface Feature Size:
500 nm
Sample 1
500 nm
500 nm
Sample 2
Sample 2
100 mm
Sample 3
Conclusions:
•At the lowest concentration of NaOH, the high density large,
well-defined, pyramid-shaped crystals seem to correlate with
the least amount of whisker growth.
•When the NaOH concentration was raised to 10g/L, the
smaller crystals and smoother base layer resulted in a higher
density of whiskers, though the average lengths were
relatively short.
•At 15g/L of NaOH, the surface became less uniform with
varying sizes of pyramid-shaped features and a rougher base
layer than sample 2. This morphology resulted in the densest
and longest whisker growths.
Acknowledgements:
Sample 3
Sample 1 has large, relatively uniform crystals. For sample 2,
the crystals became noticeably smaller. For sample 3, the
crystal sizes became uneven. Some crystals became larger
while others remained relatively small.
Base Layer Morphology:
Special thanks to Hi-Rel Laboratories Spokane for their
support
SEM Observations:
The three plated samples were observed under the scanning
electron microscope within 24 hours of plating to observe the
morphology of the Sn films. The samples were observed again
after 16 days of aging for tin whisker growth.
2 mm
Sample 1
100 mm
On all three samples, whiskers generally grow larger and in
higher densities on parts farther to the right side. Sample 1
has the least number of whiskers overall. The whiskers on
sample 1 ranged anywhere between 16-75 microns in length
and 1-3 microns in thickness. Sample 2 has a higher whisker
count compared to sample 1, but the whiskers appear to be
shorter. Whiskers on sample 2 range between 8-40 microns in
length and 1-1.5 microns in thickness. Sample 3 appear to
have the highest whisker growth concentrations compared to
samples 1 and 2. Whiskers on sample 3 have been measured
to be as long as 80 microns long and 1.6 microns thick.
Procedures:
Three copper samples were cut into 1cm x 2.54cm pieces.
Each were grinded and polished with standard metallographic
procedures up to 0.05 microns. Prior to plating, each sample
was first cleaned in a sodium hydroxide solution, then in
sulfuric acid.
100 mm
500 nm
500 nm
Sample 1
Sample 2
500 nm
Sample 3
This work was supported by the National Science Foundation’s
REU program under grant number DMR-0755055