Dramatic morphological change of scallop-type Cu6Sn5 formed on (001) single crystal copper in reaction between molten SnPb solder and Cu

Article abstract of DOI:10.1063/1.2761840

Wetting reaction between molten Sn-based solders and Cu produces scallop-type Cu6 Sn5. In the present wetting study, a (001) single crystal Cu is used as substrate and a dramatic change in the morphology of Cu6 Sn5 is observed: instead of scallop type, the authors observed a rooftop-type Cu6 Sn5 grains, elongated along two preferred orientation directions. This was confirmed by electron beam backscattered diffraction and white beam synchrotron x-ray microdiffraction. The results indicate that the nucleation, growth, and ripening behavior of Cu6 Sn5 on single crystal substrate can be quite different from the conventional case of wetting on randomly oriented polycrystalline Cu substrates.

Full text of DOI:10.1063/1.2761840

Dramatic morphological change of scallop-type
formed on (001) single crystal
copper in reaction between molten SnPb solder and Cu
J. O. Suh and K. N. TuN. Tamura
Citation: Appl. Phys. Lett. 91, 051907 (2007); doi: 10.1063/1.2761840
View online: http://dx.doi.org/10.1063/1.2761840
View Table of Contents: http://aip.scitation.org/toc/apl/91/5
Published by the American Institute of Physics

APPLIED PHYSICS LETTERS 91, 051907 ͑2007͒
Dramatic morphological change of scallop-type Cu Sn formed
6
5
on „001… single crystal copper in reaction between molten SnPb
solder and Cu
a͒
J. O. Suh and K. N. Tu
Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095
N. Tamura
Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720
͑
Received 23 May 2007; accepted 28 June 2007; published online 31 July 2007͒
Wetting reaction between molten Sn-based solders and Cu produces scallop-type Cu Sn . In the
6
5
present wetting study, a ͑001͒ single crystal Cu is used as substrate and a dramatic change in the
morphology of Cu Sn is observed: instead of scallop type, the authors observed a rooftop-type
6
5
Cu Sn grains, elongated along two preferred orientation directions. This was confirmed by electron
6
5
beam backscattered diffraction and white beam synchrotron x-ray microdiffraction. The results
indicate that the nucleation, growth, and ripening behavior of Cu Sn on single crystal substrate can
6
5
be quite different from the conventional case of wetting on randomly oriented polycrystalline Cu
substrates. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2761840͔
Because of the wide application of solder, especially Pb-
free, in consumer electronic products, the study of the wet-
ting reaction of molten solder on Cu has attracted consider-
¯ ¯
141͒
¯
͑
ʈ
͑001͒Cu and ͓101͔
ʈ
Cu Sn
6 5
͓110͔Cu.
͑6͒
Cu Sn
6
5
1
–9
This is because a low misfit of 0.24% between the Cu atoms
able interests. Metallic bonding in solder joints is achieved
through the formation of Cu–Sn intermetallic compounds of
Cu Sn and Cu Sn at the Cu/solder interface. The Cu Sn
¯
can be achieved along the ͓101͔
direction and the
5
Cu Sn
6
6
5
3
6
5
͓110͔Cu direction. The above relationships can be classified
into two groups based on the strong pseudohexagonal sym-
metry around Cu atom in Cu Sn when projected along the
has a unique scallop-type morphology, and the Cu Sn has a
3
layer-type morphology. The latter forms between the former
and the copper substrate. Figure 1 is a top-view scanning
electron microscopy ͑SEM͒ image of Cu Sn scallops on a
6
5
¯
͓
101͔ direction ͑Fig. 2͒. Crystal planes in Eqs. ͑1͒–͑3͒ corre-
6
5
spond to edges of the Cu hexagon ͑group 1͒ and Eqs. ͑4͒–͑6͒
correspond to diagonals of the Cu hexagon ͑group 2͒.
Since the low misfit directions between Cu Sn and Cu
polycrystalline Cu substrate after the remaining solder was
etched away. The scallops appear rounded and there are deep
1
0
6
5
channels between them. The crystal structure of the low
lie on the ͑001͒ plane of Cu, it is of interest to investigate the
11
temperature phase ␩-Cu Sn5 is monoclinic. Our recent
6
behavior of Cu Sn formed on ͑001͒ single crystal Cu.
6
5
study using white beam synchrotron micro-x-ray diffraction
showed that the formation of Cu Sn on Cu has a set of
Single crystal Cu substrates were purchased from Goodfel-
low and they have a diameter of 1 cm and a thickness of
6
5
1
2
preferred orientation relationships. There are six types of
0
.25 cm. The surface was carefully polished, cleaned, and
etched before being immersed in flux. Wetting samples were
preferred orientation relationships between the two phases,
¯
and in all cases the ͓101͔ direction of Cu Sn is parallel to
6
5
prepared by reacting small beads ͑ϳ0.5 mg͒ of 55Sn45Pb
the ͓110͔ direction of Cu. The six orientation relationships
͑
in wt %͒ solder with the ͑001͒ single crystal copper in flux
are as follows:
at 200 °C with different reaction times ranging from 30 s to
¯
͑
͑
010͒
343͒
ʈ
ʈ
ʈ
ʈ
ʈ
͑001͒Cu and ͓101͔
ʈ
ʈ
ʈ
ʈ
ʈ
͓110͔Cu,
͓110͔Cu,
͓110͔Cu,
͓110͔Cu,
͓110͔Cu,
͑1͒
͑2͒
͑3͒
͑4͒
͑5͒
Cu Sn
6
Cu Sn
6
5
5
5
5
¯
͑001͒Cu and ͓101͔
Cu Sn
6
Cu Sn
6
¯
¯
343͒
¯
͑
͑
͑
͑001͒Cu and ͓101͔
Cu Sn
6
Cu Sn
6
5
5
¯
101͒
141͒
͑001͒Cu and ͓101͔
Cu Sn
Cu Sn
6
5
6
5
¯
͑001͒Cu and ͓101͔
Cu Sn
6
Cu Sn
6
5
5
FIG. 1. Top-view scanning electron microscopy ͑SEM͒ image of Cu Sn
6
5
a͒Present address: Jet Propulsion Laboratory, Pasadena, CA.
003-6951/2007/91͑5͒/051907/3/$23.00
scallop-type grains on a Cu.
0
91, 051907-1
© 2007 American Institute of Physics

051907-2
Suh, Tu, and Tamura
Appl. Phys. Lett. 91, 051907 ͑2007͒
FIG. 3. Morphology of rooftop-type Cu Sn formed on ͑001͒ Cu.
6
5
parallel to the ͑001͒ plane of Cu. For the Kikuchi pattern in
¯
Fig. 4͑c͒ the ͓101͔ direction of Cu Sn was perpendicular to
6
5
¯
the ͓101͔ direction of the other Cu Sn and was parallel to
6
5
¯
¯
the ͓110͔ direction of Cu. Again, one of the group 2 planes
was parallel to the ͑001͒ plane of Cu. In summary, from
EBSD, it was confirmed that Cu Sn grains were elongated
FIG. 2. ͑Color online͒ Structure of Cu Sn projected from the ͓101͔
6
5
direction.
6
5
¯
along their ͓101͔ direction, parallel to the ͗110͘ directions of
the single crystal Cu.
4
min. The molten solder forms a cap on the Cu upon wet-
ting. After a given time of reaction, samples were quenched
to room temperature by dipping them into acetone. After
solidification, the unreacted solder was removed by mechani-
cal polishing, followed by selective chemical etching in or-
der to expose the interfacial Cu Sn scallops.
To obtain statistical data of the orientation distribution,
synchrotron white beam micro-x-ray diffraction was per-
formed. The micro-x-ray diffraction experiments were con-
ducted on beamline 7.3.3 at the Advanced Light Source in
6
5
1
3
Figure 3 shows the morphology of Cu Sn formed on
Lawrence Berkeley National Laboratory. The sample with
4 min reaction time was scanned under a micron size x-ray
beam. At each step of scan, a Laue pattern was collected
using an x-ray charge coupled device detector. The micro-x-
ray beam can penetrate through the Cu6Sn5 and reach the Cu.
Therefore, each Laue pattern is a composite of diffraction
spots from Cu6Sn5 and Cu. The strongest Laue spots came
from the Cu due to its larger grain size and thus contribution
to the signal. The Cu reflections were indexed first to yield
Cu grain orientation and then removed from the list of
reflections for the subsequent analysis of the Cu6Sn5
reflections.
6
5
͑
001͒ Cu substrate. Elongated and rooftop-type Cu Sn
6 5
grains were distributed on the entire surface. The elongations
go along two perpendicular directions. We hypothesized that
Cu Sn should elongate along the low misfit direction in or-
6
5
der to minimize interfacial energy with Cu. Electron beam
backscattered diffraction ͑EBSD͒ analysis was performed to
verify the elongation direction. Because of the high rough-
ness of the sample ͑see Fig. 3͒, performing EBSD mapping
is an issue. However, since each grain is a single crystal, a
single Kikuchi pattern coming from a selected spot of an
elongated grain is enough to determine its orientation. Kiku-
chi patterns were obtained separately from the ͑001͒ single
crystal Cu substrate and Cu Sn grains, and we found that
Figure 5͑a͒ is a histogram of the angles between the
͓001͔ direction of Cu and the ͓101͔ direction of Cu Sn , after
the 4 min reaction at 200 °C. An area of 25ϫ30 ␮m was
scanned with a step size of 0.5 ␮m in both x and y direc-
¯
6
5
6
5
Cu Sn grains are elongated along two different ͗110͘ direc-
2
6
5
tions. Figure 4͑a͒ is a Kikuchi pattern from the Cu substrate.
Figures 4͑b͒ and 4͑c͒ are the respective Kikuchi patterns of
an elongated grain and of another grain elongated perpen-
dicularly to the first one. The analysis indicated that the
¯
tions. There is a strong peak at 90°, indicating that the ͓101͔
directions of most of the Cu Sn scallops are lying on the
001͒ plane of Cu. Figure 5͑b͒ is a histogram of the angles
between the ͓010͔ direction of Cu and the ͓101͔ direction of
Cu Sn . There are two peaks: one peak is at 45° and the
6
5
͑
͑
001͒ plane of Cu is perpendicular to the surface normal, and
¯
that the ͗110͘ directions are nearly parallel ͑within 4°͒ to the
laboratory x and y axes. The Kikuchi pattern shown in Fig.
6
5
¯
¯
other is at 135°. It indicates that the ͓101͔ directions of most
4
͑b͒ indicated that the ͓101͔ direction of Cu Sn is parallel to
6 5
¯
the ͓110͔ direction of Cu. One of the “group 2” planes was
of the Cu Sn grains are parallel to the ͓110͔ or ͓110͔ direc-
6
5
FIG. 4. ͑Color online͒ Kikuchi patterns from the EBSD
analysis. ͑a͒ Kikuchi pattern from the Cu substrate.
͑
b͒ and ͑c͒ are Kikuchi patterns of a Cu Sn grain
6 5
and another grain perpendicular to the first scallop,
respectively.

051907-3
Suh, Tu, and Tamura
Appl. Phys. Lett. 91, 051907 ͑2007͒
FIG. 5. ͑Color online͒ ͑a͒ Histogram of angle between
¯
the ͓001͔ direction of Cu and the ͓101͔ direction of
Cu Sn , after 4 min reflow at 200 °C. ͑b͒ Histogram of
6
5
¯
angle between the ͓010͔ direction of Cu and the ͓101͔
direction of Cu Sn .
6
5
tion of Cu. The above results confirm the existence of a
strong preferred orientation relationship between Cu Sn and
Due to the strong orientation relationship between the
elongated Cu Sn and ͑001͒ Cu, as shown in Fig. 3, we ex-
6
5
6
5
͑
001͒ Cu on the bases of EBSD and synchrotron micro-x-ray
diffraction.
The dramatic change in morphology of Cu Sn suggests
pect a lower interfacial energy between them than that be-
tween the round scallop-type Cu6Sn5 and polycrystalline Cu,
as shown in Fig. 1. Indeed when we etched the Cu6Sn5, we
found that the elongated Cu6Sn5 on ͑001͒ has lasted much
longer in the etchant. The lower interfacial energy will im-
prove the impact fracture toughness of the interface.
6
5
that nucleation, growth, and ripening mechanisms of the
elongated Cu Sn can be different from the rounded scallop-
6
5
type Cu Sn . Figure 6 is a SEM image of Cu Sn scallops on
6
5
6
5
In summary, a dramatic change in morphology of
͑
001͒ Cu after 30 s reflow. Clearly Cu Sn already has very
6 5
Cu Sn was found when Sn-based solder was reacted with a
strong texture. The strong texture of Cu Sn indicates that
6
5
6
5
͑
001͒ single crystal Cu. Grains of Cu Sn become elongated
nucleation of the Cu Sn is not random but rather oriented
6 5
6
5
along the two low misfit directions between Cu Sn and Cu.
when ͑001͒ Cu is used as a substrate. Soldering is a reactive
6
5
The relationship between the morphology and the crystallo-
graphic orientation was verified by EBSD study. Statistical
distribution data obtained by white beam synchrotron micro-
x-ray diffraction agreed with the EBSD study. The Cu Sn
wetting. When molten solder spreads on copper, dissolution
1
4
of copper substrate takes place at the interface. If the ori-
entation of substrate copper is a high-index ͑hkl͒ plane, more
copper will be required to be dissolved away in order to
expose the low misfit ͗110͘ crystal directions and ͕001͖
planes of Cu. Therefore, the nucleation of Cu Sn will not
6
5
already showed strong texture at 30 s of wetting reaction,
indicating that the grains tend to nucleate with texture.
6
5
have enough time to nucleate with the preferred orientation if
the Cu substrate is a high-index plane, and random nucle-
ation will become dominant. However, if the substrate is a
This study was performed at UCLA, supported by SRC
under Contract No. NJ-1080 and NSF under Project No.
0
503726. The Advanced Light Source is supported by the
͑
001͒ single crystal Cu, Cu Sn can directly nucleate on the
6 5
Director, Office of Science, Office of Basic Energy Sciences,
of the U.S. Department of Energy under Contract No.
DE-AC02-05CH11231.
low misfit direction and plane. As a result, Cu Sn grains
6
5
will have an oriented nucleation and textured growth.
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FIG. 6. ͑Color online͒ SEM image of Cu Sn grains on ͑001͒ Cu, after 30 s
6
5
reflow.