Ripening-assisted asymmetric spalling of Cu-Sn compound spheroids in solder joints on Si wafers

Article abstract of DOI:10.1063/1.116013

In reacting eutectic SnPb solder with Ti/Cu and Cr/Cu/Au thin film metallization on Si wafers, we have observed spalling of Cu6Sn5 spheroids when the solder consumes the Cu. The formation of the spheroids is assisted by the ripening reaction among the compound grains. In addition we have observed an asymmetric spalling phenomenon using a sandwich structure, in which two wafers were soldered face-to-face. The spalling occurs predominantly at the interface at the bottom of the solder joint. It suggests that gravity plays a role.

Full text of DOI:10.1063/1.116013

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Ripeningassisted asymmetric spalling of CuSn compound spheroids in solder joints on
Si wafers
H. K. Kim, K. N. Tu, and P. A. Totta
Citation: Applied Physics Letters 68, 2204 (1996); doi: 10.1063/1.116013
View online: http://dx.doi.org/10.1063/1.116013
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/68/16?ver=pdfcov
Published by the AIP Publishing
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Ripening-assisted asymmetric spalling of Cu-Sn compound spheroids
in solder joints on Si wafers
H. K. Kim and K. N. Tu
Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles,
California 90095-1595
P. A. Totta
IBM East Fishkill Facility, Hopewell Junction, New York 12533
͑Received 14 December 1995; accepted for publication 12 February 1996͒
In reacting eutectic SnPb solder with Ti/Cu and Cr/Cu/Au thin film metallization on Si wafers, we
have observed spalling of Cu6Sn5 spheroids when the solder consumes the Cu. The formation of the
spheroids is assisted by the ripening reaction among the compound grains. In addition we have
observed an asymmetric spalling phenomenon using a sandwich structure, in which two wafers were
soldered face-to-face. The spalling occurs predominantly at the interface at the bottom of the solder
joint. It suggests that gravity plays a role. © 1996 American Institute of Physics.
͓S0003-6951͑96͒02316-9͔
To synchronize with the advance in very large scale in-
tegration ͑VLSI͒ of Si technology, a high density of input/
output ͑I/O͒ connections is required in packaging the Si
chip.1–3 The trend in low power electronic packaging is to
shift from peripheral wire bonds to area array solder bumps,
especially in the flip-chip technology.4–6 In this technology,
small solder bumps are arrayed in a grid pattern on the face
of a chip. The chip is flipped and aligned to a module sub-
strate having a matching footprint of wettable I/O terminals.
Then, the solder bumps are reflowed to achieve the con-
trolled collapse chip connection ͑C4͒.
During the solder reflow process, intermetallic com-
pounds are formed at the solder/Cu interface. Since the
amount of Cu is limited in thin film metallization, spalling of
the intermetallic compounds occurs after the Cu is
consumed.7 The spalling tends to dewet and weaken the sol-
der joint. However, the phenomenon and mechanism of spal-
ling are unclear. In this letter, we report that the spalling is
assisted by the ripening reaction among the compound
grains. The grains become spheroids before they detach from
the substrate.
direct chip-on-card technology, for example, smart cards in
the future.
In our study, a small ͑ϳ0.5 mg͒ solder ball was reflowed
on the Ti/Cu film in the mildly activated rosin flux ͑RMA͒ at
200 °C with Ϯ3 °C control. To remove the Cu-oxide, the Cu
film samples were etched using a 50% ammonium hydroxide
solution diluted with water before reflowed.10 Figures 1͑a͒
and 1͑b͒ show the cross-sectioned views in scanning electron
microscope ͑SEM͒ of interface between the solder and Cu
film after reflowed for 90 and 600 s, respectively. Using
energy dispersive x-ray ͑EDX͒ analysis, the scallop-like
grains at the interface in Fig. 1͑a͒ were determined to be
Cu6Sn5. We found that there is a ripening reaction among
them; they grow bigger but fewer with annealing time. Even
after all Cu has been consumed by the Cu-Sn reaction, the
ripening continues but the morphology of the Cu6Sn5 scal-
Since the C4 solder joints have two interfaces; chip-to-
solder and solder-to-substrate, the spalling at these two inter-
faces may be different. In this letter, we report the use of a
sandwich structure to study them. We found that the spalling
seems to depend on whether the compounds form above or
below the solder. The asymmetric phenomenon suggests that
gravity may play a role in the spalling.
Two kinds of Cu film samples were used in this study;
Ti/Cu and Cr/Cu/Au evaporated on oxidized Si wafers. The
Ti/Cu films were prepared by a consecutive deposition of
2500 Å of Ti by e-gun and 8500 Å of Cu by thermal evapo-
ration. The Cr/Cu/Au films were prepared at IBM East Fish-
kill by e-gun evaporation, having thicknesses of 2500 Å Cr/
5000 Å Cu / 1100 Å Au. For the solder, we have used the
eutectic ͑63Sn37Pb, wt %͒ SnPb solder. Although high-Pb
͑e.g., 5Sn95Pb, wt %͒ solders are being used in chip-level
interconnection, we chose the eutectic solder for a compari-
son to earlier soldering studies on Cu sheet.8,9 also because
the low melting solder/Cu film system is of interest in the
FIG. 1. Backscattered electron SEM images of the cross-section of interface
between the eutectic SnPb solder and Ti/Cu film reflowed at 200 °C ͑a͒ for
90 s, ͑b͒ for 600 s.
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FIG. 3. Enlarged SEM images of interface shown in Fig. 2͑b͒ ͑a͒ top inter-
face, ͑b͒ bottom interface.
FIG. 2. ͑a͒ Schematic of sandwich structure, ͑b͒ cross-section of the sample
reflowed at 200 °C for 600 s using eutectic SnPb solder on Cr/Cu/Au film.
posed Ti or Cr surface, it exerts a lifting force to the
Cu6Sn5 grains due to gravity or density difference. When the
grains transform to spheroids which have a nonwetting con-
tact angle ͑180°͒ to the Ti or Cr surfaces, they may be lift up
from the bottom interface by the gravity force. Although the
detailed mechanism of spalling such as shape change of the
Cu6Sn5 grains and their wetting behavior on Ti or Cr sur-
faces remains to be studied, the gravity effect seems in
agreement with the observed asymmetric behavior.
On the wetting behavior of Cu6Sn5 on Ti or Cr surfaces,
we expect some adhesion between them since they are all
metals. Nevertheless, the dewetting of the intermetallic from
the metal surface is due to the total surface energy changes in
the shape change from a scallop to a spheroid. It includes
their interfaces with the molten solder.
The avoidance of ripening assisted spalling in product
may be achieved by locking the intermetallic compounds
with a ‘‘phased’’ region between Cr and Cu.11 In phasing Cr
and Cu, the crystallites of Cr and Cu become intermixed and
the intermetallics of Cu become locked into the Cr so that
they will not separate easily. Follow-on studies are planned
to examine intermetallic ripening on phased structures.
In summary, the spalling of the Cu-Sn compounds at the
solder interface on thin film metallization is an important
reliability issue of flip-chip technology. We have found that
the ripening among the Cu6Sn5 grains assisted the spalling.
Under the influence of gravity, the spalling tends to occur
predominantly at the bottom interface of a solder joint.
This research was supported by SRC contract No. 93-
NJ-356 and NSF contract No. DMR-9320769.
lops begins to change into spheroids. The change is accom-
panied by decreasing the contact area between the grains and
bottom metallization. This is because there is no more Cu;
the ripening becomes conservative. The smaller grains will
be dissolved into the bigger grains. The contact area de-
creases and it exposes the Ti or Cr surface to the liquid
solder, in turn, spalling will lead to more exposed surface as
shown in Fig. 1͑b͒. If the Ti and Cr surfaces are oxidized,
they can not be wetted by the solder. It can present a serious
dewetting problem and the interface will be vulnerable to
mechanical failure. In the earlier report on solution-assisted
spalling by Berry et al.7 no ripening reaction among the
Cu6Sn5 grains was mentioned. We show that it is the conser-
vative ripening which dissolves the smaller grains and results
in spalling. We have observed the same type of spalling in
the reflow of solder on the Cr/Cu/Au films.
In the face-to-face samples of two wafers with a solder
joint in between them, the schematic in Fig. 2͑a͒ shows the
sandwiched structure while the micrograph in Fig. 2͑b͒
shows the cross-section of a reacted sample. The small ͑ϳ0.5
mg͒ solder disk with a thickness of about 120 ␮m was sand-
wiched by two wafers and the wafers were supported by
spacers with the same thickness to maintain the spacing. The
cross-section of the sandwich was polished using SiC paper
and alumina powder, and a light etching was performed us-
ing a mixture of 5% HCl in methanol to delineate the inter-
facial structure of the solder joint.10 Figures 3͑a͒ and 3͑b͒ are
the enlarged micrographs of the top and bottom interfaces of
the joints. They clearly show an asymmetric spalling behav-
ior of the two solder interfaces. After 10 min reflow at
200 °C, although a severe spalling has already occurred at
the bottom interface, the Cu-Sn compounds at the top inter-
face do not show any spalling. Because of the selective re-
action of Cu with Sn, the composition of solder next to the
Cu-Sn compounds will be enriched in Pb. Therefore, there is
a concentration gradient of Pb in the molten solder to mix the
solder. However, when the molten solder flows to the ex-
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2206 Appl. Phys. Lett., Vol. 68, No. 16, 15 April 1996 Kim, Tu, and Totta
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