A Comparative Study on the Microstructure
and Mechanical Properties of Cu
Joints Formed by TLP Soldering With/Without
the Assistance of Ultrasonic Waves
H.Y. ZHAO, J.H. LIU, Z.L. LI, X.G. SONG, Y.X. ZHAO, H.W. NIU, H. TIAN,
H.J. DONG, and J.C. FENG
In this study, the microstructure and mechanical properties of Cu
joints, formed by the transient liquid phase (TLP) soldering process with and without the
assistance of ultrasonic waves (USWs), were compared. After the application of USWs in the
TLP soldering process, Cu-Sn intermetallic compounds (IMCs) exhibited a novel noninterfacial
growth pattern in the molten solder interlayer. The resulting Cu
Sn joints consisted
of reﬁned equiaxed IMC grains with average sizes of 3 and 2.3 lm, respectively. The Cu
grains in the ultrasonically soldered intermetallic joints demonstrated uniform mechanical
properties with elastic modulus and hardness values of 123.0 and 5.98 GPa, respectively, while
those of Cu
Sn grains were 133.9 and 5.08 GPa, respectively. The shear strengths of
ultrasonically soldered Cu
Sn joints were measured to be 60 and 65 MPa,
respectively, higher than that for reﬂow-soldered intermetallic joints. Ultrasonically soldered
Sn joints both exhibited a combination of transgranular and intergranular
fractures during shear testing.
Ó The Minerals, Metals & Materials Society and ASM International 2018
facilitate integration and miniaturization, the
power density of electronic devices has been dramati-
cally increasing, leading to increased junction tempera-
tures. This is especially apparent in high-temperature
power devices utilized in the ﬁelds of space exploration,
petrochemical, electric power, and geological explo-
ration, where junction temperatures resulting from
power dissipation can exceed 300 °C.
conventional bonding materials used in the packaging
of electronic devices, such as Pb-free solders, typically
have melting temperatures below 250 °C, which can lead
to solder interconnection failure if used in high-temper-
ature power devices. Thus, the development of
suitable bonding materials or methods that can demon-
strate reliable operation at high temperatures and in
harsh environments remains a challenge.
Recently, TLP soldering has been demonstrated as a
potential interconnection method for fabricating joints
with high melting points for application in the packag-
ing of high-temperature power devices.
systems, the melted Sn interlayer reacts with Cu to form
Cu-Sn intermetallic compounds (IMCs) (i.e.,Cu
Sn with melting points of 415 and 676 °C,
). With the increasing reaction time, the
Sn interlayer can be fully consumed and completely
transformed to Cu-Sn IMCs. Unavoidably, the fabrica-
tion of complete Cu-Sn IMC joints is highly time
consuming (> 60 minutes) in the TLP soldering process,
which can introduce additional thermal stress, resulting
in decreased reliability of packaging systems.
fore, the development of methods that can reduce the
processing time for TLP soldering is desirable, not only
for the reliability of packaging systems but also for
economic viability of mass production.
An eﬀective approach to solve this challenge involves
the use of ultrasonic waves during the TLP soldering
process. Acoustic cavitation and streaming can occur
when ultrasonic waves (USWs) are propagated in the
The solid–liquid interface reaction
can be promoted by acoustic cavitation, leading to the
H.Y. ZHAO, J.H. LIU, X.G. SONG, Y.X. ZHAO, and J.C. FENG
are with the State Key Laboratory of Advanced Welding and Joining,
Harbin Institute of Technology, Harbin 150001, China and also with
the Shandong Provincial Key Lab of Special Welding Technology,
Harbin Institute of Technology at Weihai, Weihai 264209, China.
Contact e-mails: email@example.com, firstname.lastname@example.org Z.L. LI, H.W.
NIU, H. TIAN, and H.J. DONG are with the Shandong Provincial
Key Lab of Special Welding Technology, Harbin Institute of
Technology at Weihai.
Manuscript submitted December 14, 2017.
Article published online May 10, 2018
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2739