Diffusion Soldering of Pb-Doped GeTe Thermoelectric Modules
with Cu Electrodes Using a Thin-Film Sn Interlayer

C.L. YANG,1 H.J. LAI,2 J.D. HWANG,2 and T.H. CHUANG1,3

1.—Institute of Materials Science and Engineering, National Taiwan University, Taipei 106,
Taiwan. 2.—Material and Chemical Research Laboratories, Industrial Technology Research
Institute, Hsinchu 31015, Taiwan. 3.—e-mail: tunghan@ntu.edu.tw

Directly coating a GeTe(Pb) thermoelectric device with a Ni barrier layer and
an Ag reaction layer and then diffusion soldering with a Cu electrode coated
with Ag and Sn leads to breakage at the GeTe(Pb)/Ni interface and low
bonding strengths of about 6 MPa. An improved process, precoating with 1 lm
Sn film and heating at 250�C for 3 min before electroplating with Ni and Ag
layers, results in satisfactory bonding strengths ranging from 12.6 MPa to
19.1 MPa. The precoated Sn film leads to the formation of a (Ni,Ge)3Sn4 layer
between the GeTe(Pb) thermoelectric material and Ni barrier layer, reducing
the thermal stress at the GeTe(Pb)/Ni interface.

Key words: Diffusion soldering, GeTe(Pb) thermoelectric modules,
intermetallic compounds, bonding strength

INTRODUCTION

Thermoelectric (TE) materials using the Seebeck
effect or the Peltier effect are being developed in
response to the energy crisis. The thermoelectric
effect, which involves the transfer of heat and elec-
tricity, can be applied in power generators, heat
dissipators, thermoelectric coolers, and waste heat
recovery. However, the thermoelectric conversion
efficiency of a single thermoelectric device is limited,
so in general, TE modules are produced through a
combination of multiple TE devices bonded with
metallic electrodes for commercial applications.
Traditionally, soldering or brazing methods are
employed for the bonding of thermoelectric materials
with electrodes. Although soldering at lower tem-
perature can reduce thermal stress, the bonding
interface may melt during subsequent operation of
thermoelectric modules at high temperature. Braz-
ing, on the other hand, allows operation at high
temperature because the filler has a higher melting
point. Unfortunately, while the bonded thermoelec-
tric modules are cooling from the high temperature
of the brazing process, high thermal stress may occur

at the bonding interface. Such stress can damage
thermoelectric devices. Consequently, since soldering
and brazing methods both entail the risk of failure in
the manufacture or operation of thermoelectric
modules, an improved method should be developed.
In the alternative method, the thermoelectric mod-
ules should be bonded at a temperature as low as
possible, and the melting point of the joints should be
higher than the working temperature of thermo-
electric devices.

A novel bonding technique, called diffusion sol-
dering, uses the principle of thin-film solid–liquid
interdiffusion. This technique might solve the above
problems. It employs a low-melting-point metallic
thin-film interlayer (LT) that melts at low temper-
ature and reacts rapidly with the sandwiched high-
melting-point (HT-1 and HT-2) layers or with the
substrates to form intermetallic compounds. The LT
layer must be thinner than the HT-1 and HT-2
layers or substrates, and the reactions of the LT
layer have to be exhausted entirely. This novel
technique is suitable for application in emergent
high-temperature electronic devices, which demand
a low bonding temperature. It has been applied to
the manufacturing of microwave packages,1 high-
power devices,2 thick-film resistors,3 GaAs/Si wafer
packages,4 ceramic multichip modules,5 and even
gold jewelry6 in past decades. Recently, diffusion

(Received June 19, 2012; accepted October 31, 2012;
published online December 8, 2012)

Journal of ELECTRONIC MATERIALS, Vol. 42, No. 3, 2013

DOI: 10.1007/s11664-012-2345-6
� 2012 TMS

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