Study of the SnZnX alloys for solder applications

1
Study of the Sn-Zn-X alloys for solder
applications in the electronic industry
WORKING TEAM IN GROUP PROJECT GP5 Portugal Fi
nland Bulgaria France
Univ. of Minho, Dep. Mechanical Eng. Physical
Dep. Univ. of Porto Materials Eng.
Dep. Peixinhos, Lda
Helsinki University of Technology, Department of
Materials Science and Rock Engineering
Laboratory of Materials Processing and Powder
Metallurgy
University of Sofia, Faculty of Chemistry
Universite Henri Poincare, Nancy 1 Faculte des
Sciences et Techniques Laboratoire de Chimie du
Solide Mineral, LCSM Groupe Thermodynamique et
Corrosion
OBJECTIVE The main objective of the project is
the development of a new lead-free alloy for
electronic application. The new alloy, based in
the system Sn-Zn-X or Sn-Bi-X, will allow the
substitution of the actual Pb-Sn solders. The
specific objectives of the project are the
selection of the alloying elements that allow the
optimization of the properties with interest to
the mentioned application. The new alloy must
have properties similar to the actually used
(melting point, ) in order to minimize the
effects at the industry level.
WORK PLAN

• Systems selection
• Univ. of Minho, Univ. of Porto, Helsinki
  University of Technology, University of Sofia,
  Universite Henri Poincare
• Two base alloy systems were selected considering
  the following aspects
• the melting temperature
• the alloy final price.

Alloy synthesis Univ. of Minho - DEM
Chemical composition, obtained by XRF
spectrometry, of the melted alloys
The samples used in this work were melted from
pure elements (99.9 wt. ) in a resistance
furnace under inert atmosphere (obtained by a
constant flow of argon) and poured in a steel
mold pre-heated up to 100 ºC. Two standard alloys
were prepared for the sake of comparison with
results obtained in the studied systems. The
first alloy, a Sn-Pb base alloy, has a
composition close to the actually used solder
alloys in electronic soldering. The second alloy,
of the Sn-Ag-Cu system, is an alternative alloy
to the traditional solder alloys and to the
compositions studied in this work. After melting
and pouring, each alloy was heat treated using
the following cycle heating up to 120 ºC at 10
ºC/min, stage of 30 min and cooling to room
temperature at 10 ºC/min.
Alloys characterisation
Alloys microstructures Univ. of Minho - DEM
Mechanical properties Univ. of Minho - DEM
The microstructure of the alloy 1, of the system
Sn-Zn-Al, reveals the presence of 3 constituents
matrix, a zinc rich phase, needles (black zone)
and an aluminum rich phase (stars). For the
smaller additions, bismuth forms a solid solution
with the tin rich phase (fig. b). For higher
additions of Bi, a separate phase, consisting
essentially of Bi (white phase in fig. c), is
formed. The solubility of Bi in the tin rich
phase is higher, around 3.7 wt in alloy 6, than
the corresponding value for the binary Sn-Bi
system (approximately 1 wt at room temperature).
The effect of the Bi content on the Sn-Zn-Al-Bi
system has been determined by tensile test
experiments. It was concluded that Bi contents
higher than 5 wt lowers the mechanical
resistance and changes the fracture behavior from
ductile to brittle.
Microstructures obtained in the alloys, with
different Bi contents a) with 0Bi b) with 0.9
wt Bi c) with 6.9 wt Bi (magnifications of
500X).
Fracture morphology of the different alloys a)
and detail of the ductile and brittle fracture b)
for, respectively, alloys 1 (0 wt. Bi), 4 (3.3
wt.Bi) and 6 (6.9 wt.Bi).
Influence of Bi contents on the yield strength,
ultimate tensile strength and elongation at
rupture obtained in produced alloys.
Chemical composition of the phases and
constituents, obtained by SEM/EDS, in alloys 1
and 6.
Diffusion experiments Univ. of Minho - DEM
Thermal and electric measurements Univ. of Minho
- DF
Thermal properties Univ. of Minho - DEM
The solder/substrate interface has been studied
by diffusion experiments, between a solid
substrate (pure copper) and the melted solder.
Two stage times, at 250 ºC, were selected 30 and
120 min. The layers obtained were studied by
optical microscopy (OM) and scanning electronic
microscopy (SEM/EDS). Several layers were
detected for the Sn-Zn-Al and Sn-Zn-Al-Bi solders.
Thermal diffusivity measurements were performed
by the photothermal beam deflection technique. In
order to improve accuracy, independent
measurements were performed under two different
surrounding media, air and CCl4. Electrical
resistivity measurements were performed by the
four probe method.
The transformation temperatures of the alloys has
been determined by DSC/TGA. The experiments were
made under inert atmosphere with a
heating/cooling velocity of 10ºC/min.
Melting temperature (or melting range) of alloys
with different chemical compositions.
Thermall fatigue experiments Univ. of Minho - DEM
The samples for thermal fatigue experiments have
the following drawing
Copper Solder
Samples will be submitted to several thermal
cycles (-40 to 120 ºC). After that the interface
zone will analysed byptical microscopy and
Scanning Electronic Microscopy. Cracks formation
and evolution at the interface will be
characterised in accordance with the thermal
cycles for several solder chemical compositions.