Strong Enhancement of π-Electron Donor/Acceptor Ability by Complementary DD/AA Hydrogen Bonding

Article abstract of DOI:10.1002/anie.201910288

π-Conjugated organic materials possess a wide range of tunable optoelectronic properties which are dictated by their molecular structure and supramolecular arrangement. While many efforts have been put into tuning the molecular structure to achieve the desired properties, rational supramolecular control remains a challenge. Here, we report a novel series of supramolecular materials formed by the co-assembly of weak π-electron donor (indolo[2,3-a]carbazole) and acceptor (aromatic o-quinones) molecules via complementary hydrogen bonding. The resulting polarization creates a drastic perturbation of the molecular energy levels, causing strong charge transfer in the weak donor–acceptor pairs. This leads to a significant lowering (up to 1.5 eV) of the band gaps, intense absorption in the near-IR region, very short π-stacking distances (≥3.15 ?), and strong ESR signals in the co-crystals. By varying the strength of the acceptor, the characteristics of the complexes can be tuned between intrinsic, gate-, or light-induced semiconductivity with a p-type or ambipolar transport mechanism.

Full text of DOI:10.1002/anie.201910288

Materials Research Express
PAPER
Microstructure and properties of Sn-3.8Ag-0.7Cu-xCe lead-free solders
with liquid-liquid structure transition and Ce addition
To cite this article: Liu Xiaoguang et al 2019 Mater. Res. Express 6 1165b8
View the article online for updates and enhancements.
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Mater. Res. Express 6 (2019) 1165b8
https://doi.org/10.1088/2053-1591/ab4d3a
PAPER
Microstructure and properties of Sn-3.8Ag-0.7Cu-xCe lead-free
solders with liquid-liquid structure transition and Ce addition
RECEIVED
16 July 2019
REVISED
21 September 2019
Liu Xiaoguang1, Jiang Xiaoming1, Cao Lichao1 , Zhai Wengang2, Li Xianfen3 and Wei Zhou2
ACCEPTED FOR PUBLICATION
1
11 October 2019
Guangdong Institute of Intelligent Manufacturing, Guangdong Academy of Sciences, Guangzhou 510075, People’s Republic of China
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China
2
PUBLISHED
3
23 October 2019
E-mail: xg.liu@giim.ac.cn
Keywords: Sn-3.8Ag-0.7Cu-xCe lead-free solder, liquid-liquid structure transition, microstructure, properties
Abstract
Our previous work suggested that the Sn-3.8Ag-0.7Cu-xCe (x takes 0%, 0.2%, 0.5% and 1.0wt%)
solder melt experienced a temperature-induced liquid-liquid structure transition (L-LST) which was
reversible during two heating and cooling cycles far above the liquidus. In this work, the effects of
liquid state manipulations and rare earth element Ce addition on solidified Sn-3.8Ag-0.7Cu-xCe
structures and properties were investigated. The results revealed that the microstructure was
remarkably refined, the spread ability and the shear strength were enhanced when the melt
experienced L-LST and Ce addition. Meanwhile, 0.2 wt% Ce addition showed the most homogenous
microstructure, leading to better properties. This work elicited that melt manipulations and Ce
addition could be an effective way to optimize microstructure and properties of Sn-3.8Ag-0.7Cu-xCe
soldered joints.
1. Introduction
Ideal solidified structures lead to a better performance for materials. Many researches have pursued this goal for
decades. In order to get refined solidified crystals, various liquid manipulations like melt overheating [1, 2], melt
thermal treatment [3, 4] have been applied and sometimes it appears effective. In the past, a lot of researchers
explored the thermal history which had an important influence on the microstructure and properties of alloys. It
has been found that the liquid-liquid structure transition (L-LST) existed in some liquids [5–7]. It has been
demonstrated that temperature or pressure induced liquid structure transitions can occur in some melts of pure
elements and alloys by means of in situ synchrotron radiation [8], neutron diffraction [9], viscosity [10], x-ray
diffraction and internal friction [11], resistivity [12] and computer simulations [13] etc. The experiments proved
that the microstructure of amorphous alloys was much more homogeneous if the alloy melts were much over-
heated. It was accepted that the structure transition was related to the metastable local short range orders (SROs)
or covalent clusters remained in melts at temperature above the liquidus slightly [14].
Considering the inherent toxicity of lead and lead containing alloys to environment and human healthy,
lead-free solders have been developed to respond the trend of lead-free electronic packing [15, 16]. SnAgCu
lead-free solder with good thermal mechanical properties was regarded as one of the most promising candidates
for the traditional Sn-Pb solder in electronic assembly. As rare elements (RE) have been called the vitamin of
metals, some researchers studied the addition of pure RE like Eu [17], Sm [18], Yb [19], Pr [20], La [21], Nd [22],
Ce [23] and mixed-RE [24, 25] to enhance the properties of SnAgCu lead-free solders and great achievements
have been obtained.
In this article, the overheat temperature for melts was based on the result of our previous work [26],
solidification experiment, spread ability test and shear strength test were conducted to investigate the effects of
Ce and L-LST on microstructure and properties of Sn-3.8Ag-0.7Cu-xCe lead-free solder.
© 2019 IOP Publishing Ltd

Mater. Res. Express 6 (2019) 1165b8
L Xiaoguang et al
Table 1. Starting temperature and ending temperature of L-LST for Sn-
3.8Ag-0.7Cu-xCe alloys.
Transition temperature(°C)
Starting
Ending
Composition
temperature
temperature
Sn-3.8Ag-0.7Cu
Sn-3.8Ag-
770 2
762 2
960 2
831 2
0.7Cu-0.2Ce
Sn-3.8Ag-
771 2
601 2
988 2
626 2
0.7Cu-0.5Ce
Sn-3.8Ag-
0.7Cu-1.0Ce
Table 2. Treatment temperature and time of solidification experiment.
Holding temperature and time
Composition
Group A
Group B
Sn-3.8Ag-0.7Cu
500 °C×40 min
500 °C×40 min
500 °C×40 min
500 °C×40 min
1000 °C×30 min → 500 °C×10 min
1000 °C×30 min → 500 °C×10 min
1000 °C×30 min → 500 °C×10 min
1000 °C×30 min → 500 °C×10 min
Sn-3.8Ag-0.7Cu-0.2Ce
Sn-3.8Ag-0.7Cu-0.5Ce
Sn-3.8Ag-0.7Cu-1.0Ce
2. Experimental procedure
2.1. Solidification experiment
The Sn-3.8Ag-0.7Cu-xCe alloys were prepared by melting high purity granular Sn, Ag, Cu and Ce in a resistivity
furnace. During melting, the melts were covered with shielding flux B2O3 to prevent the evaporation and
oxidation of metals. The Sn-3.8Ag-0.7Cu-xCe melts experienced a temperature-induced L-LST which was
reversible during two heating and cooling cycles far above the liquidus while the heating temperature was above
the ending temperature [26]. The transition temperature was shown in table 1.
In this experiment, the holding temperature was shown in table 2 which was chosen according to table 1.
Taking Sn-3.8Ag-0.7Cu as an example, the group A was heated to 500 °C with a dwelling time of 40 min. Group
B was firstly heated to 1000 °C with a dwelling time of 30 min. Then, the melt was shifted expeditiously to a
500 °C furnace for 10 min to unify the purring condition. After heat treatment, the melts were poured into an
iron mold. The specimens for microstructures were prepared by standard metallographic procedures with the
etchant of FeCl3+HCl+H2O solution and observed under optical microscope.
2.2. Spread ability test
Copper with a purity of 99.97 wt% was used as substrate, which was cut into the size of
40 mm×40 mm×2 mm. The Cu substrate was polished before cleaning with ethanol. The solder was cut into
0.2 0.0002 gram with the same diameter. Spread ability tests were conducted at 260 °C with a dwelling time of
120 s after melting in a resistance furnace. The activated rosin was used as flux. The same measurement
conditions were applied to all samples for a reliable comparison of their wettability. Spreading area was
measured to judge the wettability by Image Pro Plus assisted by Photoshop. Six specimens were made for each
condition.
2.3. Tensile test
Several soldered joints were prepared by using the solidified solders as specimens for tensile test, the Cu plate as
substrate. Dimensions of specimen for tensile testis were shown in figure 1. The average values of every tensile
strength were estimated from 5 measurements.
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Mater. Res. Express 6 (2019) 1165b8
L Xiaoguang et al
Figure 1. Dimensions of specimen for tensile test (unit in mm).
Figure 2. Microstructure of Sn-3.8Ag-0.7Cu-xCe solders.
3. Results and discussion
3.1. Solidification microstructure
The microstructures of Sn-3.8Ag-0.7Cu-xCe are represented in figure 2. It is worth pointing out that no obvious
porosity/hole was observed in all the solidified samples. The microstructure of Sn-3.8Ag-0.7Cu-xCe consists of
the light colored β-Sn phase surrounded by dark eutectic networks of β-Sn and inter metallics phase (Cu6Sn5
and Ag3Sn) [27]. It can be clearly seen that, after the melts experienced L-LST, the microstructure of β-Sn is
dramatically refined. It is generally accepted that lots of residual micro-clusters or shortrange orders (SROs) or
covalent clusters still remain in the melt near liquidus [5–7]. These SROs are disrupted and dispersed into
smaller clusters which could offer more nuclear sites, leading to a much homogenous microstructure. It can be
3

Mater. Res. Express 6 (2019) 1165b8
L Xiaoguang et al
Figure 3. Spreading area of Sn-3.8Ag-0.7Cu-xCe solders.
Figure 4. Schematic diagram of the wettability test.
observed in figures 2(a), (c), (e) and (g), with 0.2 wt% Ce addition, coarse β-Sn grains are remarkably refined. It is
known that the standard Gibbs free energy of formation of Sn-RE IMCs is lower than those for Ag-RE and Cu-
RE [28]. Accordingly, in this work, Ce has a higher affinity for Sn, Ce atoms are more easily attach the growing
interface of Sn, therefore, the growth steps were blocked and this explains the refinement of β-Sn. However,
dendrites appear again as Ce addition reaches to 0.5 and 1%. It can be concluded that manipulating parent liquid
state and Ce addition can be effective for optimizing materials solidified structures.
3.2. Wettability
The result of the spread ability testing of the solders is shown in figure 3. It can be seen from figure 3 that the
wettability of solders is increased with Ce addition and after solder melts experienced liquid structure transition.
After the melts experienced L-LST, the wetting area of Sn-3.8Ag-0.7Cu was increased by 5.78% and Sn-3.8Ag-
0.7Cu-0.5Ce was increased by 1.32% compared with that no Ce addition. With 0.5 wt% Ce addition, the wetting
area of Sn-3.8Ag-0.7Cu-0.5Ce increased by 8.54% compared with Sn-3.8Ag-0.7Cu.
As shown in Figure 4, wettability can be represented by contact angle θ which is defined by the Young-Dupre
equation [12],
gSF - gSL
cos q =
gLF
where gSF is the surface tensions of the substrate/flux;
interfacial area and
g
SL is the surface tensions of the substrate/liquid solder
is the surface tensions of the liquid solder/flux. gSF can be regarded as a constant which is
gLF
determined by same substrate and flux. According to the Ce–Cu binary alloy phase diagram, the interaction
between Ce and Cu is very low when the temperature is below 300 °C [28]. Therefore, gSL, the surface tension of
Cu substrate and solder, is impossible to reduce. That is to say, in such a situation, gSF and gSL are constants.
gLF
is determined by the property of the molten alloy. The melts which experienced L-LST are more disordered and
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Mater. Res. Express 6 (2019) 1165b8
L Xiaoguang et al
Figure 5. Shear strength of Sn-3.8Ag-0.7Cu-xCe solders.
homogeneous than that not, which resulted in a more active state of atoms and smaller SROs, so the free energy
is increased, which means that the solders would melt with less energy absorbed. For these two reasons, the
wettability of solders is increased after melts experienced L-LST. Ce is a surface active element, so the surface
tension of Sn-3.8Ag-0.7Cu-xCe melts is decreased with addition of Ce. However, more Ce-based intermetallic
and dioxide is generated with increasing Ce addition, which would prevent molten alloy from spreading on the
Cu substrate, and the wettability is decreased at last. The L-LST and Ce doping decreased gLF, and the contact
angle θ is decreased, so the wettability is increased.
3.3. Shear strength
Figure 5 shows the shear strength of soldered joints fabricated by Sn-3.8Ag-0.7Cu-xCe solders before and after
L-LST. It can be seen from figure 5, shear strength is enhanced by L-LST and Ce addition. Shear strength of Sn-
3.8Ag-0.7Cu is 33.0 MPa. With 0.2 wt% Ce addition, the shear strength of Sn-3.8Ag-0.7Cu-0.2Ce reached
36.3 MPa, increased by 10%. When Ce content increased to 0.5 and 1.0 wt%, the shear strength decreased.
Nevertheless, it is still better than that of no Ce addition. After the melts experienced L-LST, shear strength is
increased about 3.0%–4.7%. One reason of this enhancement is the refinement of β-Sn. On the other hand,
CeSn3 tends to be absorbed at grain boundary and decrease boundary movement [29]. In addition, the strain
field of large RE atoms and dislocations interact and restrain the movement of dislocations, thus increasing the
tensile strength of a solder joint [28]. Excessive intermetallic compound growth in solder joints will significantly
decrease the reliability of the joints [30], so when Ce addition Reached 0.5 and 1.0 wt%, shear strength was
decreased.
4. Conclusions
Microstructure and properties of Sn-3.8Ag-0.7Cu-xCe lead-free solders with L-LST and Ce addition are
investigated in this work. In the summary, the microstructure was refined by L-LST and Ce addition, leading to
the enhancement of spread ability and shear strength of Sn-3.8Ag-0.7Cu-xCe lead-free solders. Consequently,
this work confirmed that melts manipulations and Ce addition could be an effective way to optimize Sn-3.8Ag-
0.7Cu-xCe lead-free solders microstructures and properties.
ORCID iDs
Cao Lichao https://orcid.org/0000-0002-9560-5429
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