11 research outputs found
Effect of Recrystallization on β to α-Sn Allotropic Transition in 99.3Sn-0.7Cu wt. % Solder Alloy Inoculated with InSb
The effect of recrystallization of 99.3Sn-0.7Cu wt % solder alloy on the allotropic transition of β to α-Sn (so-called tin pest phenomenon) was investigated. Bulk samples were prepared, and an InSb inoculator was mechanically applied to their surfaces to enhance the transition. Half of the samples were used as the reference material and the other half were annealed at 180 °C for 72 hours, which caused the recrystallization of the alloy. The samples were stored at -10 °C and -20 °C. The -Sn to α-Sn transition was monitored using electrical resistance measurements. The expansion and separation of the tin grains directly during the -Sn to α-Sn transition process were studied using scanning electron microscopy. The recrystallization of the alloy suppresses the tin pest phenomenon considerably since it decreased the number of defects in the crystal structure where heterogeneous nucleation of -Sn to α-Sn transition could occur. In the case of InSb inoculation, the spreading of the transition towards the bulk was as fast as the spreading parallel to the surface of the sample
Whisker Development from SAC0307-Mn07 Solder Alloy
In this study, the whisker growth from SAC0307-Mn07 solder alloy was investigated. To get fast information, an ultra-thin film was vacuum evaporated from the examined solder alloy onto Cu substrates. The evaporated layer was ~100-150nm thick in average with the same grain size. The layers were kept at room temperature for 28 days. Whisker growth was monitored by scanning electron microscope with some days frequency, and the layer structure under the whisker was examined on focused ion beam cuts. The large mechanical stress on the Sn layer due to the intermetallic layer growth initiated the whisker growth right after the layer deposition. However, it stopped after 2 weeks since the intermetallic grains consumed the ultra-thin Sn layer. The whisker density was extremely high (~24000 pcs./mm2). Most of the whiskers were the short nodule and hillock type, which does not mean risk in microelectronics. Therefore, the alloying of Mn into the high Sn content solder alloys have a positive effect on the reliability of the solder joints
ELECTRICAL CHARACTERIZATION OF ß→α-Sn TRANSITION IN HIGH TIN CONTNET SOLDER ALLOYS WITH DIFFERENT INOCULATORS
The identification of allotropic transition of the metallic β-Sn to non-metallic α-Sn in Sn-rich solders joints is crucial for electronics working in sub-zero temperatures. This phenomenon was characterized by electrical resistance measurements in SnCu1 and Sn99Ag0.3Cu0.7 alloys inoculated with InSb, CdTe and α-Sn. Samples were stored at -18°C for 10 weeks. The transition showed characteristic differences at the different alloys and inoculators, like different nucleation, growth and the saturation stages. Although the presence of α-Sn initiates the transition faster than the other inoculators, but the higher diffusion rate of the non-tin inoculators results in much more serious destruction of the samples. The results of the electrical resistance measurements were validated with metallurgical cross-sections
Early Stage Whisker Development from Sn Thin Film on Cu Substrate
The Sn thin film on Cu substrate has high Sn whisker growth susceptibility because of the intensive Cu6Sn5 intermetallic formation at the Sn-Cu interface. In this study, the whisker development on vacuum evaporated Sn thin film deposited on Cu substrates was investigated, starting directly from the Sn deposition. The aim was to obtain more information about the whiskering behavior of Sn thin film in the early stage of the life cycle. For the study, 99.99% pure tin was vacuum evaporated onto Cu substrates. Two different Cu substrates were applied with different surface roughness to investigate the effect of surface roughness on the whisker development. The average thickness of the evaporated Sn layer was ~2 ÎĽm. Samples were stored at room temperature for 10 weeks. Whisker development was observed by scanning electron microscope. It was found that the large compressive stress in the Sn layer because of the intermetallic formation initiates the whisker development even after 1 day of the Sn layer deposition. Almost only filament type whiskers were detected. The characteristics of the whisker density showed exponential saturation up to 10 days of the study, while the length of the whiskers was growing further still the end of the study. It was also found that the surface roughness of the Cu substrate affects the rate of whisker growt
Characterization of the microstructure of tin-silver lead free solder
Reliability and lifetime are the two most relevant design considerations in the production
of safety critical assemblies. For example in a modern automobile dozens of electronic assemblies
are integrated in which thousands of solder joints are mounting the
electronic components to the
printed circuit boards. There
exists no standardised and universal observation method
for
characterising the fine micro
structure of such solder joints.
Previously we have developed a new
method for the quantitative characteriz
ation of lead
-
free solder alloys and in
present study the
validity of the proposed method is demonstrated.
M
icrostructure of Sn
-
3.5Ag lead free solder alloy
was investigated by electrochemical impedance spectroscopy.
Solder samples were solidified with
dif
ferent cooling rates in order to induce differences in the microstructure.
M
icrostructure of the
ingots was revealed by selective electrochemical etching. Electrochemical impedance spectr
a (EIS)
were
measured before and after
the
selective etching process.
The complex impedance
spectra
contain
information about microstructure of the solder alloys.
Comparison and mode
l
ling
of two
EIS spectra allowed
obtaining
a
characteristic parameter of
surface structure of the etched
specimens.
The EIS measurements were c
omplemented with small angle neutron scattering
measurements
and scanning electron microscopy
, in order to correlate the EIS parameter with the
magnitude of the interface of the
β
-
Sn and Ag
3
Sn
phases
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Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 , Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal
Kinetics of Sn whisker growth from Sn thin-films on Cu substrate
The kinetics of Sn whisker growth was investigated on vacuum evaporated Sn thin-films. Sn film layers were deposited on a Cu substrate with 0.5 and 1 µm thicknesses. The samples were stored in room conditions (22±1°C / 50±5RH%) for sixty days. The Sn whiskers and the Cu-Sn layer structure underneath them were investigated with both scanning electron and ion microscopy. Fast Cu-Sn intermetallic formation resulted in considerable mechanical stress in the Sn layer, which initiated intensive whisker growth right after the layer deposition. The thinner Sn layer produced twice many whiskers compared to the thicker one. The lengths of the filament-type whiskers were similar, but the growth characteristics differed. The thinner Sn layer performed the highest whisker growth rates during the first seven days, while the thicker Sn layer increased the growth rate only after seven days. This phenomenon was explained by the cross-correlation of the stress relaxation ability of Sn layers and the amount of Sn atoms for whisker growth. The very high filament whisker growth rates might be caused by the interface flow mechanism, which could be initiated by the intermetallic layer growth itself. Furthermore, a correlation was found between the type of the whiskers and the morphology of the intermetallic layer underneath
Investigation of the Mechanical Properties of Mn-Alloyed Tin-Silver-Copper Solder Solidified with Different Cooling Rates
Manganese can be an optimal alloying addition in lead-free SAC (SnAgCu) solder alloys
because of its low price and harmless nature. In this research, the mechanical properties of the novel
SAC0307 (Sn/Ag0.3/Cu0.7) alloyed with 0.7 wt.% Mn (designated as SAC0307-Mn07) and those of the
traditionally used SAC305 (Sn96.5/Ag3/Cu0.5) solder alloys were investigated by analyzing the shear
force and Vickers hardness of reflowed solder balls. During the preparation of the reflowed solder
balls, di�erent cooling rates were used in the range from 2.7 K/s to 14.7 K/s. After measuring the
shear force and the Vickers hardness, the structures of the fracture surfaces and the intermetallic layer
were investigated by SEM (Scanning Electron Microscopy). The mechanical property measurements
showed lower shear force for the SAC0307-Mn07 alloy (20–25 N) compared with the SAC305 alloy
(27–35 N), independent of the cooling rate. However, the SAC0307-Mn07 alloy was softer; its Vickers
hardness was between 12 and 13 HV, whereas the Vickers hardness of the SAC305 alloy was between
19 and 20 HV. In addition, structural analyses revealed rougher intermetallic compound layers in the
case of the SAC0307-Mn07 alloy, which can inhibit the propagation of cracks at the solder–substrate
interface. These two properties of SAC0307-Mn07 alloy, the softer nature and the rougher intermetallic
layer, might result in better thermomechanical behavior of the solder joints during the lifetime of
electronic devices