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Allotropic Transformation

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Kazuhiro Nogita – One of the best experts on this subject based on the ideXlab platform.

  • Thermal expansion of Cu6Sn5 and (Cu,Ni)(6)Sn-5
    Journal of Materials Research, 2011
    Co-Authors: Mu Dekui, J. Read, Yafeng Yang, Kazuhiro Nogita
    Abstract:

    Cu6Sn5 is a common intermetallic compound formed during electrical packaging. It has an Allotropic Transformation from the low-temperature monoclinic eta’-Cu6Sn5 to high-temperature hexagonal eta-Cu6Sn5 at equilibrium temperature 186 degrees C. In this research, the effects of this Allotropic Transformation and Ni addition on the thermal expansion of eta’- and/or eta-Cu6Sn5 were characterized using synchrotron x-ray diffraction and dilatometry. A volume expansion during the monoclinic to hexagonal Transformation was found. The addition of Ni was found to decrease the undesirable thermal expansion by stabilizing the hexagonal Cu6Sn5 at temperatures below 186 degrees C and reducing the overall thermal expansion of Cu6Sn5.

  • nickel stabilized hexagonal cu ni 6sn5 in sn cu ni lead free solder alloys
    Scripta Materialia, 2008
    Co-Authors: Kazuhiro Nogita, Tetsuro Nishimura
    Abstract:

    Cu6Sn5 is an important intermetallic compound (IMC) in lead-free solder alloys. Cu6Sn5 exists in two crystal structures with an Allotropic Transformation from monoclinic η′-Cu6Sn5 at temperatures lower than 186 °C to hexagonal η-Cu6Sn5. A detailed analysis by transmission elecelectron microscopy (TEM) in Sn–0.7 wt.% Cu–0.06 wt.% Ni reveals that when the Ni content in (Cu, Ni)6Sn5 is ∼9 at.% Ni, the hexagonal allotrope of (Cu, Ni)6Sn5 becomes stable at room temperature.

Christopher Hunt – One of the best experts on this subject based on the ideXlab platform.

  • On the absence of the β to α Sn Allotropic Transformation in solder joints made from paste and metal powder
    Microelectronic Engineering, 2011
    Co-Authors: D. Di Maio, Christopher Hunt
    Abstract:

    Until July 2006, most solder joints in the electronics industry were made of the alloy 63Sn37Pb or 62Sn36Pb2Ag. After this date, the European environmental Restriction of Hazardous Substances directive (RoHS) forced many manufacturers to use Pb-free alloys. These substitutes for SnPb are Sn-rich alloys (over 90% Sn) of various compositions. Below 13.2^oC, Sn potentially transforms into a different phase. This occurs with catastrophic effects, as the transforming material becomes extremely brittle and falls apart. The purpose of this paper is to investigate if this Allotropic Transformation also occurs in samples prepared from solder paste or metal powder. This work compares the Transformation propensity of samples prepared with bulk solder, solder paste, and tin powder. Different conditions and geometries are used in the investigation and experiments with both commercial and specifically prepared solder pastes are carried out. Samples prepared from bulk solder transform into the @a phase as expected, whilst samples prepared from solder paste and tin powder do not transform. The residual organic compounds from the flux are believed to be responsible for this behaviour. The tin oxide (SnO”2) retained in the bulk after melting could also play a role. This paper shows, for the first time, a relationship between the ability of tin to transform and the nature of the starting material and in particular that the tin @b/@a Allotropic Transformation does not occur when samples are prepared from paste or powders. The new lead-free alloys can therefore be used with more confidence in mission-critical applications.

  • Allotropic Transformation in tin and lead free solder alloys: Measuring the effect and implications for industry.
    , 2009
    Co-Authors: Christopher Hunt, D. Di Maio
    Abstract:

    This jointly funded industry and UK government project, addressed concerns with the Allotropic Transformation in tin. s-Sn is stable between 232 oC and 13 oC. Below this temperature s-Sn becomes the equilibrium phase. This Transformation has catastrophic consequences on the transforming material, mainly because of the 26% volume expansion that accompanies the change from the BCT to the diamond cubic structure. This phenomenon has recently become of interest in the field of electronic interconnections, due to the high tin content of the soldering alloys used for the manufacture of printed circuit boards. Previous studies have shown that this Transformation can occur on soldering alloys, if the right conditions are met. There are many variables that can influence the formation or suppression of the s-Sn and in this study some of them will be investigated. In particular, this work will look at the effect of one thermal cycle to room temperature, the effect of cubic ice as a Transformation seed, and the effect of thermal-cycling to generate susceptibility to subsequent Transformation to s-Sn. The incubation time (nucleation) plays an important role in determining the total time of the Transformation. Cubic ice is an Allotropic form of ice that can naturally form; this type of crystal can enhance the nucleation speed of s-Sn in pure tin. When a seed promotes the Transformation, the nucleation time is greatly reduced. Findings indicate that this nucleation process can propagate from tin, which could be a tin plated termination, and into a tin alloy. Stresses, such as the thermal stresses that occur on PCBs during service, can also accelerate the Transformation. This was investigated here comparing the Transformation time of samples that were thermally cycled with samples that were not. An important part of this study will also look at the propensity to Transformation of 3 commercial Sn alloys, the Sn / 0.5-0.7 Cu / < 0.1 Ni (Sn100C), the Sn / 3.0 Ag / 0.5 Cu (SAC305) and the Sn 0.3 Ag 0.7 Cu 0.1 Bi (SACX).

  • Time-Lapse photography of the beta-Sn/alpha-Sn Allotropic Transformation.
    , 2009
    Co-Authors: D. Di Maio, Christopher Hunt
    Abstract:

    When tin transforms from the to phase it undergoes a dramatic process. The crystalstructure changes from tetragonal to diamond cubic; the material properties transform from a ductile metal to a brittle semiconductor; however the most notable change is the decrease in density, which goes from 7.31 g/cm3 to 5.77 g/cm3 [1]. It can be calculated that this decrease in density is equivalent to an increase in volume of about 26% [2]. Due to this volume increase and the brittleness, the transformed material progressively cracks and eventually falls apart. This could potentially be a threat for tin-rich alloys used in electronics in low temperature applications. Due to the optimal Transformation temperature of approximately 240K and the the long time required for the Transformation, a direct observation of the phenomenon has not been possible. This study proposes a new method for observing the / Transformation in situ using a time-lapse photographic technique. This study concentrates on pure tin, but the applicability of the method opens new possibilities for studying the phenomenon for other tin alloys, such as the two commonly encountered eutectics of SnCu and SnAgCu.

Satoshi Hirano – One of the best experts on this subject based on the ideXlab platform.

  • stir zone microstructure of commercial purity titanium friction stir welded using pcbn tool
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008
    Co-Authors: Yangjun Zhang, Yutaka S. Sato, Hiroyuki Kokawa, Seung Hwan C. Park, Satoshi Hirano
    Abstract:

    Abstract In the present study, friction stir welding was applied to commercial purity titanium using a polycrystalline cubic boroboronride tool, and microstructure and hardness in the weld were examined. Additionally, the microstructural evolution during friction stir welding was also discussed. The stir zone consisted of fine equiaxed α grains surrounded by serrate grain boundaries, which were produced through the β → α Allotropic Transformation during the cooling cycle of friction stir welding. The fine α grains caused higher hardness than that in the base material. A lath-shaped α grain structure containing Ti borides and tool debris was observed in the surface region of the stir zone, whose hardness was the highest in the weld.

David C. Dunand – One of the best experts on this subject based on the ideXlab platform.

  • Strain ratchetting of titanium upon reversible alloying with hydrogen
    Philosophical Magazine A, 2001
    Co-Authors: Megan Frary, Christopher A. Schuh, David C. Dunand
    Abstract:

    Abstract During cyclic hydrogen charging (e.g., in metal–hydride systems), internal stresses and strains can be developed due to lattice swelling and/or phase Transformation (e.g., Allotropic Transformation or hydride precipitation). We examine macroscopic plastic deformation due to such internal stresses (strain ratctetting) in the Ti–H system, where gaseous hydrogen is alloyed with Ti, causing the Ti α–β Allotropic Transformation, and subsequently removed, producing the β–α Transformation. Cyclic hydrogen charging is found to cause macroscopic plastic shrinkage strains in directions normal to the hydrogen concentration gradient. Furthermore, increasing the charging time leads to larger ratchetting strains. A simple adaptation of diffusion theory is used to describe the kinetics of strain evolution, and the contributions to total ratchetting from both the α–β phase Transformation and the lattice swelling strains are quantified.

  • Phase Transformation and thermal expansion of Cu/ZrW2O8 metal matrix composites
    Journal of Materials Research, 1999
    Co-Authors: Hermann Holzer, David C. Dunand
    Abstract:

    Powder metametallurgy was used to fabricate fully dense, unreacted composites consisting of a copper matrix containing 50–60 vol% ZrW2O8 particles with negative thermal expansion. Upon cycling between 25 and 300 °C, the composites showed coefficients of thermal expansion varying rapidly with temperature and significantly larger than predicted from theory. The anomalously large expansion on heating and contraction on cooling are attributed to the volume change associated with the Allotropic Transformation of ZrW2O8 between its high-pressure γ-phase and its low-pressure α- or β-phases. Based on calorimetry and diffraction experiments and on simple stress estimations, this Allotropic Transformation is shown to result from the hydrostatic thermal stresses in the particles due to the thermal expansion mismatch between matrix and reinforcement.

D. Di Maio – One of the best experts on this subject based on the ideXlab platform.

  • On the absence of the β to α Sn Allotropic Transformation in solder joints made from paste and metal powder
    Microelectronic Engineering, 2011
    Co-Authors: D. Di Maio, Christopher Hunt
    Abstract:

    Until July 2006, most solder joints in the electronics industry were made of the alloy 63Sn37Pb or 62Sn36Pb2Ag. After this date, the European environmental Restriction of Hazardous Substances directive (RoHS) forced many manufacturers to use Pb-free alloys. These substitutes for SnPb are Sn-rich alloys (over 90% Sn) of various compositions. Below 13.2^oC, Sn potentially transforms into a different phase. This occurs with catastrophic effects, as the transforming material becomes extremely brittle and falls apart. The purpose of this paper is to investigate if this Allotropic Transformation also occurs in samples prepared from solder paste or metal powder. This work compares the Transformation propensity of samples prepared with bulk solder, solder paste, and tin powder. Different conditions and geometries are used in the investigation and experiments with both commercial and specifically prepared solder pastes are carried out. Samples prepared from bulk solder transform into the @a phase as expected, whilst samples prepared from solder paste and tin powder do not transform. The residual organic compounds from the flux are believed to be responsible for this behaviour. The tin oxide (SnO”2) retained in the bulk after melting could also play a role. This paper shows, for the first time, a relationship between the ability of tin to transform and the nature of the starting material and in particular that the tin @b/@a Allotropic Transformation does not occur when samples are prepared from paste or powders. The new lead-free alloys can therefore be used with more confidence in mission-critical applications.

  • Allotropic Transformation in tin and lead free solder alloys: Measuring the effect and implications for industry.
    , 2009
    Co-Authors: Christopher Hunt, D. Di Maio
    Abstract:

    This jointly funded industry and UK government project, addressed concerns with the Allotropic Transformation in tin. s-Sn is stable between 232 oC and 13 oC. Below this temperature s-Sn becomes the equilibrium phase. This Transformation has catastrophic consequences on the transforming material, mainly because of the 26% volume expansion that accompanies the change from the BCT to the diamond cubic structure. This phenomenon has recently become of interest in the field of electronic interconnections, due to the high tin content of the soldering alloys used for the manufacture of printed circuit boards. Previous studies have shown that this Transformation can occur on soldering alloys, if the right conditions are met. There are many variables that can influence the formation or suppression of the s-Sn and in this study some of them will be investigated. In particular, this work will look at the effect of one thermal cycle to room temperature, the effect of cubic ice as a Transformation seed, and the effect of thermal-cycling to generate susceptibility to subsequent Transformation to s-Sn. The incubation time (nucleation) plays an important role in determining the total time of the Transformation. Cubic ice is an Allotropic form of ice that can naturally form; this type of crystal can enhance the nucleation speed of s-Sn in pure tin. When a seed promotes the Transformation, the nucleation time is greatly reduced. Findings indicate that this nucleation process can propagate from tin, which could be a tin plated termination, and into a tin alloy. Stresses, such as the thermal stresses that occur on PCBs during service, can also accelerate the Transformation. This was investigated here comparing the Transformation time of samples that were thermally cycled with samples that were not. An important part of this study will also look at the propensity to Transformation of 3 commercial Sn alloys, the Sn / 0.5-0.7 Cu / < 0.1 Ni (Sn100C), the Sn / 3.0 Ag / 0.5 Cu (SAC305) and the Sn 0.3 Ag 0.7 Cu 0.1 Bi (SACX).

  • Time-Lapse photography of the beta-Sn/alpha-Sn Allotropic Transformation.
    , 2009
    Co-Authors: D. Di Maio, Christopher Hunt
    Abstract:

    When tin transforms from the to phase it undergoes a dramatic process. The crystalstructure changes from tetragonal to diamond cubic; the material properties transform from a ductile metal to a brittle semiconductor; however the most notable change is the decrease in density, which goes from 7.31 g/cm3 to 5.77 g/cm3 [1]. It can be calculated that this decrease in density is equivalent to an increase in volume of about 26% [2]. Due to this volume increase and the brittleness, the transformed material progressively cracks and eventually falls apart. This could potentially be a threat for tin-rich alloys used in electronics in low temperature applications. Due to the optimal Transformation temperature of approximately 240K and the the long time required for the Transformation, a direct observation of the phenomenon has not been possible. This study proposes a new method for observing the / Transformation in situ using a time-lapse photographic technique. This study concentrates on pure tin, but the applicability of the method opens new possibilities for studying the phenomenon for other tin alloys, such as the two commonly encountered eutectics of SnCu and SnAgCu.