THERMAL PROPERTIES OF Cu-Ga-In-Sn DIFFUSIVE-HARDENING ALLOYS

V. Bykov, T. Kulikova, A. Shubin, K. ShunyaevInstitute of Metallurgy, Ural Branch of RAS, Ekaterinburg

AbstractDiffusion-hardening solders (DHS) based on gallium not contain

lead. These alloys have specific rheological properties. The main kind of DHS synthesis is a mechanical mixing of the starting components such as metal powders (fillers) and liquid gallium alloys. The obtained metal pastes are undergoing irreversible phase transformations as a result is formed a hard alloy (composite) with a special structure. In this work, microstructure and thermal properties of diffusion-hardening solders Cu-Ga-In-Sn by scanning electron microscopy, X-ray microanalysis, laser flash method and dilatometry were investigated.

IntroductionRare earth alloys and trace elements with special properties are of

particular interest of researchers for many years [1]. Previously [2-4], we have studied a series of mechanical, thermal and rheological characteristics of gallium pastes and cured samples.

These alloys demonstrate unique properties (e.g., solidification at room temperature) which makes them extremely promising for use as solders, dental filling materials and etc.

The purpose of this work is to investigate the microstructure and thermal properties of copper-gallium-indium-tin alloys prepared by mechanochemical mixing solid (copper and its alloys) and liquid (melt Ga-In) components followed by curing.

The structure of such materials based on gallium was studied in [5, 6] by scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction. Thermal effects taking place before and after solidification of metal pastes by the differential scanning calorimetry (DSC) were studied, in particular, the authors of work [7].

For copper-tin alloys, the main process which leads to curing of composite pastes is due the reaction of the following type:

Cu-Sn + Ga → CuGa2 + Sn (1)

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Complicated phase composition of received material is related to the presence of tin or indium in liquid gallium. Factors as the ratio of a solid-liquid paste, the geometric shape, size of the particles of the filler alloy and the soaking temperature have a great influence on the diffusion rate of solidification.

If the average diameter of the copper-tin spherical particles (one of the most common fillers) is approximately 40 microns, the semi-solid alloy Cu-Ga-In-Sn is solidified at room temperature for several minutes. The further phase transformations becomes relatively slow and can be take place during 24 hours or more at normal temperature. The obtained alloy will have a stable mechanical strength and thermal properties after 2-3 days of excerpt at 25 oC. Exposure time at 37 oC speeds up of solidification and sample reaches constant physicochemical characteristics after 1 day aging.

The alloy obtained by a «cold» hardening Cu-Ga-In-Sn paste is a metastable and includes more than two phases. Nevertheless, the technological properties of these alloys is relatively high, they can be used, in particular, to create an enough strong and vacuum-tight connection of metallic and nonmetallic materials.

Thermal properties of the solidified alloys are largely due to their relatively difficult structure. Usually, the samples consist of the remainder of the precursor particles of filler alloy (e.g., copper, and tin), the crystalline phase (such as CuGa2) and low melting phase (for example, a solid solution gallium in tin or indium). Interaction of the components in non-equilibrium system with difficult composition is restarted at heating to temperatures about 100 oC and above.

ExperimentalThe samples have a gross composition (% by weight.): Cu-63.7,

Ga-25.9; In-7.1; Sn-rest 3.3). The ratio of components in this case is due to the optimal rheological properties of obtained pastes.

The composite was synthesized using the powder (spherical particles Cu-5 % wt. Sn alloy) (fraction – 40 microns). Furthermore, the starting mixture had included of gallium-indium eutectic alloy (21.4 % wt. In, Ga - rest).

The paste was prepared by intense mechanical mixing of these components. The mechanical activation leads to intensification of interaction between the components. The material was then placed in a

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special form. The cured samples are cylinders with a diameter of 10 mm and a height of 3-4 mm. The unpolished surface of the sample is shown in Fig. 1a. The treated surface of microsection is demonstrated in Fig. 1b.

As can be seen in Fig. 1, composites have a rather complex structure comprising various phases.

a

bFig. 1 a – SEM micrograph of the cured sample;

b – SEM micrograph of the polished surface of sample. Detector back-scattered electrons.

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The microstructure of gallium solidified paste usually is composed of residual particles of the filler-powder (clearly visible in Fig. 1) and the phases are formed by diffusion of eutectic Ga-In in particles of the solid alloy. In work [2] was shown that the compressive strength of such metal composite can exceed 400 MPa.

According to the EPMA, the cured samples are consisting from the residual particles of the starting powder (darkest areas of phase contrast in Fig. 1) containing up to 2 wt %. gallium, Cu-Ga alloy (gray areas) and phases containing indium (white local area of microsection in Fig. 1b).

The alloy of copper and gallium, apparently, is the two phase region consisting of the intermetallics CuGa2 and Cu0.875Ga0.115. According to the stoichiometric calculations, such two phase regions are contained in mole fractions approximately 0.75 of CuGa2 and 0.25 of Cu0.875Ga0.115. Areas rich in indium have an average composition close to the Cu - 30; gallium - 50, Indium-15, tin - the rest (at.%).

The structural phase formation of the cured samples Cu-Ga-In-Sn was studied by SEM (electron microscope Carl Zeiss EVO 40), EPMA (prefix Oxford Instruments INCA X-Act). Thermal expansion was investigated on the NETZSCH DIL 402C dilatometer using high-sensitive detector (linear variable displacement transformer - LVDT). The experiment was carried out in the high-purity helium atmosphere. The heating speed was constant and equal to 2 K/min. The thermal conductivity was calculated from thermal diffusivity and specific heat which are measured using LFA (Laser Flash Analysis) technique with instrument Netzsch LFA 457 over a temperature range from room temperature to 100 °C using (1):

(T) = a(T)・d(T)・Cp (T) (1)

where - thermal conductivity; a - thermal diffusivity; Cp - specific heat; d - density of the sample; T – temperature

Results and discussionApplied for soldering metals and alloys should possess a number

of specific properties, without which is impossible to obtain a reliable connection. One of the main characteristics of the solder defining as a

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destination and the method of its application is the coefficient of thermal expansion.

Firstly, the coefficient of thermal expansion of the solder must not significantly differ from the coefficient of expansion of the metal base. If all the components inside the devices have a same coefficient of linear thermal expansion (CLTE) and heat transfer is instantaneous, they will expand and contract at the same rate and thermal EMF will not arise. Otherwise, there is an uneven distribution of heat in the soldered seam, which leads to an increase of the thermal resistance and the decrease of the strength of soldered connections. Most of the solder alloys have CLTE in the low 20∙10-6·K-1 range with the exception of Bi-42Sn, which has a CLTE of 15∙10-6·K-1. Cu (used as lead frames) and FR-4 (the most common printed circuit board material) have CLTE 16-18·10-6·K-1 and 11.0-15.0·10-6·K-1. Therefore, if CLTE of soldiers and components of printed circuit boards are essentially different, such as solder materials used does not seem possible.

The study of the thermal expansion of three fully hardened samples Cu-Ga-In-Sn in the temperature range from 25 to 100oC was performed. The samples have a cylindrical shape with plane parallel end faces. The results obtained values of thermal expansion are shown in Fig. 2. The differential thermal expansion coefficient was calculated by the equation (2):

(2)

where - thermal expansion; Lo - initial length of the sample; T – temperature; L - length of the sample.

The calculation results of linear expansion coefficient α(T) in the temperature range from 35 to 100 oC are shown in Fig. 3.

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Fig. 2. Thermal expansion over temperature of three Cu-Ga-In-Sn samples

Fig. 3. CLTE over temperature of three Cu-Ga-In-Sn samples

The curves α(T) are the result of polynomial interpolation of the discrete data set received from equation 2. On the curve L/Lo(T) when heated is observed mainly monotonic change of length samples, indicating the absence of phase transitions in the investigated

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temperature range. At the same time, it should be noted that for alloys are observed decrease values of CLTE since 90 oC.

The average CTLE taken from three measurements of each sample is calculated in the temperature range from 25 to 100 oC and equaled to 19.8·10-6/K.

The coefficient of thermal conductivity for the two alloys T67 calculated by the equation 1, taking into account obtained data by thermal diffusivity (Fig. 4), heat capacity (Fig. 5) and thermal expansion is shown in Fig. 7.

As can be seen in Fig. 6, the calculated values of thermal conductivity in the range from 25 to 100°C decrease linearly with increasing temperature.

The one of the criteria for the technological capability of application solder is comparison of the experimental values of the thermal conductivity Cu-Ga-In-Sn alloys with a thermal conductivity of pure tin and its alloys. The thermal conductivity of Cu-Ga-In-Sn alloys is different by 8-10 percent from the thermal conductivity of pure tin, which is an excellent indicator for the this class of solders.

Fig. 4. Thermal diffusivity over temperature of three Cu-Ga-In-Sn samples

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Fig. 5. Heat capacity over temperature of three Cu-Ga-In-Sn samples

Fig. 6. Thermal diffusivity over temperature of three Cu-Ga-In-Sn samples

ConclusionIn this present work, Cu63.7-Ga25.9-In7.1-Sn3.3 have been

prepared using mechanochemical mixing solid (copper and its alloys) and liquid (melt Ga-In) components at a temperature of 25 °C. Microstructure and thermal properties of diffusion-hardening solders

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Cu-Ga-In-Sn by scanning electron microscopy, X-ray microanalysis, laser flash method and dilatometry were investigated. The thermal conductivity of Cu-Ga-In-Sn alloys is different by 8-10 percent from the thermal conductivity of pure tin.

AcknowledgmentAuthors are grateful for support to the Presidium of RAS

(research program 12-Р-3-1032)

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