Effect of sintering process on microstructure and properties of Al–Si alloy made by powder metallurgy for electronic packaging application

The impact of the sintering temperature on the microstructure and characteristics of Al–Si composite was investigated in this study. Al-60Si (wt%) composites were sintered at different temperatures, and the microstructural features were evaluated. Optimal microstructural characteristics were observed when the composites were sintered at 850 °C for 2 h. The resulting material exhibited a thermal conductivity, coefficient of thermal expansion, and relative density of 131.4 W/(m·K), 10.02 × 10–6 K–1, and 99.87%, respectively. Elevating the temperature facilitated better wettability of the material, which in turn aided the filling of voids within the material by liquid Al. In addition, a decrease in the total number of interfaces within the material and the establishment of a connected network by the Al matrix contributed to enhanced thermal conductivity and coefficient of thermal expansion.


Introduction
The rapid advancement of modern electronic information technology has led to an increased demand for electronic packaging materials, which are essential in the design and manufacturing of electronic devices [1]. Nevertheless, due to the growing complexity and density of components in these devices, there is an urgent necessity for the development of new electronic packaging materials with exceptional performance. Highquality packaging materials should possess strong mechanical properties, low coefficients of thermal expansion (CTE), and high thermal conductivity (TC). Aluminum-high Silicon (Al-Si) alloy exhibits favorable TC, high specific strength and stiffness, good plating compatibility with gold, silver, copper, and nickel, weldability to substrates, and ease of precision machining [2,3]. Furthermore, it is readily available, environmentally benign, non-toxic, and easily recyclable [4][5][6]. This material is primarily employed in aerospace, space technology, portable electronic devices, and other high-tech domains.
At present, numerous methods are available for preparing Al-Si composite materials, including spray deposition [7], powder metallurgy [8], pressure infiltration [9], hot pressing sintering [10], and pressureless infiltration [11]. The British company Osprey Metals has developed a series of alloys with Si content ranging from 12% to 70% using jet deposition and hot isostatic pressing. They have named this the CE alloy, which exhibits a CTE of 7 × 10 -6 to 20 × 10 -6 K -1 and high TC [12,13]. The Si particle size increases as the Si content in CE alloys is enhanced [14]. Osprey's CE alloys can be employed in various applications; for instance, CE7 and CE9 alloys are suitable for cathode RF and microwave circuit packaging and aerospace electronic systems due to their CTE compatibility with Si and GaAs [15]. Compared to the spray deposition method, the powder metallurgy method is not limited by the matrix and secondary phase, allowing for adjustments in the ratio of the two phases within a certain range, resulting in a more uniform powder distribution. Sumitomo Electric Corporation in Japan utilizes powder metallurgy hot extrusion to create Al-40Si (wt%) alloy, which has a CTE of 13 × 10 −6 · K −1 , TC of 126 W/(m·K), and density of 2.53 g cm −3 [16]. However, the composite material Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. prepared via the powder metallurgy hot extrusion method contains numerous voids, which significantly affect the material's structure and properties. In contrast, the hot pressing sintering method eliminates these voids, producing a denser material. Appropriate sintering temperatures can effectively enhance the material's wettability, thus, promoting the filling of voids within the material by liquid Al. In this study, composites were sintered at various temperatures and subsequently hot extruded to determine the optimal sintering temperature [17].

Experimental procedure
Al and Si powders were utilized for fabricating Al-60Si (wt%) composite material, which had a median particle size of 30 μm (table 1). The shape and structure of the powders can be visualized in figure 1.
To prepare the composite material, a combination of Al and Si powders was filled into a pure Φ70 × 70 mm Al jacket. The powder was compacted using a vibrating technique and preheated for 20 min at 300°C. The powder is preheated, it can occur greater plastic deformation under less pressure, the resistance between the mixed powder is smaller, the contact between the powder is closer, and even metallurgical bonding. In the process of extrusion and release, the friction between the mold wall and the billet will not cause pressing failure. The preheated powder was then placed into a mold and kept at 300°C for an hour, after which it was subjected to a pressure of 900 MPa on a 500 t four-column hydraulic press to form the ingot. Subsequently, the pressed blank was transferred to a furnace cavity under a pressure of 10 -3 Pa. For one set of experiments, the samples were heated to the desired temperatures of 750°C, 800°C, 850°C, and 900°C, with a heating rate of 5°C min −1 for 2 h. Eventually, all the samples were cooled inside the furnace. It is noteworthy that the entire process was carried out under a vacuum. Figure 2 shows the macroscopic sample of the sintered material.
The original powder's composition and morphology were examined using a TM3030 scanning electron microscope (SEM), and its microstructure was observed using an OLYMPUS GX51 optical microscope. The samples were polished with 400#, 1000#, and 2000# sandpapers, followed by etching with Kroll's solution. The etching solution, containing HF, HNO 3 , and H 2 O in a volumetric ratio of 1:3:7, was used for a 10 s corrosion duration. The DXF-200 thermal conductivity meter (sample size: Φ12.72 mm) was used to measure the material's thermal conductivity. An electronic balance, with a precision of 0.0001 g, was used to measure the density. The Unitherna Dilatometer System Series 1000 thermal expansion tester was used to measure the material's CTE (sample size Φ520 mm) from room temperature (RT) to 150°C at a heating rate of 2°C min −1 , with a high purity argon gas atmosphere.  3. Results and discussion 3.1. Influence of sintering temperature on microstructure and properties of the materials The powder was compressed at a temperature of 300°C, with a mold temperature of 300°C, and subjected to a pressure of 900 MPa. Subsequently, the compacted ingot was placed inside a vacuum sintering furnace and sintered at varying temperatures for 2 h at a rate of 5°C min −1 , resulting in sintered composites at different temperatures (figure 3). At low temperature, a large number of temperatures, numerous small voids, and discontinuous Si phases were observed in the composite, which significantly decreased as the temperature increased to 850°C, resulting in the formation of a continuous Al matrix network. When the temperature further increased to 900°C,  although the Al matrix maintained its continuous network, the number of pores within the material began to rise and increased in size. The primary explanation for this phenomenon is as follows: the melting point of Al is 660°C, so when the sintering temperature was set at 750°C and 800°C, the Al particles merely melted, leading to poor fluidity of the liquid phase and weak wettability between the liquid phase and the silicon particles. As a result, the pore-filling effect through liquid phase flow was insufficient. Furthermore, particle rearrangement was not fully achieved; thus, impacting the densification degree, which in turn caused a large number of pores in the material and a considerably smaller Si phase. However, as the sintering temperature increased, the viscosity of the liquid phase Al decreased, fluidity improved, and wettability between the liquid phase and silicon particles significantly enhanced. When the liquid phase adheres to the solid phase surface, a contact angle exists between the two phases, also known as the wetting angle. The wettability of the liquid phase to the solid phase is determined by this wetting angle. The liquid phase wets the solid phase if the wetting angle is less than 90°, while it does not wet the solid phase if the angle is greater than 90°. The wettability under equilibrium conditions can be expressed by Young-Dupre's formula (1).
The surface tension of the solid phase is denoted by γ S , that of the liquid phase by γ L , and the interfacial tension between the solid and liquid phases by γ SL , while θ represents the angle of wetting. When the liquid phase of Al comes into contact with the solid phase of Si, the contact surface between them tends to expand, resulting in the infiltration phenomenon. Conversely, when the liquid phase contacts the solid phase, the contact surface tends to shrink, resulting in no infiltration phenomenon. A larger contact surface between the liquid and solid phases facilitates better filling of the voids between solid particles, leading to a better sintering effect and higher relative density of the materials. Therefore, when the wetting angle is less than 90°, the liquid phase can immerse into the voids or even the inter-granular gaps of the solid particles during infiltration. Figure 4 depicts the equilibrium diagram of a solid phase wetting by a liquid.
Numerous factors can influence the wetting effect. The relationship between adhesive work and the wetting process is illustrated by Formula (2): Among them: W SL -adhesion work. Substitute equation (1) into the above equation to get Only when W SL > 0, that is, the sum of the surface energy of the solid phase and liquid phase is greater than the solid-liquid interface energy, the liquid phase can wet the solid phase. Therefore, reducing g SL or reducing the wetting Angle is beneficial to wetting, and changing the temperature can change the g . SL With an increase in the sintering temperature, the interfacial tension between the liquid aluminum and silicon particles decreases, leading to enhanced wettability of these materials. The filling of the gaps between the Si particles with liquid Al becomes comparatively more efficient as well. As a result, when the temperature was maintained at 850°C, the internal pores of the material reduced considerably, and the Al matrix exhibited a continuous network. However, with a further rise in the sintering temperature, although the wettability of the system improved, the Si particles became more prone to agglomeration and segregation, resulting in a local concentration of Si particles that obstructed the flow channel of liquid Al. Therefore, when the temperature reached 900°C, the number of pores increased due to Si particle agglomeration, thereby adversely affecting the properties of the composite. The solidification of liquid aluminum also influences this process. Once the temperature exceeds a specific range, the liquid aluminum gets lost and exuded from the sintered body. This results in a reduction in the aluminum content of the material, leading to poor filling and an increase in material defects.
Additionally, as the sintering temperature increased while maintaining all other conditions constant, the density and relative density of the composite initially experienced an increase, followed by a decline ( figure 5).
During the sintering process, temperature plays a crucial role in determining the structure and properties of the material. In the case of liquid phase sintering, as the temperature increased, the fluidity and wettability of the liquid phase with respect to the solid phase progressively improved, thereby promoting the rearrangement of particles during the sintering process within the liquid phase and ultimately enhancing the density and relative density of the bulk material. As previously discussed, due to the poor wettability and fluidity of the liquid phase at low sintering temperatures, the inadequate filling effect of Al led to a high number of pores, resulting in lower density and relative density values for the material. However, when the temperature was raised to 850°C, Al demonstrated an improved pore filling effect for evident reasons, leading to the maximum density and relative density values for the sintered material, 2.467 g cm −3 and 99.87%, respectively. Conversely, an inverse trend was observed when the temperature was further increased to 900°C. Although high sintering temperatures are advantageous for enhancing the wettability of the liquid to the solid phase, in the case of composites with a high volume fraction of the second phase, elevated sintering temperatures caused agglomeration and segregation of the Si phase. Consequently, the local enrichment of Si particles impeded the flow of Al, which created larger voids in the material, thereby reducing the density and relative density.
The results indicated that the TC exhibited a similar pattern to density, as it initially increased and then decreased with the rising temperature ( figure 6). When the sintering temperature reached 850°C, the material's TC achieved its maximum value of 131.4 W/(m·K).
In this study, the inadequate filling effect and particle arrangement in the Al matrix were found to be the primary reasons for the poor fluidity and wettability of the liquid phase compared to the solid phase. As a result, the composite lacked the necessary density, and many pores were present in the material. The presence of these pores increased the interface thermal resistance of the material and lowered its TC, as air trapped in the pores had a low TC at low sintering temperatures. Additionally, the Al matrix could not form a continuous network due to the presence of numerous fine Si particles that inhibited the expansion of the Al matrix. Therefore, the TC of the material was very low at 750°C and 800°C. However, at 850°C, the wettability of the liquid phase to the  solid phase increased, allowing the Al liquid to fill the internal pores effectively, thereby reducing their number. Additionally, Al formed a continuous matrix that facilitated heat conduction in the composite, resulting in the maximum TC of the material at 850°C. Subsequently, the continuous network of the Al matrix inside the material further improved the TC of the material as the temperature increased. Nonetheless, extremely high sintering temperatures caused the agglomeration and segregation of Si particles, increasing the number and size of the pores, and reducing the TC of the material. Therefore, the TC of the composite at 900°C was slightly lower than that at 850°C.
The average CTE of the sintered material at different sintering temperatures was determined while maintaining all other conditions constant. The results indicated that the addition of a large number of lowexpansion Si particles significantly reduced the CTE of the Al matrix (23.6 × 10 -6 K -1 at RT), a role that the second phase often plays in most electronic packaging materials. Furthermore, as the sintering temperature increased, the CTE initially increased, followed by a decrease similar to the TC and density values, as shown in figure 7.
The Al matrix did not form a completely connected network at 750°C and 800°C, resulting in a relatively small CTE. Additionally, a large number of dispersed small Si particles further suppressed the expansion of the matrix. Furthermore, during low-temperature sintering, the wettability of the Al liquid to solid Si particles was insufficient, and the filling effect of liquid pores was inadequate. A considerable number of pores were present in the material, which did not expand upon heating and had a strong inhibitory effect on the expansion of the Al matrix, resulting in the lowest CTE value. When the temperature increased to 850°C, the Al matrix formed a continuous network inside the material, exhibiting high thermal expansion properties. At this temperature, the Si phase grew significantly, and its inhibitory effect on the expansion of the Al matrix was reduced. Moreover, a few pores were left in the material, which had a limited restraint on the thermal expansion of the Al matrix. Therefore, the CTE achieved the maximum value of 10.02 × 10 -6 ·K -1 at 850°C. As the temperature further increased to 900°C, although the Al matrix still had a continuous distribution that increased the CTE of the material, the high temperature led to the agglomeration of the Si phase and an increase in the number of pores in the material. The existence of pores significantly reduced the CTE of the material. As a result, the CTE of the material was slightly lower than that at 850°C due to the combined effect of these two factors.
The Brinell hardness test was conducted on the sintered samples prepared at different sintering temperatures, and the relationship between them was determined from the experimental data, as shown in figure 8. The results indicated that the hardness value of the sintered composites initially increased and then decreased with an increase in the sintering temperature. At low temperatures, the hardness value of the material was relatively low and reached the maximum value of 125.3 HBW at 850°C. However, as the temperature continued to increase, the Brinell hardness of the material decreased.
At low sintering temperatures, the poor wettability of the liquid phase in relation to the solid phase and insufficient liquid phase fluidity resulted in a weak pore filling effect in the liquid phase. Concurrently, the particle rearrangement during the liquid phase sintering process was also inadequate, leading to the formation of numerous pores and a lower Brinell hardness value for the material. As the temperature increased to 850°C, the Brinell hardness of the material reached its maximum value of 125.3 HBW due to enhanced wettability of the liquid relative to the solid phase and other associated events. However, a further rise in the sintering temperature caused the second phase particles to agglomerate and segregate as a consequence of their high volume fraction. This impeded the flow of liquid Al, increasing the number and size of voids in the material, and subsequently, decreasing the microhardness of the composites.

Changes in
Al-Si composites mechanism, Si phase morphology, and size during heating Composite powders of Al-60Si (wt%) were obtained at different sintering temperatures, including 750°C, 800°C, 850°C, and 900°C, and their corresponding SEM micrographs are displayed in figure 9.
The SEM results showed that with the increase in the sintering temperature, the size of the Si phase also gradually increased.
The SEM results revealed that the size of the Si phase increased gradually with an increase in the sintering temperature. According to the Lifshitz-Lyozov theory, small crystal particles produced in the solution tend to dissolve into the surrounding medium due to their high energy and large curvature and then re-precipitate on the surface of larger crystal particles, leading to their further growth. Figures 2 and 7 illustrate that when the sintering temperature is low, most of the silicon phase in the material's microstructure is very small. As the sintering temperature increases, the size of the silicon phase in the microstructure also starts to grow, and small  particles are dissolved, resulting in a difference in the matrix's concentration after dissolution. Subsequently, the dissolved Si phase continues to form large particles, leading to the continued growth of the bulk particles, consistent with the Lifshitz-Lyozov theory.
In the case of Al-Si composites, as the heating temperature increases or the heating time extends, the dissolution and precipitation process in the liquid phase become more efficient. The small Si particles in the system and the region with a small radius of curvature of the particles tend to dissolve preferentially in the liquid phase and grow through the Al matrix to the large particles, while the Al matrix migrates and precipitates to the large particles and the relatively smooth part of the particle surface. Consequently, small particles gradually disappear, and large particles continue to grow, accompanied by the passivation of particle morphology.
According to the Gibbs-Thompson formula: In this expression, Cα(r) represents the solute equilibrium concentration of the base metal at radius r, while Cα (∞) denotes the equilibrium concentration of the matrix metal solute at a distance far from r. Furthermore, R is the gas constant, T refers to the temperature, and V corresponds to the molar volume of the precipitate. According to mathematical approximation, ( ) + = x ln 1 x ; therefore, equation (4) converts to equation (5) as follows: Equation (5) establishes a correlation between the surface concentration of precipitated particles within the matrix and the radius of the particles (r). It indicates that a smaller particle radius corresponds to a higher solute concentration on the surface. Consequently, in Al-Si composite materials, during isothermal heat treatment, the smaller Si phase and regions with a reduced radius of curvature r of the bulk Si phase dissolve into the adjacent matrix metal, increasing the solute concentration in the surrounding matrix. This occurs due to their high surface free energy and instability. In contrast, larger Si phases and areas with a greater radius of curvature r of the bulk Si phase possess lower surface free energy and are less prone to decomposition. As a result, the solute concentration in the surrounding matrix remains low. In both small and large Si phases with diminished curvatures, the concentration of the matrix metal solute in the proximate area is elevated. This encourages the diffusion of Si atoms towards the large-sized Si particles and areas where these particles exhibit a reduced radius of curvature. Consequently, the matrix solute concentration near the sharp edges of both the small and large Si phases decreases, falling below the equilibrium concentration. In the large Si phase and those with a smaller radius of curvature, the neighboring matrix concentration surpasses the equilibrium concentration. Thus, driven by the chemical potential difference, the Si atoms in the small-sized Si phase and the sharp corners of the large-sized Si phase with reduced curvature diffuse into the adjacent matrix. Conversely, the large-sized Si phase and areas with minimal curvature of the bulk Si phase spontaneously absorb Si atoms from the nearby matrix, progressively forming granulated and stable particles. As a result, the graining and coarsening of the Si phase in the Al-Si composite occur concurrently during the heating process.
During particle coarsening, solute atoms from smaller particles dissolve into the matrix and diffuse through it to larger particles. Consequently, small particles continue to diminish in size, while large particles persist in growing. Assuming the total volume of the second phase particles remains constant throughout the isothermal process, the growth rate of these particles can be represented by equation (6): when = r r, / = dr dt 0; r < r, / dr dt < 0, small particles are dissolved, whereas when r > r, / dr dt > 0, large particles grow up; also when = r 2r, / dr dt is maximum and grow rapidly. During the growth process, the small particles dissolve, and the large particles grow, leading to a decrease in the total number of particles. In fact, at r > 2r, there is rarely any particle left in the system. Figure 10 shows the relationship between the particle growth rate and particle radius.
According to equation (6), / dr dt drops with the temperature rise. According to equation (7), it can be observed that the value of D increases exponentially with an increase in C∞ with increasing temperature. Thus, as per the combined effect of equations (6) and (7), / dr dt increases with the temperature rise. As a result of the increase in temperature, the atomic diffusion coefficient shows an exponential increase, leading to an improvement in the growth of the Si phase and passivation in Al-Si composite materials. Moreover, for a specific temperature, a longer sintering time provides sufficient time for Si atoms to diffuse through the liquid phase.

Conclusions
In conclusion, the impact of sintering temperature and time on the microstructure and properties of Al-60Si (wt%) composites was thoroughly examined. The samples were sintered at temperatures of 750°C, 800°C, 850°C, and 900°C for a duration of 2 h. The findings revealed that the composite displayed an optimal structure and superior performance when sintered at 850°C for 2 h. Furthermore, the TC, CTE, density, and relative density reached maximum values of 131.4 W/(m·K), 10.02 × 10 −6· K −1 , 2.467 g cm −13 , and 99.87%, respectively, under the same conditions. Therefore, the composite demonstrated the best overall performance at 850°C.