Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiC_p/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO₂ as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiC_p/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10^–6/K were obtained in 50–71 vol% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.     

Silicon carbide whisker copper-matrix composites fabricated by hot pressing copper coated whiskers

Copper-matrix SiC whisker composites containing 33–54 vol % SiC whiskers and with < 5 vol % porosity were fabricated by hot pressing SiC whiskers that had been coated with copper by electroless plating followed by electroplating. The highest Brinell hardness of 260 was attained at 50 vol % SiC whiskers. The lowest coefficient of thermal expansion (CTE) of 9.6 × 10^–6°C^–1 (at 25–150°C) was attained at 54 vol % SiC whiskers. The composites exhibited lower porosity, higher hardness, higher compressive yield strength, lower CTE, lower electrical resistivity and higher thermal conductivity than the corresponding composites made by hot pressing mixtures of copper powder and bare SiC whiskers.     

Thermal expansion of a novel hybrid SiC foam–SiC particles–Al composites

A new type of hybrid SiC foam–SiC particles–Al composites (V_SiC = 53, 56.2 and 59.9%) to be used as an electronic packaging substrate material were fabricated by squeeze casting technique, and their thermal expansion behavior was evaluated. The coefficients of thermal expansion (CTEs) of the hybrid composites in the range of 20–100 °C were found to be between 6.6 and 7.7 ppm/°C. The measured CTEs are much lower than those of SiC particle-reinforced aluminum (SiC_p–Al) composites with the same content of SiC because of the characteristic interpenetrating structure of the hybrid composites. A material of such a low CTE is ideal for electronic packaging because of the low thermal mismatch (and therefore, low thermal stresses) between the electronic component and the substrate. To achieve similar CTEs in SiC_p–Al composites, the volume fraction of SiC would be much higher than that in the hybrid composites.     

Al/stainless-invar composites with tailored anisotropy for thermal management in light weight electronic packaging

Composite plates in which the low CTE phase has the shape of a honeycomb are anticipated to present optimum anisotropy of thermal expansion and thermal conductivity for baseplates in electronic packaging. This design is explored by choosing an invar alloy for the low CTE phase. In order to allow the formation of a passivation layer protecting from reaction with liquid Al during squeeze casting, the honeycomb is made of the Cr-rich alloy commonly called “stainless-invar”. Composite plates containing 20 vol.% and 38 vol.% stainless invar were processed using honeycombs with the same thickness over cell side ratio. Experimental CTE values are significantly lower than the predictions of three different thermo-elastic models. The very limited amplitude of the strain hysteresis precludes the occurrence of global plastic yielding in the matrix. It appears that, owing to the high contiguity of the low CTE phase, the low value of the experimental CTE results from void closing and opening by localised plastic flow. A honeycomb volume fraction of 38% is necessary for bringing the average CTE down to the level suitable for packaging applications. The ratio of transverse thermal conductivity to density then amounts to about half of the performance of the best Al/SiC composites.     

Hot-pressed AlN-Cu metal matrix composites and their thermal properties

AlN-Cu metal matrix composites containing AlN volume fractions between 0.1 and 0.5 were fabricated firstly by liquid phase sintering of AlN using Y₂O₃ as a sintering aid and then by hot pressing the powder mixtures of sintered AlN and Cu at 1050°C with a pressure of 40 MPa under flowing nitrogen. With Y₂O₃ additions of 1.5 to 10 wt%, the densification of AlN could be achieved by liquid phase sintering at 1900°C for 3 h and subsequently slow cooling. The sintered AlN showed a maximum thermal conductivity of 166 W/m/K at a Y₂O₃ level of 6 wt%. Dense AlN-Cu composites with AlN contents up to 40 vol% were achieved by hot pressing. The thermal conductivity and the coefficient of the thermal expansion (CTE) of the composites decreased with increasing AlN volume fractions, giving typical values of 235 W/m/K and 12.6 × 10^–6/K at an AlN content of 40 vol%.     

Influence of processing route on electrical and thermal conductivity of Al/SiC composites with bimodal particle distribution

Al/SiC composites with volume fractions of SiC between 0.55 and 0.71 were made from identical tapped and vibrated powder preforms by squeeze casting (SC) and by two different setups for gas pressure infiltration (GPI), one that allows short (1–2 min) liquid metal/ceramic contact time (fast GPI) and the other that operates with rather long contact time, i.e., 10–15 min, (slow GPI). Increased liquid metal–ceramic contact time is shown to be the key parameter for the resulting thermal and electrical conductivity in the Al/SiC composites for a given preform. While for the squeeze cast samples neither dissolution of the SiC nor formation of Al₄C₃ was observed, the gas pressure assisted infiltration led inevitably to a reduced electrical and thermal conductivity of the matrix due to partial decomposition of SiC leading to Si in the matrix. Concomitantly, formation of Al₄C₃ at the interface was observed in both sets of gas pressure infiltrated samples. Longer contact times lead to much higher levels of Si in the matrix and to more Al₄C₃ formation at the interface. The difference in thermal conductivity between the SC samples and the fast GPI samples could be rationalized by the reduced matrix thermal conductivity only. On the other hand, in order to rationalize the thermal conductivity of the slow GPI a reduction in the metal/ceramic interface thermal conductance due to excessive Al₄C₃-formation had to be invoked. The CTE of the composites generally tended to decrease with increasing volume fraction of SiC except for the samples in which a large expansive drift was observed during the CTE measurement by thermal cycles. Such drift was essentially observed in the SC samples with high volume fraction of SiC while it was much smaller for the GPI samples.     

Thermal expansion of isotropic Duralcan metal–matrix composites

The thermal expansion behaviour of Duralcan composites having a matrix of hypoeutectic Al–Si alloy containing SiC reinforcements ranging from 10–40 vol% was investigated. The coefficient of thermal expansion (CTE) of the MMCs was measured between 25 and 350 °C by a high-precision thermomechanical analyser, and compared to the predictions of three theoretical models. At low temperature, the experimental CTEs show substantial deviation from the predictions of the elastic analysis derived by Schapery, while the Kerner model agrees relatively well at high temperature. The overall measured CTE, in the range of 25–350 °C, as a function of the volume fraction of SiC is well predicted using Schapery's lower bound. We interpret these features as being an effect of reinforcement phase geometry and the modified microstructure derived from the Duralcan process and subsequent heat treatments. © 1998 Kluwer Academic Publishers     

Effects of Sip size and volume fraction on properties of Al/Sip composites

Al–12 wt.% Si alloy matrix composites reinforced with high volume fraction of Si_p were fabricated by squeeze infiltration. The effects of the compacting pressure on the volume fraction of Si_p in preforms, and the influences of Si_p size and volume fraction on the properties of Al/Si_p composites were examined through this study. Si particles were compacted at different pressure of 40–130 MPa followed by sintered at 1000 °C for 7 h to obtain preforms containing 60–70 volume fraction (vol.%) of Si_p. The sintered preforms were then infiltrated with Al–12 wt.% Si alloy at 750 °C under a 75 MPa squeeze infiltration pressure. It was found that lower coefficient of thermal expansion (CTE) and smaller density may be obtained with higher Si_p volume fraction, yet increasing Si_p volume fraction leads to higher amount of porosities in the composites and thus lowers the thermal conductivity (TC) and flexural strength. Besides, with the same Si_p volume fraction, coarse Si particles result in higher CTE and TC, while finer Si particles may lower CTE and enhance the flexural strength of the composites effectively. From the results obtained in this study, it is expected that the high volume fraction Si_p reinforced Al/Si_p composites posses good potential in electronic packaging applications.     

Effect of copper content on the thermal conductivity and thermal expansion of Al–Cu/diamond composites

Al–Cu matrix composites reinforced with diamond particles (Al–Cu/diamond composites) have been produced by a squeeze casting method. Cu content added to Al matrix was varied from 0 to 3.0 wt.% to detect the effect on thermal conductivity and thermal expansion behavior of the resultant Al–Cu/diamond composites. The measured thermal conductivity for the Al–Cu/diamond composites increased from 210 to 330 W/m/K with increasing Cu content from 0 to 3.0 wt.%. Accordingly, the coefficient of thermal expansion (CTE) was tailored from 13 × 10⁻⁶ to 6 × 10⁻⁶/K, which is compatible with the CTE of semiconductors in electronic packaging applications. The enhanced thermal conductivity and reduced coefficient of thermal expansion were ascribed to strong interface bonding in the Al–Cu/diamond composites. Cu addition has lowered the melting point and resulted in the formation of Al₂Cu phase in Al matrix. This is the underlying mechanism responsible for the strengthening of Al–Cu/diamond interface. The results show that Cu alloying is an effective approach to promoting interface bonding between Al and diamond.     

Thermally conducting aluminum nitride polymer-matrix composites

Thermally conducting, but electrically insulating, polymer-matrix composites that exhibit low values of the dielectric constant and the coefficient of thermal expansion (CTE) are needed for electronic packaging. For developing such composites, this work used aluminum nitride whiskers (and/or particles) and/or silicon carbide whiskers as fillers(s) and polyvinylidene fluoride (PVDF) or epoxy as matrix. The highest thermal conductivity of 11.5 W/(m K) was attained by using PVDF, AlN whiskers and AlN particles (7 μm), such that the total filler volume fraction was 60% and the AlN whisker–particle ratio was 1:25.7. When AlN particles were used as the sole filler, the thermal conductivity was highest for the largest AlN particle size (115 μm), but the porosity increased with increasing AlN particle size. The thermal conductivity of AlN particle epoxy-matrix composite was increased by up to 97% by silane surface treatment of the particles prior to composite fabrication. The increase in thermal conductivity is due to decrease in the filler–matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 60 vol.% silane-treated AlN particles only, the thermal conductivity of epoxy-matrix composite reached 11.0 W/(m K). The dielectric constant was quite high (up to 10 at 2 MHz) for the PVDF composites. The change of the filler from AlN to SiC greatly increased the dielectric constant. Combined use of whiskers and particles in an appropriate ratio gave composites with higher thermal conductivity and low CTE than the use of whiskers alone or particles alone. However, AlN addition caused the tensile strength, modulus and ductility to decrease from the values of the neat polymer, and caused degradation after water immersion.     

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi₂ and 18 vol % SiC whisker-reinforced MoSi₂ were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi₂ showed an increase from about 0.44 Wsg^–11K^–1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm^–1 K^–1 at room temperature to 0.28 Wem^–1 K^–1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C^–1 between room temperature and 200 °C to 9.13 °C^–1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi₂ are discussed.     

Whisker reinforcement of glass-ceramics

Silicon carbide whisker reinforcement of anorthite and cordierite glass ceramics has been studied. At 25 vol% whisker loading the flexural strengths increased from 65–103 MPa to 380–410 MPa, the fracture toughnesses increased from 1.0–1.5 MPa m^1/2 to 5.2–5.5 MPa m^1/2. The strengths decline to 240–276 MPa at 1200 °C. The reasons for the decrease in strength with temperature are discussed. Whiskers from two different sources with differences in diameters and aspect ratios were evaluated and the effect of the whisker morphology on the composite properties was studied. It was found that larger diameter, higher aspect ratio whiskers result in improved composite performance. The composites were also characterized in terms of their thermal properties, i.e. thermal expansions and thermal conductivities. The thermal expansion coefficient from 25–1000 °C for anorthite-based composite was 4.6×10^–6 °C^–1 and that for the cordierite-based composite was 3.62×10^–6 °C^–1. The thermal conductivities at 1000 °C were 3.75 and 4.1 Wm^–1 K^–1 for cordierite and anorthite composites, respectively.     

Carbonization of coal-tar pitch into lump needle coke in a tube bomb

The carbonization of coal-tar pitches and their QI (quinoline insoluble)-free fractions was studied by evaluating their lump cokes produced in a tube bomb at various temperatures (470 to 550° C) and pressures (0 to 75 kg cm^–2 gauge). The lump coke from QI-free fractions had a comparable anisotropic development and coefficient of thermal expansion (CTE). The carbonization temperature and pressure were found to influence strongly the properties of the cokes. At the highest temperature of 550° C, the most appropriate pressure for the best needle coke was 15 kg cm^–2 G (gauge). Either higher or lower pressure increased the CTE value of coke. In contrast, at a moderate temperature of 500° C, the higher pressure produced the better coke. At the lowest temperature of 470° C, it took 10 h to complete the carbonization, and the lowest pressure allowed the best extent of uniaxial arrangement. Among the cokes prepared under the present conditions, the Carbonization at 500° C under 40 kg cm^–2 G produced the best needle coke with a CTE value as low as 0.1 × 10^–6° C^–1. The carbonization scheme leading into the needle coke is discussed for a better understanding of how the carbonization temperature and pressure cooperatively influence the quality of the resultant coke, in relation to the carbonization reactivity of coal-tar pitch.     

Thermal and dielectric properties of zirconyl phosphate compact

Experimental results are presented on the measurements of thermal expansion (up to 1500°C), thermal conductivity (up to 1000°C), dielectric constant (up to 450 °C) and tan (up to 800 °C) of zirconyl phosphate compacts obtained by sintering at 1600°C. The thermal expansion coefficient of the samples at the temperature below 1100°C was less than 1.7 × 10^–6°C^–1. The samples showed a definite shrinkage at temperatures of 1110 and 1470°C due to the phase transformations. The expansion at 1500°C was less than that at 1100°C probably because of the phase transformation. The thermal conductivity at room temperature was a very small value (0.0046 to 0.0065 cal s^–1 cm°C^–1 cm^–2). The dielectric constant was close to 9. The value of tan° (–0.0001) measured is one of the lowest values for ceramic materials.     

Characteristics of a continuous Si-Ti-C-O fibre with low oxygen content using an organometallic polymer precursor

A continuous Si-Ti-C-O fibre with 12 wt% oxygen content, which is lower than the usual 18 wt% found in the normal fibres, was synthesized by using polytitanocarbosilane which has fewer Si-Si bonds than the usual precursor polymer. The density, tensile strength, tensile modulus and thermal conductivity were found to be 2.37 g cm^–3, 3.4±0.3 GPa, 190±10 GPa and 1.40 W m^–1 K^–1, respectively. Amongst these properties, the tensile modulus was improved by 20 GPa and the thermal conductivity had a higher value in comparison with that of the ordinary Si-Ti-C-O fibre with 18 wt% oxygen content. The Si-Ti-C-O fibre with a 12 wt% oxygen content has a better heat resistance above 1400 °C in an argon atmosphere and 1300 °C in air, than the usual fibre. About 60 and 40% of its tensile strength at room temperature were retained in air at respectively, 1500 and 1600 °C. This improved ceramic fibre is considered to be useful as a reinforcing material for advanced composites such as high-temperature ceramic matrix composites and metal matrix composites.     

高体积分数SiCp/A356复合材料的显微组织和电导率

铝基复合材料在电子封装领域存在着潜在的应用前景。为获得高体积分数的铝基复合材料,利用压力浸渗法制备了高体积分数SiC颗粒增强A356复合材料(SiC_p/A356),通过金相显微镜、XRD、SEM和EDS等分析手段对其物相、显微结构和电导率进行了表征。结果表明:用该方法制备的SiC_p/A356复合材料组织致密,颗粒分布均匀,界面结合性能较好;SiC增强颗粒与A356基体界面反应控制良好,仅有少量Al4C3脆性相生成。SiC粉体经颗粒表面氧化处理在其表面生成一层SiO_2薄膜,虽抑制了界面反应的发生,但也使复合材料的收缩减小,电阻率增大,导电性能变差。     

Negative thermal expansion of laminates

Measurements have been carried out on the in-plane and through the thickness thermal expansion coefficients of glass polypropylene fibre composites of 50% volume fraction between room temperature and 120°C. Only in the temperature range 20° to 75°C are reproducible results obtained. It is confirmed that in-plane negative values may be obtained in specific directions. The physical reason for this and its connection with the necessary appearance of a large Poisson ratio is pointed out. The expansivity of the matrix material depends strongly on temperature. Very good agreement between the experimental values and those predicted from the properties of the two constituents is found provided that the value for the expansivity of the polyoropylene is that within the temperature range considered. It is shown that the polypropylene matrix does not provide a matrix which is stable enough in its properties to enable the system to yield consistent negative values of expansivity. An alternative system is proposed and a single experiment confirms that a negative value of the thermal expansivity of as large as –50 × 10^–6 K^–1 may be obtained.     

Epoxy Composites Filled with Micro-Sized AlN Particles for Microelectronic Applications

With the rapid development of the electronic information industry, better properties are required for substrate and packaging materials such as high thermal conductivity, low coefficient of thermal expansion, and low dielectric constant. Polymers are ordinarily being used for this purpose due to their high electrical resistivity and low density, but unfortunately they suffer from a disadvantage like low thermal conductivity. To offset this deficiency, adding inorganic conductive particles to polymer is a versatile method. In view of this, the present work aims at developing a class of particulate filled polymer composites with micro-sized aluminum nitride (AlN) particles having an average particle size of 60–80 µm reinforced in epoxy matrix. A set of composites, with filler content ranging from 0 to 25 vol%, have been prepared by the hand-layup technique. Effects of filler percentage on various properties like effective thermal conductivity (k_eff), coefficient of thermal expansion (CTE), glass transition temperature (T_g), and dielectric constant (ε_c) are studied. It is found that the incorporation of AlN in resin increases the k_eff and T_g, whereas CTE of the composite decreases favorably. Though dielectric constant of the matrix increases with filler content yet it remains well within the desirable limit. With modified thermal and dielectric characteristics, these composites can possibly be used for microelectronics applications.     

Thermal expansion and conductivity of magnetite flakes taken from the Oconee-2 steam generator

Flakes consisting primarily of iron oxides (magnetite) have been discovered in the spaces between tubes and support plates in steam generators, increasing flow resistance and causing abnormal increases in water levels. To aid in the determination of the effects of tube scale on steam generators, Duke Power Company and MPR Associates, Inc. arranged for the author to measure the thermal expansion and thermal conductivity of tube scale specimens from the steam generator of the Oconee-2 pressurized water reactor. The study measured the thermal expansion of the flakes directly, using miniaturized specimens, rather than deriving these data from X-ray powder diffractometry as in past studies.The flakes are composed of multiple layers, each of which exhibits different thermal behaviour. Thermal expansion was higher than that for Fe₃O₄. The average thermal conductivity for two- and three-layered flakes is 0.026 J sec^–1 cm^–1 ° C^–1. The thermal conductivity for a singlelayered flake is 0.060 J sec^–1 cm^–1 ° C^–1.The work was performed during employment at Battelle.     

Fabrication of controlled expansion Al-Si composites by pressureless and spark plasma sintering

A combination of low coefficient of thermal expansion (CTE) and decent thermal conductivity (TC) is the reason for the Al-high vol% Si system to become popular for electronic packaging material. In the present work, two process routes, firstly conventional powder metallurgy and then spark plasma sintering (SPS) were utilized for the fabrication of Al-20-60 wt.% Si composites. In addition, effect of small fraction of CNT addition on the CTE of Al-20?wt% Si was studied. Effect of process parameters on the consolidation of the composites in terms of densification, microstructure evolution along with fractographic analysis and strength was studied. CTE and TC of the sintered composites were measured and correlated with the densification, percentage of Si and morphologies of the sintered products. Overall, better densification could be achieved in SPS and the Al-30%Si and Al-40%Si composites SPSed at 550?°C showed average CTE values of 14.52?×?10^?6/K and 13.36?×?10^?6/K, respectively, in the temperature range of 30–200?°C, which were better than some of the existing alloys with higher Si content. Simultaneously, TC values were 114.4?W/mK and 107.12?W/mK, respectively, for the above two SPSed composites.