Performance of Tungsten-Copper and Molybdenum-Copper Materials for Electronic Packaging

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Performance of WCu/MoCu Electronic Packaging Materials

WCu and MoCu alloys are primarily used in large-scale integrated circuits and high-power microwave devices as insulating metal substrates, thermal control plates, heat dissipation components (heat sink materials), and lead frames. For electronic packaging materials, their quality and performance must meet extremely high requirements. Not only do they require high purity and air tightness (high density), low gas content, and uniform micro structure, but they also demand high thermal conductivity and a thermal expansion coefficient that matches the chip.

1. Basic Properties of Each Component

The melting points of W and Mo are 3410°C and 2620°C, respectively, both belonging to high-melting-point metals often used as hardening phase reinforcements. Although W and Mo have higher densities, their thermal expansion coefficients are relatively close to Si, Ga As, and GaN, and they have high thermal conductivity. The melting point of Cu is 1083.4°C, and its thermal conductivity reaches 400 W/(m·K), exhibiting excellent thermal conductivity, making it非常适合作为导热材料. Therefore, the combination of Cu and W (or Mo) with different mass ratios can complement each other, leveraging the strengths of both to obtain electronic packaging materials with excellent overall performance.

Since Cu is not solidly soluble with W or Mo, and no intermediate phase compound is formed, the composite material they form is a typical pseudo-alloy. Due to Cu's melting point of only 1083.4°C, it exists as a liquid phase during preparation. Additionally, the wetting angles of liquid Cu with high-melting-point hard phases W and Mo above 1100°C are both less than 30°, indicating good wettability between liquid Cu and W, Mo at high temperatures. This provides more technical means for the preparation of such composite materials and lays a good foundation for improving their performance.

2. Performance of WCu/MoCu Alloys

2.1 Thermal Conductivity

Thermal conductivity is the most important performance parameter for electronic packaging materials. The heat transfer mechanism varies among different materials. For insulators, the heat transfer mechanism is primarily phonon conduction; for pure metals, electron conduction is the dominant heat transfer mechanism; in alloys, both electron conduction and lattice conduction play a role. Since Cu, W, and Mo are all metallic elements, their composite materials primarily exhibit metallic characteristics. According to the mechanism of solid heat conduction, pure metals primarily conduct heat through free electrons, known as electron conduction. In WCu/MoCu pseudo-alloys, Cu and W (or Mo) are independently present at the atomic level, with a distinct crystal interface between the two phases. Therefore, the thermal conduction mechanism of these pseudo-alloys should be a combination of Cu, W (or Mo) pure metal conduction and interfacial conduction. Additionally, due to process limitations, pseudo-alloys often contain a certain amount of porosity (porosity <5%). Since gas is a poor heat carrier, the thermal conductivity decreases as the porosity increases. The overall thermal conductivity of the alloy is determined by the synergistic effect of pure Cu, W (or Mo), and the two-phase interface, as well as the porosity.

2.2 Thermal Expansion Coefficient

The thermal expansion coefficient of WCu/MoCu is directly related to the inherent properties of its matrix and reinforcement phases, especially the content of the reinforcement phase. Therefore, by controlling the phase composition and content of the alloy, it is easy to adjust the thermal expansion coefficient of WCu/MoCu to match the thermal expansion coefficient of the chip material. The thermal expansion coefficients of W and Mo are very close to those of Si and Ga As, making them ideal for packaging material properties. Therefore, it is typically necessary to maintain a high content of W and Mo in WCu/MoCu alloys. Under the premise of matching the thermal expansion coefficient, improving thermal conductivity is an inevitable trend in the development of WCu/MoCu alloy performance.

Since Cu has a significantly higher thermal conductivity than W and Mo, alloys with a higher Cu content typically exhibit higher thermal conductivity. However, an excessively high Cu content inevitably leads to a decrease in W and Mo content, increasing the thermal expansion coefficient of the alloy, which is unfavorable for electronic packaging materials. For WCu alloys used in electronic packaging materials, WCu10 and WCu15 compositions are commonly selected, which have a linear expansion rate matched to the chip material and high thermal conductivity. This type of WCu material requires a density greater than 98% of the theoretical density to ensure high thermal conductivity. Although the thermal conductivity of MoCu alloys is lower than that of WCu, their lower density and ease of pressure processing into thin sheets make them advantageous for lightweight packaging materials in aerospace and portable instrumentation applications.