Advanced interconnect materials play a critical role in semiconductor packaging, enabling high-performance, reliable, and miniaturized electronic systems. As device scaling continues, the demand for finer pitch interconnects, improved electrical conductivity, and enhanced thermal management has driven innovation in materials such as copper-pillar bumps, anisotropic conductive films, and low-temperature solders. These materials must meet stringent electrical, mechanical, and thermal requirements while ensuring compatibility with advanced packaging architectures.

Copper-pillar bumps have emerged as a leading solution for high-density interconnects, replacing traditional solder bumps in many applications. The primary advantage of copper pillars lies in their superior electrical conductivity, with copper exhibiting a resistivity of approximately 1.68 µΩ·cm, significantly lower than conventional lead-free solders, which typically range between 10-15 µΩ·cm. This reduction in resistance minimizes signal loss and power dissipation, making copper pillars ideal for high-speed and high-power applications. Mechanically, copper pillars offer greater structural stability due to their higher Young's modulus (around 110-130 GPa) compared to solder alloys (20-50 GPa). This stiffness reduces the risk of deformation under mechanical stress, improving reliability in stacked die and 3D IC configurations. However, the thermal expansion mismatch between copper (CTE ~17 ppm/°C) and silicon (CTE ~2.6 ppm/°C) necessitates careful design to mitigate thermomechanical stress. To address this, copper pillars are often capped with a thin solder layer, forming a hybrid interconnect that combines electrical performance with improved stress absorption. Fine-pitch compatibility is another key benefit, with copper pillars enabling pitches below 40 µm, compared to the 100-150 µm limits of conventional solder bumps.

Anisotropic conductive films (ACFs) are widely used in display interconnects, flip-chip packaging, and flexible electronics due to their ability to provide z-axis conductivity while maintaining x-y insulation. These films consist of conductive particles, typically nickel or gold-coated polymer spheres, dispersed in an adhesive polymer matrix. When compressed between two surfaces, the particles form conductive pathways vertically while remaining isolated laterally. The electrical resistance of ACF interconnects depends on particle density and compression force, with typical values ranging from 10-100 mΩ per connection. Mechanically, ACFs offer flexibility and strain relief, making them suitable for applications involving thermal cycling or bending, such as flexible displays. The polymer matrix provides adhesion and stress absorption, with elastic moduli generally between 1-5 GPa, balancing rigidity and compliance. Thermal stability is a critical consideration, as most ACFs are limited to operating temperatures below 150°C due to polymer degradation. However, advanced formulations incorporating thermally stable resins can withstand temperatures up to 200°C. ACFs excel in fine-pitch applications, supporting pitches as small as 20 µm, though alignment precision becomes increasingly critical at these dimensions.

Low-temperature solders have gained prominence as packaging solutions for heat-sensitive components and heterogeneous integration. Traditional lead-free solders like SAC305 (Sn-3Ag-0.5Cu) require reflow temperatures around 240-260°C, which can damage organic substrates or adjacent devices. In contrast, low-temperature solders such as Sn-Bi (melting point ~138°C) and Sn-In (melting point ~118°C) enable processing at 150-200°C, reducing thermal stress and energy consumption. The electrical resistivity of these alloys ranges from 12-15 µΩ·cm, slightly higher than SAC305 but still acceptable for most applications. Mechanical properties vary significantly with composition; for example, Sn-58Bi exhibits high strength but brittleness due to Bi precipitation, while Sn-51In offers better ductility but lower creep resistance. Thermal cycling reliability is a key challenge, as the low melting point correlates with reduced high-temperature performance. Alloy modifications, such as adding small amounts of Ag or Cu, can improve mechanical stability without substantially increasing processing temperatures. Fine-pitch compatibility is achievable with particle-free solder pastes and precise jetting techniques, enabling pitches below 50 µm. However, interfacial reactions between low-temperature solders and copper or nickel surfaces require careful evaluation to prevent excessive intermetallic growth during operation.

Thermal management is a critical consideration for all advanced interconnect materials. Copper-pillar bumps provide excellent thermal conductivity (~400 W/m·K), facilitating heat dissipation from high-power devices. ACFs, with their polymer-dominated composition, exhibit much lower thermal conductivity (0.2-0.5 W/m·K), making them less suitable for thermally demanding applications unless supplemented with thermally conductive fillers. Low-temperature solders typically have thermal conductivities between 30-60 W/m·K, adequate for many applications but inferior to copper. The choice of interconnect material must balance thermal performance with other requirements, such as mechanical compliance or processing constraints.

Reliability under environmental stress is another key differentiator. Copper pillars demonstrate excellent electromigration resistance due to copper's high melting point, with current-carrying capacities exceeding 10^5 A/cm². ACFs are susceptible to moisture absorption and thermal degradation over time, though advanced formulations mitigate these issues through hermetic fillers and stable polymer chemistries. Low-temperature solders face challenges in thermal cycling due to their lower creep resistance, requiring alloy engineering or underfill materials to enhance durability.

The evolution of advanced interconnect materials continues to address the demands of next-generation packaging. Copper-pillar technology is extending into hybrid bonds with direct copper-to-copper connections at room temperature, enabling sub-10 µm pitches. ACFs are incorporating nanoscale conductive fillers to improve current density while maintaining flexibility. Low-temperature solders are being optimized with microalloying to enhance reliability without compromising processability. Each material system offers distinct advantages, and the optimal choice depends on specific application requirements including pitch density, power handling, thermal environment, and mechanical constraints. As packaging architectures become more complex, the interplay between these interconnect solutions will shape the future of semiconductor integration.