Bridging the Thermal Gap: Ag-MWCNT Reinforced Silver Paste as an Interfacial Interconnect for High-Power Electronic Packaging

Modern electronics are getting smaller and more powerful, packing immense energy into tiny spaces. This progress brings a stubborn problem: heat. When a computer chip or a high-power device works, it generates intense warmth that must be moved away quickly, or the device will fail. To solve this, engineers use special materials to bridge the gap between the hot chip and a cooler metal piece designed to draw the heat away. However, even when these two surfaces are pressed together, they are never perfectly smooth at a microscopic level. Tiny gaps and rough spots trap air, which acts like a blanket, blocking the flow of heat. This creates a bottleneck, preventing the device from cooling down efficiently. For years, scientists have looked for a way to fill these microscopic gaps with a material that conducts heat better than the metals themselves, hoping to unlock the full potential of next-generation electronics.

Researchers at Shanghai University have developed a new approach to this problem by creating a specialized paste that acts as a thermal bridge. They started with a base of nano-silver paste, a material made of tiny silver particles that can be fused together at low temperatures to form a solid, conductive layer. While silver is excellent at moving heat, the paste alone still struggles with the microscopic imperfections between the heat source and the cooling surface. To fix this, the team introduced a second ingredient: multi-walled carbon nanotubes. These are incredibly thin, tube-like structures made of carbon atoms, known for their ability to transport heat very efficiently. However, simply mixing these tubes into the paste causes them to clump together, which actually blocks heat flow rather than helping it.

To solve the clumping issue, the researchers first treated the carbon nanotubes with a mixture of strong acids and ultrasonic vibrations. This process cleaned the tubes and added tiny chemical groups to their surfaces, making them more receptive to bonding. They then coated these treated tubes with silver nanoparticles, essentially dressing the carbon tubes in a silver jacket. This decoration served two purposes: it kept the tubes from sticking to each other, and it created a strong chemical bond between the carbon tubes and the silver paste when they were heated together. The team tested different amounts of these silver-coated tubes, mixing them into the paste in varying quantities to find the perfect balance.

The results showed that adding these reinforced tubes dramatically improved the material's ability to move heat. When the paste contained five percent of these silver-coated tubes by weight, its ability to conduct heat reached a peak value of 10.16 watts per meter-kelvin. This is a massive improvement, representing a 175 percent increase compared to the plain silver paste without any tubes. The silver coating on the tubes acted as a secure handshake between the carbon structure and the silver matrix, allowing heat to jump easily from one material to the other without getting stuck at the boundary.

However, the story of the best performance depends on where the material is used. When the researchers applied this paste to connect a graphene film—a thin, two-dimensional sheet of carbon known for its high heat conductivity—to a copper substrate, the ideal amount of reinforcement changed. In this specific setup, adding just one percent of the silver-coated tubes reduced the total resistance to heat flow by 59 percent, bringing it down to a very low level. This suggests that while a higher concentration of tubes makes the paste itself a better conductor, a smaller amount is better at smoothing out the interface between two different materials. The tubes in this lower concentration acted like tiny bridges, spanning the microscopic gaps between the silver particles and the metal surfaces to create a continuous path for heat to travel.

The study also examined how strong these new connections were. The paste with one percent of the silver-coated tubes was not only better at moving heat but also stronger mechanically. It could withstand a shearing force of 23.36 megapascals before breaking, which is significantly higher than the plain silver paste. This indicates that the tubes helped hold the structure together more tightly. However, adding too many tubes had the opposite effect. When the amount reached ten percent, the tubes began to clump together again, creating weak spots and cracks in the material. This caused both the heat conductivity and the mechanical strength to drop sharply, proving that there is a precise limit to how much reinforcement can be added before the material becomes flawed.

This work offers a clear path forward for managing heat in advanced electronic packaging. By carefully tuning the surface chemistry of carbon nanotubes and controlling how much of them are added to a silver paste, engineers can create materials that fill the microscopic gaps between components more effectively than ever before. The research demonstrates that the key to unlocking high-performance thermal management lies not just in the materials themselves, but in how they are prepared and combined to ensure they work together seamlessly.