Characterization of nano-scale materials for interconnect and thermal dissipation application in electronics packaging

Doktorsavhandling, 2014

This thesis focuses on studies of nano-scale materials in electronic packaging applications with respect to following aspects: surface analysis of nano-scale oxide of lead-free solder particles, and thermal performance and mechanical property studies of nano-scale fiber and metal composite-based thermal interface materials. The composition and thickness of the solder oxide have a direct impact on the quality of interconnects and the reliability of a packaged system. The characterization of the nano-scale oxide of lead-free solder particles is investigated by transmission electron microscopy and scanning transmission electron microscopy. The solder powders are exposed to air at 150 oC for 0, 120 and 240 h. The oxide thickness is 6 nm and 50 nm measured by STEM for 0 h and 120 h samples, respectively. The increase in oxide thickness of solder particles is proportional to the rooting of the oxidation time. The intersection analysis method for analyzing Auger electron spectroscopy depth profiles is also presented which could be expand to analyze oxide of other alloy, i.e. Cu, Ag or stainless steel. In the next part of this thesis, a new composite design consisting of electrospun polyimide fiber networks and infiltrated metal matrix is presented. Three composites are fabricated including polyimide fiber-InSnBi, polyimide fiber-indium and polyimide fiber-SnAgCu composites. The microstructure of the composite is investigated by scanning electron microscopy, energy dispersive X-ray detector and X-ray diffraction, showing a good bonding between the fibers and the metal matrix. These composites demonstrate high thermal conductivity, low thermal contact resistance and reliable thermomechanical performance during thermal cycling. The polyimide fiber-indium composites are sandwiched between chips and heat spreaders with different packaged sizes to detect the junction temperature and junction-to-case thermal resistance. The shear strength of the polyimide fiber-indium composite between Sn surfaces can reach 4 MPa which is larger than that with Au and Cu surfaces. Both composites present good reliability during the humidity-heat aging tests. The polyimide fiber-indium composite’s ultimate tensile strength at 20 °C is five times higher than that of the pure indium, and the tensile strength of the composite exceeds the summation of those from its individual components. With the increase in temperature, the ultimate tensile strength declines but still precedes pure indium and the elongation at fracture increases. Contrary to most metallic materials, the ultimate tensile strength of the composite is inversely proportional to the logarithmic strain rate in a certain range. Finally, a new strengthening mechanism is presented based on mutually reinforcing structures formed by the indium and surrounding fibers, underlining the effect of compressing at the fiber-indium interfaces by dislocation pileups and slip pinning. The creep threshold of the composite corresponds to the fracture strength of the polyimide fiber, and the step-like sudden increases of the composite’s creep strain are due to the breakage of fibers. The fiber-indium interfaces are also beneficial to the composite’s creep resistance. In the final part of the thesis, two novel thermal interface materials are developed and characterized including boron nitride fiber-indium composite and carbon fiber-SnAgCu composite. Thermally conductive boron nitride fiber or carbon fiber is prepared via electrospinning and heat treatment. Afterward, the boron nitride/carbon fibers are sputtered with Ti/Au coatings and infiltrated with metal matrix. Good in-plane and through-plane thermal conductivity of the thermally conductive fiber-metal matrix composite are obtained using a laser flash apparatus.