Metal/carbon nanotube composite materials and their application in advanced electronic packaging

Abstract

Miniaturization and lead-free solders have been two dominant trends in advanced electronic packaging for a number of years. However, along with their implementation, a large number of new reliability issues have arisen. Failures can occur in any part of the electronic devices, but typically occur in a small portion of the components. Failures can be caused by electrical, chemical, thermal and mechanical effects - and result in irreversible hardware damage, even though that damage may be microscopic. With the continuous downsizing of feature sizes, more failure mechanisms will occur in solders or in under-bump area. When applying nano-scale materials to electronic packaging, nano-composite solders have been considered as a solution to the above-mentioned problems. Solders with intentionally incorporated secondary phases are called composite solders. Recently, with the development of miniaturization and other novel packaging methods, the limitations of conventional lead-free solders are becoming more obvious. Due to the high soldering and service temperature, the microstructure is less stable and thermal stress will accumulate. According to previous research, composite solder materials show better mechanical properties, better microstructure stability and creep resistance, since the secondary phase can inhibit dislocation and cracks that take place at grain boundaries. In the present study, carbon nanotubes (CNTs) have been chosen as the reinforcing secondary phase in the fabrication of nano-composite solders. Because CNTs have unique and attractive electrical, thermal and mechanical properties, and CNTs are considered as one of the perfect materials for the fabrication of composites. First of all, effective and reproducible pre-treatment and dispersion methods for CNTs have been investigated. It is well-known that the strong dispersion of CNTs is a key factor for their successful incorporation, since CNTs has strong tendency to agglomerate. Nitric acid (HNO3) treatment and hydrochloric acid (HCl) treatment were employed in this experiment to shorten longer CNTs, since short CNT fibers have fewer tendencies to agglomerate. Surfactant and ultrasonic agitation were also introduced to disperse these nano-scale CNT fibers. Both surfactant and ultrasonic agitation can accelerate the breakup of CNT clusters, which was necessary to maintain a stable and uniform distribution of CNTs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the dispersion after adopting the aforementioned treatments, as well as the surface morphology of the electrolessly deposited layers. It was verified that the CNT fiber dispersion can be significantly improved by the treatment process proposed in this study. Secondly, after stable dispersed CNTs were obtained, the CNTs were incorporated into lead-free solder paste. The CNTs with various weight percentages of 0.01 wt.%, 0.05 wt.%, 0.1 wt.% were incorporated into a tin-gold-copper (Sn-3.5Ag-0.5Cu) (SAC) solder matrix by mechanical blending, followed by a reflow process. The microstructure with interfacial intermetallic (IMC) growth behavior was investigated after aging at 100 °C, with a current density of 1.2 × 104 A/cm2 for 336 hours. CNTs were found to be homogeneously dispersed into the solder matrix by EDX analysis. Experimental results also revealed that micro-porosities existed in some areas of doped solders, which retarded the formation of silver-tin (Ag3Sn) IMCs. The IMC growth of CNT reinforced solders was less significantly than that of the plain solder, because CNTs affected the reaction mechanisms of both Cu5Sn6 and Cu3Sn at the anode side - while alleviating IMC growth and solder dissolution into the Cu substrate at the cathode side. Thirdly, CNTs were incorporated into nickel-phosphorus (Ni-P) under-bump metallization (UBM) using electroless plating.Electroless Ni-P UBM has the advantages of an even surface, low cost and simplicity in deposition, but their mechanical strength, corrosion resistance and stability still face challenges at high soldering temperatures. Incorporating CNTs into electroless Ni-P UBM can generate Ni-P-CNT composite UBM with high mechanical strength and stability. Ni-P-CNT composite deposits and Ni-P deposits were both fabricated using an electroless plating process. In this study, an SEM was used to observe the morphology, and it was found that the CNTs were uniformly distributed in Ni-P-CNT deposits in both SEM and AFM observation. It was verified that the surface of the Ni-P-CNT composite deposits was quite smooth and continuous; that CNTs are evenly embedded in the matrix, which is advantageous for conductivity, mechanical strength and corrosion resistance. Moreover, the deposition mechanism for CNTs with Ni was analyzed and confirmed by TEM. Factors that affect composite plating quality were also discussed, and an optimum plating condition was suggested by this study. In the fourth part of the thesis, the mechanical properties and reliability were studied. A slight decrease was measured in the melting temperature of CNT reinforced solders and compared with plain solders. The CNT doped solders demonstrated improved shear strength and hardness. Shear strength tests were also conducted to judge the influence of CNT additions on the mechanical characteristics of interconnections. The results showed that the joints with CNT additions exhibited higher shear strength, at different reflow cycles. CNT doped solders have also demonstrated high electromigration resistance - and an optimum percentage of CNT doping for better electromigration reliability has been found. CNT doping is an effective way to control the solder joint reliability by enhancing the IMC growth, phase- delamination and shear stress.

Date of Award	16 Feb 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yan Cheong CHAN (Supervisor)