Abstract
Present day electronic products are developing rapidly towards multifunctionality, high-performance and high levels of integration and miniaturization, which requires the development of advanced electronic packaging technologies. Solders are applied as essential joining materials in the packaging of electronic products – and provide mechanical and electrical interconnection between various components. Recently, there has been a demand for low temperature packaging processes for some special electronic products, thereby enhancing reliability. With lower packaging temperatures, the mismatch of the coefficients of thermal expansion (CTE) and the thermal shock on sensitive devices are both reduced. Low melting temperature solders, which require a low heating temperature to form interconnections, are therefore promising for application in advanced low temperature electronic packaging. In the thesis, microstructure and reliability of novel low melting temperature In–Sn and Sn–Bi–Ag solders has been investigated. On the other hand, in order to improve the reliability of low melting temperature solder interconnects, nanoparticles reinforcement – an effective way to enhance alloy materials – has been applied to reinforce low melting temperature solders.The electromigration of In–48Sn solder interconnects with Cu pads and Au/Ni/Cu pads, respectively, has been investigated. The evolution of microstructure in Cu/In– 48Sn/Cu solder bump interconnects at a current density of 0.7 × 104 A/cm2 and ambient temperature of 55 °C has been investigated. During electromigration, tin (Sn) atoms migrated from cathode to anode, while indium (In) atoms migrated from anode to cathode. As a result, the segregation of the Sn-rich phase and the In-rich phase occurred. A Sn-rich layer and an In-rich layer were formed at the anode and the cathode, respectively. The product of the diffusivity and the effective charge number of Sn was determined to be approximately 3.13 × 10−10 cm2/s. The In–48Sn/Cu IMC showed a two layer structure of Cu6(Sn,In)5, adjacent to the Cu, and Cu(In,Sn)2, adjacent to the solder. During electromigration, the Cu(In,Sn)2 at the cathode interface thickened significantly, with a spalling characteristic, due to the accumulation of In-rich layer and the migration of Cu atoms – while the Cu(In,Sn)2 at the anode interface reduced obviously, due to the accumulation of Sn-rich layer. The mechanism of electromigration-induced failure in Cu/In–48Sn/Cu interconnects was the cathode Cu dissolution-induced solder melt, which led to the rapid consumption of Cu in the cathode pad during liquid-state electromigration and this finally led to the failure. On the other hand, the microstructural evolution in In–48Sn solder interconnects with Au/Ni/Cu pads have been investigated as well. The phase segregation showed similar characteristics to those of Cu/In–48Sn/Cu interconnects. The products of the diffusivity and the effective charge number of Sn in In–48Sn solder was determined to be 1.17 × 10−10 cm2/s at 25 °C and 3.39 × 10−10 cm2/s at 55 °C, respectively. However, it is notable that the interfacial IMC layers at both the anode and cathode interfaces grew very slowly, by applying the Au/Ni/Cu pads, because of the slow reaction rate between In48Sn solder and Ni metallization. Therefore even after long EM time, there was no obvious consumption of cathode pad, indicating an enhanced EM reliability.
Different from the effect of the category and concentration of nanoparticles on nanocomposite solder as reported by other researchers, in this thesis, the effect of Ag nanoparticle size on the microstructure of solder matrix, microhardness, growth of solder/Cu interfacial intermetallic compounds (IMC) and shear strength were investigated. Ag nanoparticles with sizes of 31 nm, 76 nm and 133 nm were added to Sn58Bi to prepare composite solders. The experimental results showed that Ag played
a positive role in improving the microstructure and mechanical properties of Sn58Bi solder and the extent of the improvement differed with the size of the doped Ag particles. The best improvement resulted from the addition of 76 nm Ag particles: the microstructure refined by 49.1% and the microhardness was enhanced by 12.2% for the as-prepared solders; the Cu–Sn IMC growth exponent reduced from 0.394 to 0.339, the IMC thickness reduced by 39.7% and the shear strength reinforced by 18.9% after liquid reaction at 220 °C for 180 min. However, the improvements resulted from the addition of larger (133 nm) or smaller (31 nm) Ag particles were not as obvious. The mechanism of particle size effect on nano-composite solder properties was analyzed and an optimal particle size is proposed.
Sn57.6Bi0.4Ag solder has been reinforced successfully through the addition of tungsten (W) nanoparticles at a concentration of 0.5 wt%. With the addition of W nanoparticles, the solder matrix lamellar interphase spacing was reduced by 31.0%. Due to the dispersion of W nanoparticles and the consequently refined microstructure, the mechanical properties of the solder alloy were enhanced, as indicated by a 6.2% improvement in the microhardness. During the reflow of solder on Au/Ni/Cu pads, the entire Au layer dissolved into the molten solder rapidly and a large number of (Au,Ni)(Sn,Bi)4 particles were formed. The fracture path of the as-reflowed joint was within the solder region, showing ductile characteristic, and the shear strength was reinforced by 8.2%, due to the enhanced mechanical properties of the solder. During the subsequent aging process, the Au migrated back towards the interface and a thick layer of interfacial (Au,Ni)(Sn,Bi)4 IMC was formed, leading to the shift of the fracture path to the interfacial IMC region, the transformation to brittle fracture and the deterioration of the strength of the joint, due to Au embrittlement. By adding W nanoparticles, the migration of Au was mitigated and the thickness of the (Au,Ni)(Sn,Bi)4 layer was reduced significantly, which reduced the Au embrittlement-induced deterioration of the strength of the joint. During electromigration, the segregation of the Bi-rich and Sn-rich phases and the accumulation of the (Au,Ni)(Sn,Bi)4 layer at cathode interface were mitigated by the addition of W nanoparticles, which improved the electromigration resistance.
| Date of Award | 5 Sept 2016 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Yan Cheong CHAN (Supervisor) |