Study of nanomaterial reinforced under bump metallization (UBM) and lead-free solder matrix for advanced electronic packaging
Student thesis: Doctoral Thesis
Related Research Unit(s)
Semiconductor technology has shown a continuing progress in the past decades. So the higher performance of electronic devices and dimensional scaling down of electronic components are in the mainstream of the electronics industry. To face the challenges of miniaturization, electronic packaging technology has also made remarkable progress. Various packaging processes and materials have been promoted in order to keep up with Moore’s law. However, the interfacial reaction is always a critical point for the ultimate reliability. Miniaturization sharply increases current density and thermal aggregation, thereby putting great pressure on the interfacial reactions in solder joints. Finally it becomes a significant threat for reliability. In another aspect, lead-containing materials are no longer suitable for use in electronic products. These hazardous materials are highly likely to cause environment pollution and have a negative effect on human health as well. In 2003, the European Union has issued Restriction of Hazardous Substances Directive (RoHS) and The Waste Electrical and Electronic Equipment Directive (WEEE) to forbid the use of Pb contained materials in electronic products. Research on lead-free solder material has been conducted extensively including Sn-Ag, Sn-Ag-Cu, Sn-Zn and Sn-Zn-Bi solder alloys. However, lead-free alloys usually have a higher melting points than lead-containing solders. The increased melting point will increase the reflow process temperature and bring challenges for thermal management. It also exacerbates of the interfacial reaction and further deteriorates the reliability of interconnection dramatically. Nowadays, nanomaterials are widely used because of its extraordinary performance. To make good use of high quality nanomaterials, two approaches have been adopted to reinforce the solder interconnections studied in this thesis including under bump metallization (UBM) modification and solder matrix modification. That is to incorporate nanoparticles to UBM and dope graphene into the solder matrix. In the first approach, TiO2 nanoparticles were incorporated into nickel-phosphorus (Ni-P) metallization by electroless deposition as a novel UBM layer to improve the solder joint performance. The interfacial reaction between the electrolessly deposited Ni-P-TiO2 layer and lead-free Sn-3.5Ag solder alloy was systematically analyzed. The prime Ni-P UBM was used for comparison in order to highlight the reinforcing effect of TiO2 nanoparticles. Both solder/Ni-P and solder/Ni-P-TiO2 joints were aged at temperatures in the range from 150°C to 190°C for different aging periods in order to study the interfacial reactions. It was found the growth of Ni3Sn4 IMC layer and void formation at the reaction interface were successfully suppressed with the help of the TiO2 nanoparticles. For the kinetic analysis, the activation energies for the growth of Ni3Sn4 on Ni-P and Ni-P-TiO2 layer were calculated to be 50.9 kJ/mol and 55.7 kJ/mol, respectively. The extensive growth of the Ni3P and Ni-Sn-P phases as well as the consumption rate of the amorphous UBM was controlled in joints with TiO2 nanoparticles. The Ni-P-TiO2 UBM blocked the Ni and Cu diffusion at the interface, thereby avoiding IMC spalling. A detailed reaction-induced diffusional mechanism was proposed to highlight the microstructural evolution. The solder/Ni-P-TiO2 solder joint consistently demonstrated higher shear strength than the solder/Ni-P joint as a function of aging time. Moreover, after the shear strength test, fracture mainly occurred in the solder matrix of the solder/Ni-P-TiO2 joint, the morphology showed a ductile fracture pattern with a large distribution of dimples on the rough surface. Then the ZrO2 nanoparticles have been used to develop an electroless Ni-P-ZrO2 (17.5 at.% of P) composite coating as the under bump metallization (UBM) for lead-free solder interconnect. The Sn-3.5Ag/Ni-P-ZrO2 solder joints were prepared and aged at various conditions to study the interfacial reaction. The Intermetallic compounds (IMCs) growth without serious spalling in the solder/Ni-P-ZrO2 joint was slowed down due to the barrier property of incorporation of ZrO2 nanoparticles which blocked the diffusion of Ni and Cu atoms. A top-view of the IMC grains shows a finer structure for Ni-P-ZrO2 UBM than that for plain joints. Based on the IMC growth, the activation energy of the solder/Ni-P-ZrO2 joint estimated to be higher than that of plain solder joint. The top-view IMC demonstrated a much finer grain size compared with that in solder/Ni-P joint. A reactive diffusion-induced compounds formation mechanism was proposed to address the microstructural evolution in detail. Moreover, the solder/Ni-P-ZrO2 joint demonstrated greater shear strength than that of solder/Ni-P joint for different aging durations. In the second approach, various weight percentages (0, 0.05 and 0.1 wt.%) of graphene nanosheets doped lead-free Sn-8Zn-3Bi solder alloys were investigated in order to analyze the electromigration induced microstructural development. The effect of electromigration on the solder joint was systematically studied by using a newly developed wire-type testing configuration. The samples were stressed under a current density of 5×10³ A/cm2 at 100°C for different aging periods in order to study the electromigration induced reliability issues. The majority of the added graphene nanosheets were found to be uniformly distributed in the β-Sn matrix by Raman spectroscopy. After the graphene addition, needle-like Zn-rich phases with a finer microstructure were discovered in the solder matrix. At the interface the growth rate of the IMC layers of the graphene doped solder was slower in comparison with that of IMC layers in plain solder. With 0.1 wt.% graphene addition, the measured IMC growth rate decreased from 30.9×10-14 to 24.9×10-14 cm2/s. The melting temperature of the doped solder measured using a differential scanning calorimeter (DSC) showed little difference from that of the plain solder. The Vickers hardness, up to 29.9 Hv with 0.1 wt.% graphene addition, is 9.1% higher than that of plain solder. The graphene doped solder consistently demonstrated higher ball shear strength as a function of aging time. The ball shear strength value increased by 10.2%±0.8% in comparison with that of plain solders during the whole aging period. The improvement was due to the dispersion-strengthening mechanism, refined microstructure and excellent intrinsic mechanical properties of graphene. Moreover, fracture occurred at the IMC interfaces of the doped samples, showing a ductile fracture pattern with a large distribution of dimples on the rough surface.
- Lead-free electronics manufacturing processes, Electronic packaging, Solder and soldering, Nanostructured materials