Abstract
The requirement for environmental-friendly green microelectronic devices has led to many investigations into printed circuit board (PCB) materials and surface-finishes. These are primarily based on selecting materials that withstand at higher temperatures during the environmental-friendly Sn-based soldering processes. It is also crucial to understand the effects of the different surface-finished PCBs and soldering systems.Several different types e.g. gold/nickel (Au/Ni), silver/nickel (Ag/Ni), and immersion-Ag of surface-finished copper (Cu) substrates were prepared using an electrolytic deposition and immersion processes. The surface morphology and plated layer thicknesses were observed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). From the SEM images, it was confirmed that the plated layers, i.e. Au/Ni and Ag/Ni, were well and uniformly deposited on the Cu substrates. Further, the plated layer thicknesses increased with increasing plating temperature. From the AFM observations, it was confirmed that the plated layer had a very smooth surface without any defects such as cracks, delamination etc. confirming the successful application of the specially developed electrolytic process. To evaluate the interfacial microstructure, low melting point tin-bismuth-silver (Sn-Bi-Ag) solder and different surface-finishes i.e., immersion Ag-plated and Ag/Ni-plated Cu substrates, were utilized. From SEM observation, it was observed that, at the interface of immersion Ag-plated Cu-substrate/Sn-Bi-Ag solders, scallop-shaped Cu6Sn5 and thin Cu3Sn intermetallic compound (IMC) layers were found. However, in Ag/Ni-plated Cu-substrate/Sn-Bi-Ag solder system, a (Cu, Ni)-Sn IMC layer was found to adhere to the interface. In order to identify the effect on the mechanical properties and behavior of tin-zinc-bismuth (Sn-8Zn-3Bi) based solders produced by adding nickel (Ni) nano-particles, the interfacial microstructure between plain and composite solders with newly developed immersion Ag-plated Cu substrates has been investigated with reaction time and temperatures. For plain Sn-8Zn-3Bi solder system, a Cu-Zn-Ag IMC layer was found to adhere to the surface of the immersion Ag-plated Cu substrate. However, after adding Ni nano-particles into the Sn-8Zn-3Bi solder, a layer type Cu-Zn-Ag (at the bottom) and (Cu, Ni)-Zn (at the top) IMCs were observed. These IMC layer thicknesses increased substantially with the reaction time and temperature. Moreover, in the solder ball region, needle-shaped α-Zn rich phase and spherical-shaped Bi-particles appeared to be homogeneously distributed throughout a beta-tin (β-Sn) matrix. However, after adding Ni nano-particles, α-Zn rich phase appeared with a fine microstructure due to the heterogeneous nucleation of Ni nano-particles. The calculated activation energy for the Cu-Zn-Ag IMC layer for the plain Sn-8Zn-3Bi solder/immersion Ag-plated Cu system was 29.95 kJ/mol while the activation energy for the total [Cu-Zn-Ag + (Cu, Ni)-Zn] IMC layers formed by composite solder/immersion Ag-plated Cu system was 27.95 kJ/mol.
Sn-3.0Ag-0.5Cu-0.5Ni (wt%) composite solder has been made by adding Ni nano-particles. A Cu6Sn5 IMC layer that adhered to the substrate surface was appeared at the interfaces of the plain Sn-Ag-Cu solder system during the early reflow cycles. Further, a very thin Cu3Sn IMC layer was found between the Cu6Sn5 IMC layer and the substrates after a lengthy reflow cycle and solid-state aging process. However, after adding Ni nano-particles, a scallop-shaped (Cu, Ni)-Sn IMC layer was found at both of the substrate surfaces without any Cu3Sn IMC layer formation. In the solder-ball region, needle-shaped Ag3Sn and sphere-shaped Cu6Sn5 IMC particles were clearly observed in the β-Sn matrix of the plain Sn-Ag-Cu solder system. Additional fine (Cu, Ni)-Sn IMC particles were homogeneously distributed in the β-Sn matrix of in the composite solder joints. The composite solder joints showed higher hardness values than the plain solder system for any specific number of reflow cycles on both substrates due to their well-controlled, fine network-type microstructures and the fine (Cu, Ni)-Sn IMC particles which acted as second-phase strengthening mechanisms. The composite solder system formed by adding Ni nano-particles improved about 18% as compare to the plain Sn-Ag-Cu solder system.
Nano-sized, non-reacting, non-coarsening CeO2 particles with a density close to that of solder alloy were incorporated into Sn-Ag-Cu solder paste. After the initial reaction, an island-shaped Cu6Sn5 IMC layer was clearly found at the interfaces of the Sn-Ag-Cu based solders/Ag-plated Cu substrate system. However, after a prolonged reaction, firmly adhering thin Cu3Sn IMC layer was observed. Rod-like Ag3Sn IMC particles were also detected at the interfaces. On the other hand, in the Sn-Ag-Cu based solder-Ag/Ni-plated Cu pad system, a (Cu, Ni)-Sn IMC layer was found. Rod-like Ag3Sn and needle-shaped Cu6Sn5 IMC particles were also found on surface of the (Cu, Ni)-Sn IMC layer. Further, these IMC layers thicknesses were increased with the temperature and reaction time. In the solder matrix of both systems, a finer structure of Ag3Sn, Cu6Sn5 IMC particles formed in the β-Sn matrix. However, the growth of the IMC layers of composite solder doped was much lower due to an accumulation of surface-active CeO2 nano-particles at the grain boundaries or in the IMC layers. In addition, the composite solder joint had a higher hardness value than the plain solder joints due to a well-controlled fine microstructure and uniformly distributed CeO2 nano-particles. After 5 minutes of reaction on immersion Ag-plated Cu substrates at 250°C, the micro-hardness values of the plain and the composite solder joints were approximately 16.6 HV and 18.6 HV, respectively, while their micro-hardness values on Ag/Ni-plated Cu substrates after 5 minutes of reaction at 250 °C were approximately 15.9 HV and 17.4 HV, respectively.
The reliability of lead-free composite solders reinforced with nano-size different metal and ceramic particles on various surface-finished substrates has been evaluated under high temperature aging and high temperature/humidity (85 °C/85 %) testing. From these reliability studies, it was revealed that composite solders improved the oxidation resistance as well as suppressing the formation of IMC layers. Additionally, composite solders doped with nano-particles enhanced the mechanical properties of the solder joints, due to refining of the microstructure and well controlled formation of the IMC layer. The newly developed environmental-friendly composite solder alloys with higher mechanical reliability and fine microstructure can increase the life-span of next-generation green electronic devices.
| Date of Award | 24 Nov 2015 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Yan Cheong CHAN (Supervisor) |