Minoru Ueshima

Thermal and mechanical stability of soldering QFP with Sn-Bi-Ag lead-free alloy

Thermal stability of the circuit boards with a quad flat package (QFP) soldered with Sn-58wt%Bi-(0, 0.5 and 1.0) wt% Ag and their microstructural features were evaluated. The addition of 1.0 wt% Ag causes the formation of large primary Ag/sub 3/Sn precipitates in the solder while no primary Ag/sub 3/Sn is found in Sn-57Bi-0.5Ag. Thermo-Calc calculation indicates that the lowest limit content for the formation of primary Ag/sub 3/Sn is about 0.8 wt%. Heat-exposure below 100/spl deg/C has no serious degradation on the joint structure for all solders. Heat-exposure at 125/spl deg/C caused serious degradation in joint strength for all alloys. The contamination of Pb from Sn-Pb surface plating on the components reduces the interface tolerance by forming ternary Sn-Pb-Bi phase melting at low temperature. Thermal fatigue between -20 and 80/spl deg/C does not have any significant influence on joint structure.

Refinement of the Microstructure of Sn-Ag-Bi-In Solder, by Addition of SiC Nanoparticles, to Reduce Electromigration Damage Under High Electric Current

The trends of miniaturization, multi-functionality, and high performance in advanced electronic devices require higher densities of I/O gates and reduced area of soldering of interconnections. This increases the electric current density flowing through the interconnections, increasing the risk of interconnection failure caused by electromigration (EM). Accelerated directional atomic diffusion in solder materials under high current induces substantial growth of intermetallic compounds (IMCs) at the anode, and also void and crack formation at the cathode. In the work discussed in this paper, addition of SiC nanoparticles to Sn-Ag-Bi-In (SABI) lead-free solder refined its microstructure and improved its EM reliability under high current stress. Electron backscattering diffraction analysis revealed that the added SiC nanoparticles refined solder grain size after typical reflow. Under current stress, SABI joints with added nano-SiC had lifetimes almost twice as long as those without. Comparison of results from high-temperature aging revealed direct current affected evolution of the microstructure. Observations of IMC growth indicated that diffusion of Cu in the SiC composite solder may not have been reduced. During current flow, however, only narrow voids were formed in solder containing SiC, thus preventing the current crowding caused by bulky voids in the solder without SiC.

Sn–Ag–Cu Soldering Reliability as Influenced by Process Atmosphere

To develop an optimal soldering process of Sn-Ag-Cu soldered joints, the influences of atmosphere and cooling speed during reflow treatment on soldering reliability have been examined through the use of Sn plated chip components and of Pd plated small outline packages (SOPs) on a printed circuit board (PCB) soldered with Sn-3wt.%Ag-0.5wt.%Cu, Sn-3.8wt.%Ag-0.75wt.%Cu and Sn-4wt.%Ag-0.9wt.%Cu solder pastes under air or N2 atmospheres. In the case of chip component joints, the solder compositions, cooling speed and atmospheres during reflow treatment slightly affect the dendritic microstructure. Those parameters rarely affect the wetting behavior and mechanical properties. In the case of SOP joints, however, the atmospheres during reflow treatment and fluxes strongly affect the appearances of fillet surfaces structure. Despite the types of solder fluxes, N2 process atmosphere obviously improved wettability of solder on lead-frame of the SOP.

Effects of additional Ni and Co on microstructural evolution in Sn-Ag-Bi-In solder under current stressing

With continuously shrinking process rules of electronic devices, the decreasing dimension of interconnections results in increasing electrical current density, and recalls the risk of massive electromigration (EM) of metal elements in solder joints. This paper reports that minor additions of Ni and/or Co to Sn-Ag-Bi-In lead-free solder to improve the EM behavior and microstructural evolutions in the solder material subjected to high electric current density of 10 kA/cm2 over 200 hours. With additional Co elements, EM resistance of solder joint was improved as twice before with microstructural evolution by it.

Die-bonding performance and mechanism based on the sintering of micro Ag paste for high power devices

Commercially available micron-sized Ag flake particles and submicron-sized Ag particles were used to make Ag paste. The Ag paste was used to connect Si dies to Ni-Ag plated Cu substrates to achieve die-bonding joint for high temperature application. The contact between die and Ag paste was affected by the loading pressure, and was realized with only 0.2 MPa pressure. And the shear strength depended on the bonding pressure and increased over 20 MPa with only 0.2 MPa pressure. The high shear strength achieved at low bonding pressure contributed to the tight contact between die and paste, and optimized bonding pressure.

Sn-Ag-Cu Soldering Reliability Influenced by Process Atmosphere

To develop an optimal surface mount reflow soldering process with Sn-Ag-Cu, the influences of atmosphere and cooling speed on soldering reliability have been examined by using Sn plated chip components and of Pd plated small outline packages (SOPs) on a printed circuit board (PCB). Typical three Sn-Ag-Cu alloy pastes, i.e., Sn-3.0wt%Ag-0.5wt%Cu, Sn-3.8wt%Ag-0.75wt%Cu, and Sn-4.0wt%Ag-0.9wt%Cu, were used for reflow soldering in air or N2 atmospheres. In the case of chip component joints, the solder compositions, cooling speed, and atmospheres during reflow treatment slightly affect the dendritic microstructure of the solder fillets. In contrast, these parameters rarely affect the solder wettability both on boards/components and shear strengths of the solder joints. In the case of the SOP joints, however, the atmospheres in reflow treatment and the fluxes strongly affect the appearances of solder fillet surfaces structure. Despite the types of solder fluxes, N2 process atmosphere obviously improved wettability of the solders on the lead-frames of the SOP. Moreover, the scatter in shear strengths becomes smaller and the wetting of solders on the lead-frames becomes stabler in N2 atmosphere than in air atmosphere.

Effect of electromigration on mechanical shock behavior in solder joints of surface mounted chip components

The mechanical shock behavior of Sn–Ag–Bi–In (SABI) solder has been investigated by micro-Charpy impact test after subjecting the solder to high electric current stress. A trace amount of Co added (0.1 wt %) is used as a refiner in the SABI solder, and the joint soldered by conventional reflow exhibits a refined grain microstructure. The SABI+Co joint exhibits higher toughness, i.e., 70% more energy is absorbed during impact than a typical SABI joint. After 80 h of 20 A electric current applied at 160 °C, electromigration (EM) is observed in both the SABI and SABI+Co joints. However, the SABI+Co joint exhibits a constant mechanical strength, while the typical SABI joint exhibits a strength decreased by 16% at the Charpy absorption energy. The mechanical strength improved by Co addition implies that microstructure refinement is useful for enhancing the EM resistance of a solder joint structure for surface mount packaging.

SiC die-attachment with minor elements added pure Zn under formic acid reflow

Pure Zn is one of the best candidates, as it has a high melting, good electrical/thermal conductivity, and excellent thermal shock resistance. To improve the properties of pure Zn, the effect of the addition of 0.1 mass% Cr and Ti was examined the formic acid reflow soldering. The minor elements are expected to form a surface protective oxide layer on Zn and the formic acid reflow soldering can replace the hydrogen reduction treatment. The addition of 0.1 mass% Cr or with 0.1 mass% Ti to pure Zn improves ductility and oxidation resistance. Among the solder alloys, the addition of Cr has the most effective improvement of ductility and oxidation resistance. The Pure Zn and Zn-X solders exhibit the excellent heat-cycle resistance for the DBC die-attach structure between −50 °C and 300 °C.

Material design and package-level reliability of a novel low-temperature solder based on intermetallic-compound phases with superior high-homologous temperature properties

A novel low-temperature solder which consists entirely of two phases of intermetallic compounds is developed as a reliable low-temperature interconnect. The new solder can be reflowed below 125C and yet exhibits excellent mechanical properties at homologous temperatures exceeding 0.9. Specifically, the new solder exhibits creep resistance an order of magnitude greater than conventional low-temperature solders, and the strength retention ability exceeding that of Sn-4%Ag-0.5%Cu (SAC405) at the same homologous temperatures. The mixture of two intermetallic compounds also exhibit extensive ductility comparable to conventional solders at room-temperature. Dramatic enhancement of wettability is achieved with addition of active metals via reactive wetting mechanisms. The new alloy exhibits excellent thermo-mechanical fatigue resistance with <5% failure after 1000 cycles and shock performance superior to SAC405.