Materials Science Forum Vols. 580-582

Abstract: The lead-free casting solders Sn-3.5Ag-xCo (x = 0, 0.1, 0.5 and 1.0 mass%, respectively) were subjected to isothermal aging at 150°C for 0, 1008 and 2016 h, respectively. The nanoindentation methodology was employed herein to assess the mechanical properties. In particular, the strain rate sensitivity index value was derived from the creep deformation at the dwell time of the target constant load using Mayo-Nix theory. Basically, there is no remarkable tendency of the variation of Young’s modulus after aging, while to some extent, the hardness of the alloys drops. The strain rate sensitivity index value continues to decrease with the prolonged aging time. The solder grain growth and the coarsening of the intermetallics namely, Ag3Sn and CoSn2, are responsible for the mechanical evolution of the alloys. The 1.0% (mass%) Co additive improved the hardness of the solder alloy, and caused the decrease of the strain rate sensitivity value.

Abstract: A new hybrid soldering and conductive adhesive joining technology using a fusible lowmelting- point alloy (LMPA) have been developed. A numerical method for numerical analysis of fusible particles behavior is proposed to investigate coalescence characteristics of fusible particles in solderable isotropic conductive adhesives (ICAs). For finding out suitable conditions to obtain reliable conduction paths, the present study examines the influence of process-related parameters such as volume fraction and viscosity on coalescence characteristics of fusible particles.

Abstract: A novel electrically conductive adhesives (ECAs) with solder particle have been developed. The kinetics of the curing reaction were investigated using isothermal differential scanning calorimeter (DSC). The dynamic DSC scans were also conducted to investigate the temperature dependant curing degree. The temperature dependant viscosity characteristic of polymer was investigated using rheometer. The wetting characteristic of two solders on two kinds of metals were investigated using an optical microscope. It was found that developed resin material showed good fluxing capacity against Sn-Bi solder on Ni/Au and the wetting angles was about 24º.

Abstract: A tensile test was conducted to evaluate thermal fatigue resistances of Sn-Bi (-Ag) and Sn-Ag-Bi-Cu lead-free solder alloys. The test is based on the strain rate change method to obtain a strain rate sensitivity index (SRSI), m. The m values were investigated at various strains during the tensile test, until fracture. The plots of m and strain where m is measured showed a linear relationship. Therefore, the m value at zero strain, m0, and the gradient of the fitting line, k, were obtained by extrapolation. Using m0 and k values, an estimation of the thermal fatigue resistance of the solder joint was attempted. It was believed that m0 and k can be taken as a guide for developing a new lead-free solder with the excellent thermal fatigue resistance. Moreover, the influences of the aging treatment on m0, k and the microstructure of the solder were also investigated.

Abstract: Resistance microwelding of fine crossed wires is of increasing industrial importance for medical devices, but the understanding of the process is very limited. A study has been carried on the feasibility of small-scale resistance welding of crossed TiNi fine wire. Welding mechanism is similar to that of small-scale resistance welding of crossed Ni wire, which is mainly four stages (wire cold deformation, surface melting, molten phase squeeze-out and, solid-state bonding). The optimum microstructure and welding parameters were proposed.

Abstract: The electrochemical migration (ECM) with flexible printed circuit board (PCB) was affected by factors such as distance between the electrodes and bias voltage using water drop (WD) test. The rate of migration was increased by decreasing the distance between electrodes of the opposite polarity and increasing the bias voltage.

Abstract: Solder fatigue is one of the major product reliability concerns in electric packaging design. In this paper, the application of a mesh-insensitive structural stress method, previously developed by the authors, is demonstrated for characterizing fatigue of solder joints under thermal fatigue and mechanical fatigue conditions. Three sets of experimental data from literature are analyzed to demonstrate the effectiveness of the structural stress method in its ability to correlate test data over different packaging designs, test methods, etc. As evidenced in this study, the meshinsensitive structural stress method offers a simple but effective means to correlate the fatigue behavior of solder joints among various test conditions. The results show that the structural stress method can be used as an effective alternative to existing methods in the literature, in reliability design of electronic packaging.

Abstract: Effects of isothermal aging on the microstructure and tensile behavior of Sn-3.0Ag-0.5Cu-0.2Co (SAC305-0.2Co) solder was explored, and compared with the standard Sn-3.0Ag-0.5Cu (SAC305) solder. The addition of Co resulted in the formation of CoSn2 phase in SAC305-0.2Co solder matrix. The isothermal aging treatment induced microstructural coarsening for both solders. With aging, the eutectic network has been translated into block or granular shape particles. Meanwhile, the outline of the primary Sn phase became less distinct. The tensile test results indicated, under as cast condition, the Co had little effect on the ultimate tensile strength (UTS) of SAC305-0.2Co solder, whereas it suppressed the ductility. After aging, the UTS of SAC305 was obviously decreased, whereas the elongation hardly changed. For the SAC305-0.2Co, the tensile behavior showed two trends: one was the decrease of the UTS, which was similar to that of SAC305; the other was the notable increase of elongation. The examination of the fracture surfaces indicated that isothermal aging could alter the fracture pattern of SAC305-0.2Co solder from brittle to ductile.

Abstract: The reaction between Sn-Ag (-Co) solder and electroless Ni-P plating was investigated in order to clarify the effect of the addition of Co to Sn-Ag solder on the formation of intermetallic compound (IMC) at the interface and the joint strength at the interface. Sn-Ag-Co solder was specially prepared. The results show that there is little effect of the addition of Co to the Sn-Ag solder on the IMC formation and the thickness of the IMC at the interface. For the pull strength of the solder bump joint, the addition of Co to the solder didn’t strongly affect the pull strength of the solder joints, but it affected the fracture mode of the solder joints.

Abstract: The reliability of QFP (Quad flat package) solder joint using Sn-8Zn-3Bi solder paste under the thermal shock test was investigated. Considering the environmental restriction such as ROHS, the QFP Cu LF (Lead-frame) was coated with lead-free materials (Sn, Sn-3Bi). To analyze the reliability under thermal shock treatment, the samples were placed in the thermal shock chamber (248K/423K, Dwell time: 30min). During the thermal shock test, the solder joint cross-sections were observed every 500 cycles. No crack initiation and propagation was observed through all type of plated Cu LF. The measured pull strength slightly decreased, as the number of cycles increased. After 1000 cycles, the pull strength of Sn and SnBi plated Cu LF reduced by 30% and 20%, respectively, compared with that of initial condition. Observing the fracture surface morphology by FE-SEM, the fracture mode changed and the IMC fractured area on the both fracture surface was increased. The IMC was identified as γ-Cu5Zn8 by chemical composition analysis using EDS. The reduced pull strength was affected by IMC fracture and fracture mode change. However, the pull strength does not decrease steeply but gradually. Consequently, the Sn-8Zn-3Bi solder joint shows the reliable solder joint strength, fracture mechanism, and compatibility with lead-free plated Cu LF during the thermal shock temperature of 248K to 423K.