Xicheng Wei

Coffin‐Manson constant determination for a Sn‐8Zn‐3Bi lead‐free solder joint

Purpose – To determine the Coffin‐Manson (CM) equation constants for fatigue life estimation of Sn‐8Zn‐3Bi solder joints, since Sn‐8Zn‐3Bi solder has a melting temperature of around 199°C which is close to that of the conventional Sn‐Pb solder which has previously been used in the electronics assembly industry.Design/methodology/approach – Three dimensional finite element (FE) simulation analysis was used for comparison with the experimentally measured data and to determine the CM constants. Low cycle fatigue tests and FE simulations were carried out for these lead‐free solder joints, and eutectic Sn‐37Pb solder was used as a reference.Findings – The CM equation for Sn‐8Zn‐3Bi solder joints was fitted to the lifetimes measured and the shear strains simulated. The constants were determined to be 0.0294 for C, the proportional constant, and for the fatigue exponent, β, −2.833.Originality/value – The CM equation can now be used to predict the reliability of Sn‐8Zn‐3Bi solder joints in electronics assembly an...

Study of IMC at interfaces of Sn3.0Ag0.5Cu3.0Bi0.05Cr/Cu joints during thermal ageing

Purpose – The purpose of this study was to comparatively investigate interfacial intermetallic compounds (IMCs) in the Sn3.0Ag0.5Cu3.0Bi0.05Cr/Cu (SACBC/Cu) and Sn3.0Ag0.5Cu/Cu (SAC/Cu) solder joints, and to determine any differences. Design/methodology/approach – The samples were annealed after isothermal ageing at 150°C for 0, 168 and 500 hours, and their cross-sections were observed by scanning electron microscopy and energy dispersive spectroscopy. Findings – The interfacial IMC morphology in two joints had significant differences. For the Cu/SAC/Cu joints, the granular and short rod-like Ag3Sn particles attached on the surface and boundary of interfacial Cu6Sn5 grains were detected, and they coarsened observably with ageing time at 150°C, and lastly embedded at the grain boundaries. However, for the Cu/SACBC/Cu joints, there were tiny filamentous Ag3Sn growing on the surface of interfacial Cu6Sn5 grains, and the Ag3Sn had a tendency to break into nanoparticles, which would be distributed evenly and c...

Interfacial IMC layer growth and tensile properties of low‐silver Cu/SACBE/Cu solder joints

Purpose – In a previous study, the authors proposed a new low‐silver solder alloy Sn‐ x(1.0, 1.5, 2.0)Ag‐0.3Cu‐3.0Bi‐0.05Er (wt.%) (SACBE) and the purpose of this paper is to provide additional useful information for new solder alloy development. The growth behaviour of the interfacial IMC layers for Cu/SACBE/Cu and Cu/SAC/Cu joints and their bonding strengths after thermal aging at 150°C for 0, 24, 168 and 500 hours are investigated and the effects of adding elemental Bi and Er on the growth of interfacial IMC layers in the joints, and their tensile properties, are characterized and discussed.Design/methodology/approach – The tensile properties of the Cu/Sn‐3.0Ag‐0.5Cu/Cu (Cu/SAC/Cu) and Cu/SACBE/Cu joints during thermal aging at 150°C for 0, 24, 168 and 500 hours were investigated, respectively. The thickness of interfacial IMC layer and the fracture surface of solder joint after isothermal aging were observed and analyzed by means of scanning electron micrograph (SEM) equipped with an energy dispersive...

Coffin‐Manson equation of Sn‐4.0Ag‐0.5Cu solder joint

Purpose: The purpose of this paper is to fit Coffin-Manson equation of Sn-4.0Ag-0.5Cu lead free solder joint by using results of solders joint reliability test and finite element analysis. Also to present a novel device for solder joint reliability test. Design/methodology/approach: Two-points bending test of Sn-4.0Ag-0.5Cu lead free solder joint was carried out at three deflection levels by using a special bending tester that can control displacement exactly by a cam system. The failure criterion was defined as resistance of solder joint getting 10 percent increase. The X-section was done for all failure samples to observe crack initiation and propagation in solder joint. Finite element analysis was presented with ANSYS for obtaining shear strain range, analyzing distribution of stress and strain and supporting experimental results. Findings: The experimental results indicate that the fatigue life decreased obviously with the displacement increased. By using optical microscope and SEM photographs, two kinds of failure mode were found in solder joint. The majority failure mode took place at the bottom corner of solder joint under the termination of resistor initially, and propagated into the solder matrix. Another failure mode was delamination. It appeared at the interface between the termination of resistor and its ceramic body. The distribution status of stress and strain in solder joint and the calculation results of shear strain range at different deflection levels were obtained from simulation result. The Coffin-Manson equation of Sn-4.0Ag-0.5Cu lead free solder joint was fitted by combining experimental data with result of finite element analysis. Originality/value: This paper presents Coffin-Manson equation of Sn-4.0Ag-0.5Cu solder joint with two-points bending test. An effective and economical device was designed and applied.

Isothermal low cycle fatigue behavior of Sn-8Zn-3Bi on single shear solder joint

Tin-zinc solder with the low melting temperature below 200 /spl deg/C is an attractive alternative to eutectic tin-lead. The addition of bismuth improves wettability of Sn-Zn alloy. Low cycle isothermal mechanical fatigue testing of solder joints of Sn-8Zn-3Bi (wt%) and Sn-37Pb were tested in a solder joint of single lap shear sample with a multi-functional Instron 4458-Microtester. Solder joints around 0.5mm high and 1.6mm in diameter was used. The testing was executed in a displacement -controlled mode, at three different amplitudes namely, 40, 50 and 60/spl mu/m. The test frequency was kept constant at 0.2Hz for all tests. The failure of the solder joint was defined as a 50% load reduction. Finite element (FE) modeling were simulated to analyze the strain and stress distribution in solder joint and compared with the experimental results. The cross-section showed that there were mixed phase of small cellular-shape and coarse needle-shape in the microstructure of Sn-8Zn-3Bi. SEM-EDS analysis indicated that the Au-Zn intermetallic compound was observed near the interface. The total average lifetime of Sn-8Zn-3Bi solder joint was 17% increase comparing with Sn-37Pb solder joint from low cycle fatigue testing. The ductile-fracture manner and brittle-fracture manner were both involved in fracture surface, and more features of brittle-fracture manner appeared in small displacement loading and more features of ductile-fracture manner in large displacement loading. From FE simulation, the maximum stress was found at two corners, coinciding with the experimental observation of crack initiation and propagation.

Low cycle fatigue testing and simulation of Sn‐8Zn‐3Bi and Sn‐37Pb solder joints

Purpose – Sn‐Zn based lead free solders with a melting temperature around 199°C are an attractive alternative to the conventional Sn‐Pb solder and the addition of bismuth improves its wetability. Whilst lead‐free soldering with Sn‐8Zn‐3Bi has already been used in the electronics assembly industry, it is necessary to study its low cycle fatigue properties since such data have not been reported up to now.Design/methodology/approach – In this study, displacement‐controlled low cycle fatigue testing of Sn‐8Zn‐3Bi and Sn‐37Pb solder joints was done on lap shear samples. The test amplitude was varied whilst the frequency was kept constant at 0.2 Hz and failure was defined as a 50 per cent load reduction. Finite element (FE) modelling was used for analysis and the results were compared to the experimental data.Findings – The microstructure of the Sn‐8Zn‐3Bi solder showed a mixed phase of small cellular‐shaped and coarser needle‐shaped areas. Au‐Zn intermetallic compounds were observed near the interface from the...

Studies of Solder Joint Reliability under Mechanical Bending Test on FR-4 PCBs with Sn-4.0Ag-0.5Cu Solder Paste

The mechanical bending test results, as well as the modeling and calculation data were presented in this study to characterize the solder joint reliability. Bending test was completed simply by loading a series of displacements on the FR-4 printed circuit board (PCB) with devices and solder joints in single direction. A special bending tester that can control displacement exactly by a cam system was designed and used for reliability study. The three group samples need to endure bend distortion with different deflections respectively, namely 3 mm, 4 mm, and 5 mm. The electrical resistance was probed every 3600 cycles by multimeter. When the resistance of solder joint got 10% increase, the failure can be defined. The experimental results indicate that the fatigue life decreased obviously with the displacement increased. By using optical microscope and SEM photographs, two kinds of failure model were found in solder joints. The majority failure model, which took place at the bottom corner of solder joint under the termination of resistor initially, and propagated into the solder matrix. The delamination appeared at the interface between the termination of resistor and its ceramic body. The finite element analysis was presented to support this experiment. The shear strain ranges under the different deflections were obtained from simulation. A Coffin-Mason equation of Sn-4.0Ag-0.5Cu from literature was used to compare with this experimental result and prove the feasibility of two points bending test in fatigue life prediction.

Intermetallic compound formation in Sn-Co-Cu, Sn-Ag-Cu and eutectic Sn-Cu solder joints on electroless Ni(P) immersion au surface finish after reflow soldering

The interfacial reactions between Sn-0.4Co-0.7Cu eutectic alloy and immersion Au/electroless Ni(P)/Cu substrate were investigated after reflow soldering at 260degC for 2 minutes. Common Sn-4.0Ag-0.5Cu and eutectic Sn-0.7Cu solders were used as reference. Two types of intermetallic compounds (IMC) were found in the solder matrix of the Sn-0.4Co-0.7Cu alloy, namely coarser CoSn2 and finer Cu6 Sn5 particles, while only one ternary (Cu,Ni)6 Sn5 interfacial compound was detected between the solder alloy and the ENIG (electroless nickel and immersion gold) coated substrate. The same trend was also observed for the Sn-Ag-Cu and Sn-Cu solder joints. Compared with the CoSn2 particles found in the Sn-Co-Cu solder and the Ag3Sn particles found in the Sn-Ag-Cu solder, the Cu6Sn5 particles found in both solder systems exhibited finer structure and more uniform distribution. It was noted that the thickness of the interfacial IMCs for the Sn-Co-Cu, Sn-Ag-Cu and Sn-Cu alloys was 3.5mum, 4.3mum and 4.1mum, respectively, as a result of longer reflow time above the alloys melting temperature since the Sn-Ag-Cu solder alloy has the lowest melting point

Microstructural evolution of Sn-Zn based lead free solders after temperature and humid atmosphere exposure

The Sn-Zn based lead free solders appear to be an attractive alternative with a melting temperature that is relatively close to eutectic Sn-Pb. The addition of bismuth improves wettability of Sn-Zn alloy and aluminum is found to improve the oxidation resistance of the eutectic Sn-Zn solder. The objective of this study is to explore the degradation of Sn-Zn system solders after temperature and humid atmosphere exposure. Three kinds of Sn-Zn system solders (Sn-9Zn, Sn-8Zn-3Bi and Sn-7Zn-Al(30ppm)) were soldered on copper pad with a layer of Au/Ni and the specimens were stored in 120 degC/100% relative humidity for 96 hours or 192 hours in a pressure cooker. Zn diffused to the surface and interface while coarsening at the same time. The coarser Zn had weak interface with p-Sn matrix and oxidation was formed. The mechanical fatigue test was carried in a displacement-controlled mode and the results show that high temperature humidity exposure decreases the mean life of Sn-Zn system solder joints when they are subjected to 120degC and 100% relative humidity for 192 hours after the pressure cooker test