Ji-guang Han

Development of SnAg-based lead free solders in electronics packaging

Abstract Lead free solder alloys for electronic assembly is being driven by environmental and health concerns regarding toxicity of lead and, more importantly, by the perceived economic advantage of marketing “green” products. Of the currently available lead free solders, SnAg has the greatest potential. In this solder, the Ag3Sn compound is distributed in a eutectic network throughout the β-Sn matrix and these results represent mechanical strength. In order to further improve the microstructures and properties of SnAg-based alloys, alloying elements such as rare earth, Zn, In, P, Cu, Ni and particles such as ZrO2, POSS are selected to meet the requirement of high reliability of high-density electronics devices. For SnAg solder bearing rare earth (Ce and La), the creep-rupture life of solder joints can be remarkably increased up to four times more than that of the original SnAg solder joints at room temperature, meanwhile, rare earths can dramatically reduce the thickness of IMCs layer at solder/pad interfaces and also refine the microstructure of the alloy which results in the enhancement of mechanical properties of the SnAg solder. Moreover, the addition of ZrO2 nanoparticles significantly refined the size of Ag3Sn due to the adsorption effect of the ZrO2 nanoparticles. This paper summarizes the effects of alloying elements and particles on the wettability, mechanical properties, creep behavior, microstructures, etc. of SnAg-based lead free solder alloys.

Reliability behavior of lead-free solder joints in electronic components

With more consumer products moving towards environmentally friendly packaging, making solder Pb-free has become an urgent task for electronics assemblies. Solder joints are responsible for both electrical and mechanical connections. Solder joint does not have adequate ductility to ensure the repeated relative displacements due to the mismatch between expansion coefficients of the chip carrier and the circuit board. Materials behavior of solder joints involves a creep–fatigue interaction, making it a poor material for mechanical connections. The reliability of solder joints of electronics components has been found playing a more important role in service for microelectronics components and micro-electro-mechanical systems. So many researchers in the world investigated reliability of solder joints based on finite element simulation and experiments about the electronics devices, such as CR, QFP, QFN, PLCC, BGA, CSP, FCBGA and CCGA, which were reviewed systematically and extensively. Synchronously the investigation on reliability of solder joints was improved further with the high-speed development of lead-free electronic packaging, especially the constitutive equations and the fatigue life prediction equations. In this paper, the application and research status of constitutive equations and fatigue life prediction equations were reviewed, which provide theoretic guide for the reliability of lead-free solder joints.

Microstructures and properties of SnZn-xEr lead-free solders

The Sn9Zn eutectic alloy is the nontoxic lead-free solders alternative having a melting temperature which is closest to that of the eutectic SnPb alloy. In order to improve the properties of SnZn lead-free solders, 0–0.5 wt.% of rare earth Er was added to the base alloys, and the microstructures were studied. Results showed that the addition of rare earth Er could enhance the wettability of SnZn solders, with 0.08%Er addition, the spreading area gave an 19.1% increase. And based on the mechanical testing, it was found that the tensile force and shear force of SnZn-xEr solder joints could be improved significantly. Moreover, the oxidation resistance of SnZn0.08Er solder was better than that of SnZn solder. In addition, it was found that trace amounts of rare earth Er could refine the microstructures of SnZn solders, especially for Zn-rich phases, and excessive amount of rare earth Er led to a coarse microstructure.

Anand model and FEM analysis of SnAgCuZn lead-free solder joints in wafer level chip scale packaging devices

Abstract In this paper, the constitutive response of SnAgCuZn solder was studied under uniaxial tension. Anand model was used to represent the inelastic deformation behavior of the lead-free solders. The material parameters of the constitutive relations for SnAgCuZn solders were determined from separated constitutive relations and experimental results. In addition, the constitutive model was used to analyze the thermally induced inelastic deformation of the solder joints in wafer-level chip scale packages mounted on printed circuit boards based on finite element analysis. And it is found that the addition of Zn can enhance the fatigue life of SnAgCu solder joint.

Microstructures and Properties of SnZn Lead-Free Solder Joints Bearing La for Electronic Packaging

In this paper, the effect of rare earth (RE) La on the properties of Sn9Zn solder was carried out. The results indicate that RE La plays an important role not only in the wettability and the structure of the solder but also in the mechanical property of the solder joint. The results show that adding trace amount of RE La can remarkably improve the wettability and oxidation resistance of SnZn solder and can enhance the mechanical properties of solder joints. When RE La content is 0.06%, the SnZn-based solder has the best properties. In addition, it is found that SnZn0.06La shows a finer and more uniform microstructure than SnZn, and the sizes of Zn-rich phases are obviously reduced.

Properties of SnZn lead free solders bearing rare earth Y

Abstract Rare earth (RE) Y has been incorporated into eutectic SnZn solders to study the wettability, mechanical properties, oxidation resistance, microstructures and thermal fatigue behaviours. The results indicate that adding trace amount of RE Y can remarkably improve the wettability of SnZn solders. When the content of RE Y is 0·06 wt-%, the tensile and shear forces of SnZn-xY solder joints give 24·1% and 27·2% increase after soldering. In addition, it is found that SnZn0·06Y has better oxidation resistance than that of SnZn solder, and the microstructures of SnZn solders can be refined by the addition of RE Y; the size of Zn rich phase can be reduced obviously. Moreover, SnZn0·06Y shows superior thermal fatigue property over SnZn solders. It can be concluded that the optimum content of RE Y is 0·06 wt-% for SnZn solders.

Finite Element Analysis of SnAgCu(Zn, Co, Fe) Lead-free Solder Joints for Electronic Packaging

Abstract With the addition of Zn, Co and Fe, the microstructures and properties of SnAgCu solders can be improved obviously, the constitutive response of SnAgCu solder bearing 0.8%Zn, 0.8%Co and 0.8%Fe was studied under uniaxial tension. Anand model was used to represent the inelastic deformation behavior of the lead-free solders. The material parameters of the constitutive relations for SnAgCu(Zn,Co, Fe) solders were determined from separated constitutive relations and experimental results. Combining the model, finite element analysis (FEA) was used to study the thermally induced inelastic deformation and the stress-strain response of the solder joints in wafer-level chip scale packages (WLCSP) mounted on printed circuit boards (PCB). And it is found that the addition of Zn, Co and Fe can enhance the fatigue life of SnAgCu solder joint, the superiority can be found for the addition of Fe among the three elements.

Reliability of SnAgCu/SnAgCuCe solder joints with different heights for electronic packaging

Finite element simulation and experiments were used to analyze the reliability of SnAgCu/SnAgCuCe solder joints with different heights. With the increase of height of solder joints, the tensile strength of SnAgCu and SnAgCuCe solder joints drops significantly. With the same height, the tensile strength of SnAgCuCe solder joints show enhanced reliability comparing with that of SnAgCu solder joints. The heights of lead-free solder joints can markedly influence the failure modes of SnAgCu/SnAgCuCe solder joints, three models [intermetallic compound (IMC), solder/IMC, solder] can be demonstrated with the variation of height. Based on finite element methods, it is found the maximum stress concentrated location show a relationship with heights of solder joints, smaller height can keep the high reliability of solder joints.

Reliability of Lead-free Solder Joints in WLCSP Device with Finite Element Simulation and Taguchi Method

Abstract In this paper, we research the thermo-mechanical reliability of lead-free solder joints in WLCSP assembly subjected to an accelerated thermal cyclic loading based on finite element simulation and Taguchi method. Effects of different control factors, including solder materials, height of solder ball, chip thickness and substrates thickness on the reliability of the lead-free solder joints were examined and calculated. The corner solder joint is the most critical solder joints in the assembly, the top surface of the solder joint is the crack propagated location. The optimal design in the WLCSP assembly has the combination of the SnAgCuAl, height of solder ball at 0.25 mm, chip thickness at 0.6 mm, substrate thickness at 1.0 mm. Moreover, the solder materials is the most important factor among the control factors in the WLCSP assembly.

Wettability optimization analysis of lead-free solders with Taguchi method

Abstract In this paper, the wettability of lead-free solders on substrates with fluxes was studied based on reflow process and Taguchi method. Effects of different control factors, including solder materials, fluxes, substrate materials and atmosphere on the wettability of the lead-free solder joints were examined and calculated. Moreover, the effect of rare earth Eu on the wetting of SnAgCu solder was investigated, the effect mechanism was analyzed too. The wettability optimal design in the lead-free solders system has the combination of the SnAgCu-0.04Eu solder, RMA flux, Au/Ni/Cu substrate, and vacuum atmosphere. Moreover, the flux is the most important factor among the control factors in the wettability system.