Key Engineering Materials Vols. 353-358

Abstract: An in-situ testing system for thermal-mechanical fatigue of thin metal lines was setup inside a dual-beam focused ion beam (FIB)/scanning electron microscope (SEM) system. Alternating currents (AC) were applied to narrow Au lines 200-nm-thick through nanomanipulator needles. Preliminary results show that severe thermal-mechanical fatigue damage can be generated by the action of the applied AC. The in-situ recording of the evolution of the damage has been carried out and the possible mechanism of the thermal-mechanical fatigue damage in the Au lines resulted from the joule heating was discussed.

Abstract: Porous silicon dioxide films featuring low dielectric constant are of great scientific and commercial interest because of their outstanding potential for application to microelectronic interconnect. However, some reliability problems arise in porous SiO2 films due to their poor mechanical performance. Therefore, it is very important to understand the mechanical and electrical properties of porous SiO2 films. New 2-D models with circle pores and 3-D models with cubic pores are proposed in this work. Simulated results of porous SiO2 structures in the case of periodic and random arrangement, which are implemented through ANSYS, are also provided. Critical parameters such as Young’s modulus E and dielectric constant k of porous films are investigated. Calculation results show that dielectric constant reaches as low as 2.5 when porosity of films is about 40%, while E keeps over 3 GPa if porosity remains in the range from 30% to 40%.

Abstract: Vacuum packaging is very important for some micro-electro-mechanical systems (MEMS) devices to perform their basic functions properly and to enhance their reliability by keeping these devices away from harmful external environment. In order to maintain high vacuum in a cavity of MEMS devices, residual gases and leaking gases must be eliminated by embedded getter materials. Micro/Nano film getters absorbing gases inside the tiny cavity of MEMS packaging were introduced in this paper. The fabrication and characterization of micro/nano getters for MEMS applications were also presented. Various kinds of patterned getter films were prepared for different MEMS applications. The activation temperature and sorption capacity of the nonevaporable getter (NEG[1]) films was investigated. The formation of micro/nano films on the inner surface of MEMS devices is totally compatible with Si-based MEMS process and illustrates the applicability of the technique in vacuum maintenance of MEMS devices.

Abstract: The formation and evolution of the intermetallic compound (IMCs) between SnAgCu lead-free solder and Cu substrate, after isothermal aging at 150°C for 24, 48, 120, 240 and 480 hours, were studied. Scanning electron microscope (SEM) was used to observe the microstructure evolution of solder joint during aging. The IMC phases were identified by energy dispersive X-ray (EDX). The results showed that IMCs layer of Cu6Sn5 was formed at the interface of solder and Cu during reflowing. With the increase of aging time, the grain size of the interfacial Cu6Sn5 increased and the morphology of the interfacial Cu6Sn5 column was changed from scallop-like to needle-like and then to rod-like and finally to particles. At the same time, the rod-like Ag3Sn phase formed at the interface of solder and the IMCs layer of Cu6Sn5 with the aging time increased. In addition, large Cu6Sn5 formed in the solder with the aging time increased. The tensile strength was measured for the solder joints. The results showed that the tensile strength increases slightly at beginning and then decreases with the aging time. SEM was used to observe the fracture surface and it showed that the fracture position moved from solder matrix to the interfacial Cu6Sn5 IMCs layer with the aging time increased. The weakening of the solder matrix is caused by the coarsening of the eutectic solder structures. The weakening of the interfacial IMCs layer is caused by the evolution of morphology and size of the interface Cu6Sn5 layer.

Abstract: The reliability of solder joints in flip chip assemblies with both compliant (flex) and rigid (PCB) substrates was studied by accelerated temperature cycling tests and finite element modeling (FEM). In-process electrical resistance measurements and nondestructive evaluations were conducted to monitor solder joint failure behavior, hence the fatigue failure life. Meanwhile, the predicted fatigue failure life of solder joints was obtained by Darveaux’s crack initiation and growth models. It can be concluded that the solder joints in flip chip on flex assembly (FCOF) have longer fatigue life than those in flip chip on rigid board assembly (FCOB); the maximum von Mises stress/strain and the maximum shear stress/strain of FCOB solder joints are much higher than those of FCOF solder joints; the thermal strain and stress in solder joints is reduced by flex buckling or bending and flex substrate could dissipate energy that otherwise would be absorbed by solder joint. Therefore, the substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during temperature cycling.

Abstract: The long-term hygrothermal aging will induce the irreversible damages of epoxy-based anisotropic conductive film (ACF) joints. In this study, hygrothermal environment tests were used to accelerate the degradation of ACF joints in Chip-on-Glass (COG) assemblies. The effects of aging on adhesion strengths were measured by shear mode tests and changes of molecular conformation of ACF material were analyzed by Fourier Transform Infrared Spectroscopy (FTIR). Also, the fracture surfaces of ACF joints were investigated by Scanning Electronic Microscope (SEM). The results show that the ACF adhesion strengths decrease with aging; some chemical modifications occur in the aged ACF joints; the fracture failure mechanisms change with the hygrothermal aging.

Abstract: Structural model of plastic electronic package under temperature and humidity is constructed, and the coupling of the thermal and moisture impact on structure field is implemented. The impact of thermal expansion, hygro-swelling and vapor pressure increases with increasing initial defect, especially the impact of vapor pressure. At certain crack length, KI and KII induced by thermal expansion is proportional to temperature difference, and KI and KII induced by vapor is proportional to vapor pressure. The impacts of thermal expansion and hygro-swelling on ERR decrease during crack propagation, but the impact of vapor pressure increases significantly.

Abstract: Reliability optimization design of Ball Grid Array (BGA) solder joints is a major concern in area array electronics packaging technology. In this paper, shapes of the solder joints and their reliability were predicted and analyzed. Through the variations of lower pads’ diameters, the shapes of full array BGA solder joints with different solder volumes were predicted by using surface evolver software. Based on the results of shape prediction, 3-D finite element models were established with MSC.MARC and the distribution of the stress and strain in the BGA solder joints under thermal cyclic loading were simulated. Finally, fatigue lives of the BGA solder joints with different solder volumes were calculated, and the diameter ratios of lower pad to upper pad for these two kinds of BGA assemblies with the best reliability were optimized.

Abstract: Today, reflow soldering is a commonly used technique to establish large-area joints in power electronics modules. These joints are needed to attach large-area (>1 cm2) power semiconductor chips to the substrate, e.g., a direct-bond copper substrate, and the multichip module substrate to a copper base plate for heat spreading. Thermal performance, specifically thermal conductivity and thermomechanical reliability, of these large-area joints are critical to the electrical performance and lifetime of the power modules. Soft solder alloys, including the lead-tin eutectic and lead-free alternatives, have low thermal conductivities and are highly susceptible to fatigue failure. As demands mount for higher power density, higher junction temperature, and longer lifetime out of the power modules, reliance on solder-based joining is becoming a barrier for further advancement in power electronics systems. Recently, we successfully demonstrated lowtemperature sintering of nanoscale silver paste as a lead-free solution for achieving highperformance, high-reliability, and high-temperature interconnection of small devices (<0.09 cm2). In this paper, we report the results of our study to extend the low-temperature sintering technique to large-area joints. The study involved redesigning the organic and inorganic components of the nanoscale silver paste, analyzing the burnout kinetics of the various organic species sandwiched between large-area plates, and developing desirable temperature-time profile to improve sintering and bonding strength of the joints.

Abstract: The mechanical properties of copper thin films deposited by sputtering and electroplating were compared using tensile test and nano-indentation. Both the Young’s modulus and tensile strength of the films were found to vary drastically depending on the microstructure of the deposited films. The Young’s modulus of the sputtered film was almost same as that of bulk material. However, the Young’s modulus of the electroplated thin film was about a fourth of that of bulk material. The micro structure of the electroplated film was polycrystalline and a columnar structure with a diameter of a few hundred-micron. The strength of the grain boundaries of the columnar grains seemed to be rather week. In addition, there was a sharp distribution of Young’s modulus along the thickness direction of the film. Though the modulus near the surface of the film was close to that of bulk material, it decreased drastically to about a fourth within the depth of about 1 micron. There was also a plane distribution of Young’s modulus near the surface of the film.