Bernd Michel

Fatigue life models for SnAgCu and SnPb solder joints evaluated by experiments and simulation

In recent years, many solder fatigue models have been developed to predict the fatigue life of solder joints under thermal cycle conditions. While a variety of life prediction models have been proposed for near eutectic SnPb(Ag)-solder joints in the literature, not enough work has been reported in extending these models to lead-free soldered assemblies. The development of lie prediction models requires a deep insight into failure modes, constitutive models for the themnomechanical behavior of solders and an experimental reliability database. This is needed for the correlation of experimentally determined cycles-to-failure to simulation results by fmiteelement analysis. This paper describes in detail the life-prediction models of SnPh(Ag) and SnAgCu solder joints for thermal cycle conditions. To obtain reliable FEM input and to verify simulation results, a variety of material testing and experimental fatigue data is necessary. The accuracy of lieprediction tools has also become critically important, as the designs need to he evaluated and improved with a high degree of reliability, not through relative comparison but by providing absolute numbers. This work deals with the effect of different solder interconnect alloys (Sn59Pb40Agl and Sn95.5Ag3.8Cu0.7) and the effect of different package types (PBGAs, CSPs, Flip Chip on FR-4 with and without underfill) on the fatigue life. Different temperature cycling conditions are applied.

Constitutive behaviour of lead-free solders vs. lead-containing solders-experiments on bulk specimens and flip-chip joints

Both high Pb-Sn and eutectic 63Sn-37Pb have been the suitable materials for the interconnection of electronic components because of their low cost and appropriate physical properties. Due to environmental awareness, and the health hazards caused by the lead in the solders, large efforts have been made to develop a lead-free soldering technology. Among the large variety of lead-free solders the Sn-Ag alloys are expected to be the best candidates. Furthermore, from a reliability point of view, there has been interest in the improved thermal fatigue resistance of solder interconnects. Therefore, in this study two lead-free solder alloys (Sn96.5Ag3.5, Sn95.5Ag3.8Cu0.7) and two lead-containing solder alloys(Sn63Pb37 Sn59Pb40Agl) were investigated and compared with each other in order to give an estimation of the reliability enhancement of the new lead-free soldering technology. These investigations were focused on mechanical and physical properties (coefficient of thermal expansion, stress-strain curves at different strain-rates, ultimate strength) as well as on the microstructural appearance of the solders.

Through silicon via technology — processes and reliability for wafer-level 3D system integration

3D integration is a rapidly growing topic in the semiconductor industry that encompasses different types of technologies. The paper addresses one of the most promising technologies which uses through silicon vias (TSV) for interconnecting stacked devices on wafer-level to perform high density interconnects with a good electrical performance at the smallest form factor for 3D architectures. Fraunhofer IZM developed a post frontend 3D integration process, the so- called ICV-SLID technology based on metal bonding using solid-liquid-interdiffusion (SLID) soldering. The SLID metal system provides the mechanical and the electrical connection, both in one single step. The ICV-SLID fabrication process is well suited for the cost-effective production of both, high- performance applications (e.g. 3D microprocessor) and highly miniaturized multi-functional systems. The latter preferably in combination with wafer-level die stacking, as e.g. Thin Chip Integration (TCI) or SnAg-microbump technologies. The fabrication of distributed wireless sensor systems (e. g. e-CUBESreg) is a typical example for the need of such mixed approaches.

Thermal and mechanical analysis of micromachined gas sensors

In this paper, we present a complete thermomechanical study of a micromachined gas sensor substrate. The work has been carried out combining coupled electrothermomechanical three-dimensional finite element modelling simulations with electrical, infrared thermography and interferometric microscopy experimental measurements. The performances predicted by simulations, such as the power consumption (heating efficiency in air of 5.7 °C mW−1), the time response (19 ms), the membrane deflection during operation and the preferential failure sites in the micromachined substrate have been confirmed by experience. Their good agreement validates the model, and allows us to consider the adaptability of this design as a micromachined substrate for integrated gas sensors.

High aspect ratio TSV copper filling with different seed layers

The paper addresses the through silicon via (TSV) filling using electrochemical deposition (ECD) of copper. The impact of seed layer nature on filling ratio and void formation will be discussed with respect to via diameter and via depth. Based on the spherolyte Cu200 the electrolyte for the copper electrochemical deposition was modified for good filling behavior. Thermomechanical modeling and simulation was performed for reliability assessment.

MicroDAC strain measurement for electronics packaging structures

Thermo-mechanical reliability concerns in microelectronics and microsystem technology have led to a significant need in information on stress/strain behavior inside microscopic components under load. Because of the decreasing size of structures new measurement tools have to be developed in order to assure future experimental access. Strain measurement from load state micrographs utilizing correlation techniques seems to be one of the promising approaches. Its application to thermally stressed objects in electronics packaging is the main topic of this paper. Deformation measurements are presented for solder interconnects of flip chip and chip scale packages. Emphasis is made for the experimental support of finite element simulation by means of experimental validation of mechanical modeling. Furthermore, a wide variety of new packaging materials and material compounds are to be characterized. Different material properties for bulk material and microscopic structures intensify the search for new testing methods. Again correlation algorithms can be applied to measure strains on microscopic areas and subsequently to obtain material properties. The feasibility to measure coefficients of thermal expansion and Poisson ratios on tiny specimens is demonstrated in the paper.

Reliability assessment of flip-chip assemblies with lead-free solder joints

Due to environmental awareness, and the health hazards involved in using lead in solders, large efforts to develop lead-free soldering have been made in recent years. Sn-Ag alloys are expected to be one of the best candidate lead-free solders. Furthermore, from a reliability viewpoint, there has been interest in improved thermal fatigue resistance of solder interconnects. In this study, two lead-free solder alloys (Sn96.5Ag3.5, Sn95.5Ag3.8Cu0.7) were investigated in comparison to lead-containing solder alloys (Sn63Pb37, Sn59Pb40Ag1). These investigations were focused on mechanical and physical properties (coefficient of thermal expansion, stress-strain curves at different strain-rates) as well as on the microstructural appearance of the solders. The mechanical and thermomechanical behavior of the solders were examined by TMA, DTMA, tensile tests, and creep tests. Constant-load creep tests were performed on the specimens at temperatures from 20/spl deg/C to 150/spl deg/C. Steady-state strain rates spanned seven orders of magnitude ranging from 10/sup -11/ s/sup -1/ to 10/sup -4/ s/sup -1/. The second step is a reliability study of flip-chip assemblies on FR-4 (high T/sub g/ material) with three different underfill materials and with Sn63Pb37, Sn96.5Ag3.5, and Sn95.5Ag4.0Cu0.5 bumps, undergoing thermal cycles from -55/spl deg/C to 125/spl deg/C and -55/spl deg/C to 150/spl deg/C. The deterioration (characterized by electrical resistance and SEM) are described. Furthermore, it is shown that the material parameters obtained from the tests will increase the precision of finite-element analysis for reliability studies of microelectronic packages with lead-free solder interconnects.

An efficient approach to predict solder fatigue life and its application to SM- and area array components

The paper describes theoretical predictions and experimental observations of solder fatigue in different Sn-63Pb-37 solder joints. Experimental characterisation of solder-behaviour is performed by both thermal cycling of Surface Mount (SM) solder joints and mechanical cycling of ring and plug specimen. Detailed studies of the microstructure in solders after temperature cycling as well as after mechanical cycling have shown the same type of microstructural degradation. This degradation can be described by overall coarsening, local coarsening, recrystallisation, crack initiation and propagation. The computational method to assess the cyclic damage of solder is based upon non-linear finite element calculation results. Comparison of calculation and test results have demonstrated better predictive capabilities when the Coffin-Manson criterion takes into account creep strain distribution within the joint and not only its maximum value. It is shown that Plastic Ball Grid Arrays (PBGA) achieve a high solder joint reliability and exhibit no reliability drawbacks when compared to Plastic Quad Flat Packages (PQFP) with a similar pin count. Additionally, solder bump fatigue of underfilled flip chip assemblies is investigated. It is demonstrated that the mechanical stiffness of underfill has a major impact on bump stresses.

Flow characterization and thermo-mechanical response of anisotropic conductive films

The paper reports investigations on the chip on glass (COG) bonding process using anisotropic conductive films (ACF). Experimental methods as well as theoretical analyses, by both analytical and numerical means, are applied. The assumptions concerning the thermo-mechanical and rheological properties of the polymer materials involved in the bonding process are characterized for dependence on temperature. The transient development of the temperature field during the bonding process is studied by finite element (FE) analysis for dependence on the upper and lower chuck temperatures. Analytical techniques of fluid mechanics are used to predict the flow of the conductive particles during bonding, treated as dimensionless points embedded in a viscous matrix. This analytical description allows one to estimate the number of conducting particles on a bump of a chip after bonding. Furthermore, numerical calculations are applied to characterize the influence of viscosity gradients on the particle flow. Finally, nonlinear finite element simulations are used to investigate the stress development and stress relaxation process within the ACF joints.

Measurement of residual stress by slot milling with focused ion-beam equipment

In this paper, the authors present a new approach to residual stress measurement that takes advantage of the combined imaging–milling capabilities of focused ion-beam equipment. The method is based on the measurement of the displacement field originated when a slot of a few microns is milled on the material under study. The fitting of the experimental results with an analytical model together with the independent determination of Youngs modulus allows us to find the residual stress of the layer under study. The complete experimental procedure is described and its feasibility is demonstrated on a LPCVD silicon nitride micromachined membrane. Values obtained by this new method show a good agreement with values obtained by a classical method such as the bulge test.