Rainer Dudek

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.

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.

Reliability investigations on conductive adhesive joints with emphasis on the mechanics of the conduction mechanism

The isotropic conductive adhesive (ICA) mounting technology is of growing interest, but reliability concerns are still preventing its broad application. Reports on environmental testing results are related to both high temperature storage and thermal cycling. Additionally, the influence of moisture has been investigated for both pressure cooker test and humidity storage with exposure times up to several weeks. In an ICA, the conductive particles are embedded in a polymeric matrix material, where they can form conductive paths. This mechanical part of the conductive mechanism was studied in more detail using a finite element (FE-) model, because only a little information is available on this subject. A joint of a chip resistor on an organic board was selected for the model. The conductive adhesive is not treated as a homogeneous material, but split into the polymeric matrix material and idealized conductive particles. A temperature dependent viscoelastic constitutive description has been used to model the epoxy behavior. Additionally, moisture diffusion analyzes of the adhesive joints were conducted. The contacting pressure of the particles is shown to depend on cure shrinkage, temperature changes, and moisture swelling effects.

Thermal fatigue modelling for SnAgCu and SnPb solder joints

Thermal fatigue of solder joints is investigated by means of the finite element method (FEM). During the usual thermal cycle regime with relatively slow temperature ramping rates, solder constitutive response is dominated by secondary creep. In the paper secondary creep laws are given for SnPb and SnAgCu solders, based on our own measurements and literature data. These secondary creep behaviours are compared to other data recently published. Additional primary creep terms are introduced in the creep description. To relate calculation results to fatigue life, criteria of Manson-Coffin type are proposed. These empirical laws are based on either the cyclic inelastic strain or dissipated energy. Effects of the choice of different creep laws on the calculated creep strains and energy densities are studied for two standard components on FR-4 board, a ceramic chip resistor (CC) of size 0805 and a plastic ball grid array with 225 I/O (PBGA 225). It is shown that the choice of the creep law, e.g. the inclusion of primary creep, does significantly affect only the results for the PBGA solder balls.

FE-simulation for polymeric packaging materials

Finite element (FE) simulations represent a useful tool to evaluate the thermomechanical behaviour of electronic packages. However, the use of the FE-method generates special difficulties, with particular regard to the proper constitutive modelling of materials used in the assembly. One more general problem in the numerical investigations of encapsulated silicon chips is the occurrence of interfaces between the dissimilar materials. Due to the assumption of sharp interface edges and interface crack tips, stress singularities arise which might be accounted for only approximately in the FE-calculation. The paper intends to show solutions of these simulation difficulties, also by means of materials testing. The complex material behaviour is discussed for different filled epoxy materials, with particular regard to the influence of filler content. A new solution method for the interfacial edge problem is briefly introduced. As an example, the pull strength test is used and the asymptotic solution for an interface edge is presented.

Experimental and numerical reliability investigations of FCOB assemblies with process-induced defects

To develop comprehensive design guidelines, models and experiments cannot overlook process-induced imperfections in the flip chip on board (FCOB) assemblies. The following items were noted as being significant factors which were used for modeling and by thermal cycling tests: (a) varying standoff-heights and alternative bump sizes, (b) underfill-particle settling, (c) underfill-void effects, (d) underfill-to-bump coverage, (e) asymmetrical fillets vs. symmetrical fillets. A detailed numerical and experimental reliability study of perfect and imperfect flip chip assemblies has been completed. Experimental studies of the failure modes and of the mean cycles to failure are in good agreement with the failure modes and life time, as predicted by FEM for the different technological variants. A hierarchy of influences was worked out in three levels (important, medium, negligible). Most important imperfections resulting in a strong reliability decrease are particle settling, asymmetrical fillets and small and big voids.

Fracture mechanics based crack and delamination risk evaluation and RSM/DOE concepts for advanced microelectronics applications

Fatigue and failure of advanced electronic packages and related systems is often caused by their increasing use under harsh environmental conditions - extreme temperatures, in particular. As a result, its thermomechanical reliability becomes more and more one of the most important preconditions for adopting it in industrial applications. Residual stresses from several steps of the manufacturing process, thermal and static and dynamic mechanical loading conditions along with the fact that microelectronic packages are basically compounds of materials with quite different Youngs modules and thermal expansion coefficients contribute to interface delamination, chip cracking and fatigue of interconnects. Consequently, numerical investigations by means of nonlinear parameterized FEA, fracture mechanics concepts are frequently used for design optimizations using sensitivity analyses (Auersperg et al., 2001). So, numerical design studies can help to optimize designs of electronics applications at the earlier phase of the product development processes. Unfortunately, this methodology typically accounts for classical stress/strain evaluation or life-time estimations of solder interconnects using modified Coffin-Manson approaches. Delamination or bulk fracture mechanisms usually remain unconsidered. This contribution intends to figure out and discuss ways of using fracture mechanics numerical approaches in connection with parameterized FEA based DOE/RSM. For improving such methods, the evaluation of mixed mode interface delamination phenomena of several ceramics/encapsulant-specimens under bending has been combined with experimental deformation measurements. Measured force vs. deflection curves, deformation fields as results of optical inspection and deformation analysis as well as crack tip vs. deflection curves determined using constitute the input for the delamination modeling by means of FEM. Major goal of the study is to make such a way determined interface toughness parameters applicable within DOE/RSM-approaches.

Thermomechanical design for reliability of WLPs with compliant interconnects

A wafer level packaging technology ELASTecreg has been developed; which uses a resilient bump contact system. The advantages are twofold; because on the one hand the elastic contact system simplifies wafer probing and on the other hand the elastic interconnects allow an increase in board level reliability. Excessive solder bump straining caused by the mismatch of thermal expansion coefficients (CTE) between silicon and organic board materials can be avoided because of the compliance of the contact system, which can take over the main part of the mismatch deformation. Since the electrical connection is made by an electrodeposited copper/nickel redistribution layer (RDL), placed on top of the bump surface, other failures risks than solder fatigue emerge which were avoided by parametric studies using finite element analyses (FEA). The thermo-mechanical characteristics like stress-strain behavior and fatigue resistance of the RDL metallic films are the most important parameters for reliability predictions by FEA, discussed in some detail. The FEA based prediction that the fatigue performance of a spiral RDL layout is superior is proven experimentally and other reliability test data is provided