Tommi Reinikainen

Deformation characteristics and microstructural evolution of SnAgCu solder joints

Three SnAgCu solder alloys (Sn2AgO.5Cu, Sn3.4AgO.8Cu, Sn4AgO.5Cu) have been tested to determine their deformation behavior in the temperature range 23-110/spl deg/C, strain-rates varying 10/sup -7/-10/sup -1/ 1/s. It is shown by optical micro-graphs of CSP solder joints that microstructure of SnAgCu may undergo through significant changes due to various loading conditions, which can occur during usage of microelectronic devices, such as thermal cycling, mechanical bending, and drop impact. The solidification microstructure consists typically of very large Sn-matrix colonies, with eutectic structure and intermetallic particles distributed within the colonies in cellular form. Typical observed temperature- and deformation-induced microstructural evolution includes recrystallization and twinning. The deformation mechanisms of the alloys have been predicted based on the values of measured activation energies and stress exponents. The constant stress and constant strain-rate tests have been performed in the shear configuration, which enables a stress-state of nearly pure shear in the solder joint. In the intermediate stress regime, the deformation appears to occur by the slip mechanism, and the rate is likely to be controlled by the dislocation climb process. The measured shear stress-strain data are utilized to determine the constants for the visco-plastic Anands constitutive model.

Absolute and relative fatigue life prediction methodology for virtual qualification and design enhancement of lead-free BGA

The semiconductor industry is driving toward lead-free solder due to environmental concern and legislation requirement. The industry has also concluded that SnAgCu solder alloy so far is the best lead-free alternative to SnPb solder. Therefore, most existing and new packages have to be tested and qualified using lead-free solder. One of the critical concerns is board level solder joint reliability during thermal cycling test. In this paper, the methodology for an absolute life prediction is described for virtual qualification of packages. A good absolute fatigue life prediction requires an appropriate solder creep model and actual test data on packages. Two new sets of lead-free Anands constants for SnAgCu solder are introduced for creep models. These Anands creep models are compared with other lead-free and eutectic solder model and the relative design trend is similar. A fatigue corrective factor is introduced to integrate the different solder models together for convenient relative design enhancement with acceptable range of absolute life prediction. These fatigue corrective factors can also be used to compare different finite element modeling assumptions such as element size and solution time step. Subsequently, design analysis is performed to study the effects of 11 key package dimensions and material properties. It is found that the relative design trend for packages with lead-free and eutectic solder is similar. Therefore, the design guidelines established for the previous eutectic solder is still valid for lead-free solder.

A simulation-based multi-objective design optimization of electronic packages under thermal cycling and bending

In this study, a simulation-based multi-objective design optimization methodology was developed for improving electronic packaging reliability. It was demonstrated using a generic model of an electronic package on a printed wiring board. The objective for the optimization was to improve the reliability of solder joints under both thermal cycling and bending by optimizing a group of design parameters. A parametric finite element model was developed using ANSYS for both load conditions. To improve the numerical efficiency of the optimization, a multi-quadric response surface method was implemented to approximate the response of finite element simulations for each loading condition. Subsequently, the multi-objective optimization of solder joint reliability was implemented using a Minmax principle on all response surfaces and a differential evolution algorithm as optimal search engine, which is capable of finding global minimum when local minima exist. Our study demonstrated that the reliability of the solder joints is significantly improved for this given generic model of electronic package. The proposed methodology can be effectively used in improving the reliability of electronic packages.

Guidelines to select underfills for flip chip on board assemblies and compliant interposers for chip scale package assemblies

Abstract The effect of thermomechanical properties of underfill and compliant interposer materials, such as coefficient of thermal expansion (CTE) and stiffness (Youngs modulus) on reliability of flip chip on board (FCOB) and chip scale packages (CSPs) under thermal cycling stresses is investigated in this study. Quasi-three-dimensional viscoplastic stress analysis using finite element modeling (FEM) is combined with an energy partitioning (EP) model for creep-fatigue damage accumulation to predict the fatigue durability for a given thermal cycle. Parametric FEM simulations are performed for five different CTEs and five different stiffnesses of the underfill and compliant interposer materials. The creep work dissipation due to thermal cycling is estimated with quasi 3-D model, while 3-D model is used to estimate the hydrostatic stresses. To minimize the computational effort, the 3-D analysis is conducted only for the extreme values of the two parameters (CTE and stiffness) and the results are interpolated for intermediate values. The results show that the stiffness of the underfill material as well as the CTE play important role in influencing the fatigue life of FCOB assemblies. The fatigue durability increases as underfill stiffness and CTE increase. In the case of compliant interposers, the reverse is true and durability increases as interposer stiffness decreases. Furthermore, the interposer CTE affects the fatigue durability more significantly than underfill CTE, with durability increasing as CTE decreases. The eventual goal is to define the optimum design parameters of the FCOB underfill and CSP interposer, in order to maximize the fatigue endurance of the solder joints under cyclic thermal loading environments.

Transition to Pb-free manufacturing using land grid array packaging technology

Land grid array IC-packages are gaining popularity among portable electronics, for low cost, mechanical reliability, direct Pb-free assembly process compatibility, and their low profile on the PWB. LGA technology is an excellent choice to fulfil future environmental requirements in thin and compact products. The reliability performance of 0.5 mm pitch LGA structure is compared to ball grid array (BGA). Reliability performance is evaluated through comparative tests designed for a portable environment. These tests consist of temperature cycling test for operation performance evaluation and board level drop test for mechanical shock durability performance evaluation. The stress distributions in LGA and BGA are analysed by the finite-element method (FEM). Furthermore, reliability investigation is done for LGA components using both standard SnPb- and Pb-free assembly processes. The differences in the reliability performance between the SnPb- and Pb-free assemblies are explained through microstructural analysis. Reliability issues relating to the transition from conventional assembly process to Pb-free process are discussed, based on the test and simulation results.

Application of ABAQUS/Explicit submodeling technique in drop simulation of system assembly

In this paper, an impact analysis procedure coupled with submodeling technique in ABAQUS/Explicit under version 6.3 was established. A nested submodeling technique was adapted from a system assembly level global model to an electronic package component (first level submodel) and then to a solder ball (second level submodel) in the drop simulation. As a demonstration, a dummy PWB board drop vehicle was first used to verify the proposed procedures using submodeling technique in ABAQUS/Explicit. The results show that in general, submodeling technique in ABAQUS/Explicit provides very positive solutions for displacements, stresses (strains) and accelerations. Then, based on the framework developed, this technique was implemented into system assembly drop simulation. The whole approach starts from an initial global linear analysis of the selected system assembly to identify component areas where the response to the loading is deemed vital. Then, these component areas are enhanced through remeshing and linear reanalyzing to determine crucial solder joint. Finally, the crucial solder joint is refined via remeshing and nonlinear reanalyzing.

Optimal Parameter Selection for Electronic Packaging Using Sequential Computer Simulations

Optimal parameter selection is a crucial step in improving the quality of electronic packaging processes. Traditional approaches usually start with a set of physical experiments and then employ Design of Experiment (DOE) based response surface methodology (RSM) to find the parameter settings that will optimize a desired system response. Nowadays deterministic computer simulations such as Finite Element Analysis (FEA) are often used to replace physical experiments when evaluating a system response, e.g., the stress level in an electronic packaging. However, FEA simulations are usually computationally expensive due to their inherent complexity. In order to find the optimal parameters, it is not practical to use FEA simulations to calculate system responses over a large number of parameter combinations. Nor will it be effective to blindly use DOE-based response surface methodology to analyze the deterministic FEA outputs. In this paper, we will utilize a spatial statistical method (i.e., the Kriging model) for analyzing deterministic FEA outputs from an electronic packaging process. We suggest a sequential method when using the Kriging model to search for the optimal parameter values that minimize the stress level in the electronic packaging. Compared with the traditional RSM, our sequential parameter selection method entertains several advantages: it can remarkably reduce the total number of FEA simulations required for optimization, it makes the optimal solution insensitive to the choice of the initial simulation setting, and it can also depict the response surface and the associated uncertainty over the entire parameter space.

Numerical studies of the mechanical response of solder joint to drop/impact load

The geometry of the solder ball is one of several factors which have significant effects on the reliability of interconnects of electronic packages. Intensive studies of the thermo-mechanical reliability of BGA packages with different solder balls have been reported in recent years. Only a few studies of the geometric shape effect on the reliability of solder joints under drop loading conditions could be found in the literature. Because of the different failure modes and failure mechanisms, the geometric shape effects on the reliability of solder joints could be very different under thermo-mechanical loading condition and drop loading condition. In this paper, the effects of the solder ball geometry on the reliability under drop loading conditions are systematically studied with numerical simulations. The solder ball shape is predicted by using the Surface Evolver software. The solder ball geometry is parameterized with solder volume, diameter, height, and pad size. The variation of stresses on the solder joint/pad interface with solder volume and pad diameter is obtained from numerical simulations. The effect of the solder ball geometric parameters on the drop damage is discussed.

A novel response surface method for design optimization of electronic packages

The response surface method has been widely used in practical engineering design optimization problems, where the optimal searches are based on the response surfaces mimicking the physical processes or models. Sometimes the response surface method is the only practical option to be able to perform design optimization, such as simulation-based design optimization when the simulations are usually computationally expensive. In conventional approaches, the response surfaces are usually built up using a group of sampling points based on certain design of experiment schemes. The accuracy of the response surface depends largely on the number of sampling points and their distributions in the design space, as well as the approximation functions for the response surface. Currently there is no general method that could achieve the necessary accuracy of a response surface with the minimal expensive (or the number of sampling points). In this paper, a novel sequential response surface updating approach is proposed to improve the efficiency and the accuracy of simulation-based optimizations for electronic packages. It is a dynamic and adaptive approach, which starts with a small number of initial sampling points based on Halton sequence (also called quasi Monte Carlo method), and then refines the response surface by adding more sampling points during the optimization process. This method is demonstrated with a design optimization problem of the thermo-mechanical analyses of a ceramic chip carrier assembly. A simplified thermo-mechanical model is used to perform the interfacial stress analysis of the solder bounding. The objective of is to minimize the interfacial stress by changing the bonding compliance in the specified design space. The case study indicates that, comparing with the conventional direct methods, this approach could greatly improve the computational efficiency of optimization processes with the needed accuracy for simulation-based design optimization.

Cracking phenomena on flexible‐rigid interfaces in PCBs under thermo cycling loading

Flexible printed circuit boards (fPCBs) have an important role in electronic product miniaturization due to their reduced thickness and ability to bend and adapt to various shapes. As with any new technology reliability should be assessed to guarantee quality of products that benefit from the additional features. Typical causes of cracking failure in electronic devices are related to mechanical stress, moisture, and heat from transportation and field use. Typically manufacturers carry out tests to evaluate the reliability of electronic components regarding these types of loads. Common tools used for this evaluation in early stages of design are finite element analysis and experimental laboratory tests. In this present work simulation models were proposed in order to develop tools for early stages of design. Thermomechanical tests were performed experimentally to assess reliability of parts and also provide information to evaluate the simulation models. The objective of this work is to develop simulation models for flexible printed circuit boards (fPCB) flex-rigid interfaces and to perform experimental tests in laboratory in order to evaluate the cracking phenomena when this device is submitted to thermal cycling.