Xueren Zhang

Pulse electroplating of copper film : a study of process and microstructure

The paper reports morphology and structure transitions of silicon carbide (SiC) nanowires during high temperature annealing; the as-prepared nanowires are in the form of SiC core and SiO2 shell. The transition temperature is about 1200 degrees C, 600 degrees C lower than that of SiC microfibers, and it starts with the formation of junctions of individual nanowires. The junctions grow into webs while the crystalline SiC cores of the nanowires oxidize. The growth and the oxidation eventually lead to the formation of an oxide film, when the transition completes. The thermal stability and the transition mechanisms of SiC nanowires are critical to their applications in high temperature environments.

Comprehensive hygro-thermo-mechanical modeling and testing of stacked die BGA module with molded underfill

Package reliability is a great concern in developing new advanced packages. This paper presents some of the modeling and testing activities for the design of mixed flip-chip (FC)-wire bond (WB) stacked die BGA module with molded underfill (MUF). The success of the MUF application depends on its performance in thermal shock (TS) test and pressure cooker test (PCT). Mechanical properties (modulus and adhesion strength) of MUF after post mold cure (PMC), reflow and PCT are measured. Shear strength between die and MUF under various temperature and moisture conditions are also characterized. The results show that reflow process and PCT degrade the material properties and adhesion strength. Hygro-mechanical properties, i.e. coefficient of moisture expansion (CME) and saturated moisture concentration (C/sub sat/), are also measured. Based on the measured mechanical and moisture properties, a combined hygro-mechanical and thermo-mechanical stress modeling is performed on the FC-WB stacked die BGA package to compare three types of MUF materials at various temperatures (-40/spl deg/C,25/spl deg/C,121/spl deg/C, and 150/spl deg/C) PCT condition. It is observed that MUF-D3 material induces the lowest stresses on the die active surface. Die stresses induced by MUF with that of conventional mold compound and underfill materials are also compared. The analysis helps in material selection of MUF to enhance the die and package reliability of BGA module.

Solder joint failure modes, mechanisms, and life prediction models of IC packages under board level drop impact

Drop impact performance of solder joints of IC packages becomes a great concern for handheld products, such as mobile phones and PDA. Failure modes of solder joints under drop impact depend on solder alloys, interfacial strength, intermetallic formulation, and etc. Submodeling technique is applied to model detailed structure of critical solder joint. The stress and strain concentration at different locations of solder joint correlate well with failure modes observed during testing. Eutectic solder joint is more susceptible to bulk solder failure while Sn-4Ag-0.5Cu is more susceptible to intermetallic compound (IMC) layer failure. Softness of Sn-37Pb reduces the stress in IMC while increases the plastic strains in bulk solder. Life prediction model is determined by solder joint failure mode and mechanism. Stress criteria is suitable for IMC interfacial brittle crack while plastic strain criteria should be applied for life prediction of bulk solder ductile failure.

Characterization and modeling of hygroscopic swelling and its impact on failures of a flip chip package with no-flow underfill

Moisture plays an important role in the integrity and reliability of plastic electronic packaging. While the basic procedures for characterizing the hygroscopic swelling properties has been detailed in a number of papers, the characterization of packaging materials presents unique challenges that are not adequately addressed in these papers. One of those challenges is to investigate the impact of non-uniform moisture distribution across the specimen using currently available metrologies. In this paper, an analytical coefficient of hygroscopic swelling was derived based on three dimensional actual moisture distribution using diffusion law and the corresponding hygroscopic deformation. The mathematical formulation was then transformed to analyze the experimental data to obtain the accurate material property of hygroscopic swelling. The theoretical predication showed a remarkable agreement with the experiment results. Detailed discussions are given on the effect of test specimen aspect ratio, diffusivity, and time range in collecting the test data. A practical guideline is proposed in conducting the experiments. Based on the accurate coefficient of hygroscopic swelling, the second part of this paper assessed the reliability of a flip-chip ball grid array package with no-flow underfill under the pressure cooker test (PCT) conditions. Finite element modeling implementation was used to analyze the moisture distribution, hygroscopic swelling behavior, and thermomechanical stress. The magnitude of tensile hygroswelling stress acting on UBM is found to be greater than the compressive thermal stress, and may cause the UBM opening failure during the PCT. Finite element results give an insight of the failure mechanism associated with moisture absorption

Transient Analysis on Hygroscopic Swelling Characterization Using Sequentially Coupled Moisture Diffusion and Hygroscopic Stress Modeling Approach

One of the challenges for the hygroscopic swelling characterization is to investigate the impact of non-uniform moisture distribution across the specimen using currently available metrologies. Our recent studies have shown that the current averaged approach might overestimate the coefficient of hygroscopic swelling as much as 250%. In our previous analysis, however, the hygroscopic stress induced deformation has not been taken into consideration. As a matter of fact, due to non-uniform moisture distribution during the test, the total measured deformation includes two parts—one is from the hygroscopic swelling itself, and the other is from the hygroscopic stress induced deformation. This paper provides a comprehensive study on the impacts of non-uniform moisture distribution and the effect of hygroscopic stress by using a sequentially coupled moisture diffusion and hygroscopic stress modeling approach. The results show that the hygroscopic stress induced displacement is relatively small, although the elastic strain caused by the hygroscopic stress accounts for about one third of the total strain. The results in our previous study are proved to be acceptably accurate, even through the hygroscopic stress induced deformation is neglected in the previous analysis.Copyright

Comprehensive Warpage Analysis of Stacked Die MEMS Package in Accelerometer Application

Packaging of MEMS (micro-electro-mechanical system) devices poses more challenges than conventional TC packaging, since the performance of the MEMS devices is highly dependent on packaging processes. A land grid array (LGA) package is introduced for MEMS technology based linear multi-axis accelerometers. Finite element modeling is conducted to simulate the warpage behavior of the LGA packages. A method to correlate the package warpage to matrix block warpage has been developed. Warpage for both package and sensor substrate are obtained. Warpage predicted by simulation correlates very well with experimental measurements. Based on this validated method, detailed design analysis with different package geometrical variations are carried out to optimize the package design. With the optimized package structure, the packaging effect on accelerometer signal performance is well controlled

4-dimensional design analysis and optimization of system-in-package

This paper is an overview of applications of CAE (computer-aided-engineering) in design for package and board level reliability of system-in-package (SiP). CAE is an efficient tool for virtual prototyping of complex SiP to save the development time and cost with understanding on the physics of failures. The paper highlights on five major reliability issues frequently encountered in the development of SiP, such as thermomechanical related package failures, matrix package warpage, moisture-induced package failures, board level solder joint reliability under thermal cycling test and drop test. For each individual topic, introduction, brief theory, and an example of application with correlation to experiment are given

Copper wirebond pull test and reliability characterization with finite element simulation

In this paper, we will compare Cu wire and Au wire behavior during pull test and package reliability test through thermo-mechanical simulation. Relationship between wire pull test and package reliability test, i.e. thermal cycling, is also evaluated in term of die stress underneath the wire bond pad area. A new stress index concept is proposed to characterize the overall die stress level underneath bond pad. Based on this concept, a new method to evaluate Cu pull test limit is established with benchmark to current Au wire standard. The methodology is demonstrated through a Cu wire bonded power package, with the extensive work of process development, reliability test, and stress simulation etc.

Hygro-thermo-mechanical modeling of mixed flip-chip and wire bond stacked die BGA module with molded underfill

Package reliability needs to be considered for the design of mixed flip-chip (FC)-wire bond (WB) stacked die BGA module with molded underfill (MUF). The success of the MUF application depends on its performance in thermal shock (TS) test and pressure cooker test (PCT). Mechanical properties (modulus and adhesion strength) of MUF after post mold cure (PMC), reflow and PCT are measured. Shear strength between die and MUF under various temperature and moisture conditions are also characterized. The results show that reflow process and PCT degrade the material properties and adhesion strength. Hygro-mechanical properties, i.e. coefficient of moisture expansion (CME) and saturated moisture concentration (Csat), are also measured. Based on the measured mechanical and moisture properties, a combined hygro-mechanical and thermo-mechanical stress modeling is performed on the FC-WB stacked die BGA package to compare three types of MUF materials at various temperatures (-40degC, 25degC, 121degC and 150degC) and PCT condition. It is observed that MUF-D3 material induces the lowest stresses on the die active surface. Die stresses induced by MUF with that of conventional mold compound and underfill materials are also compared. The analysis helps in material selection of MUF to enhance the die and package reliability of BGA module

Robust power package development with mechanical simulation and reliability validation

Thermo-mechanical reliability is one of the major concerns for electronic packages, especially for power packages operating in extremely harsh environment. As the trends towards high density and function integration, advanced power device becomes more sensitive to environmental stress. Comprehensive study is needed from design, process to test towards robust power package with high reliability. In this paper, we will demonstrate the successful application of simulation in the development of a series of robust leaded power packages. Firstly, finite element analysis(FEA) has been carried out to understand die stress behavior inside the package during assembly and reliability tests, i.e. from die attach, post mold cure, reflow to thermal cycling etc. Then DOE matrix is run to obtain the critical responses to different factors, which leads to guidelines on package design and material selection. A series of robust power packages have been developed with optimized package geometry and bill of materials.