D. Wheeler

An integrated modeling approach to solder joint formation

The attachment of electronic components to printed circuit boards using solder material is a complex process. This paper presents a novel modeling methodology, which integrates the governing physics taking place. Multiphysics modeling technology, imbedded into the simulation tool-PHYSICA is used to simulate fluid flow, heat transfer, solidification and stress evolution in an integrated manner. Results using this code are presented, detailing the mechanical response of two solder materials as they cool, solidify and then deform. The shape that a solder joint takes upon melting is predicted using the SURFACE EVOLVER code. Details are given on how these predictions can be used in the PHYSICA code to provide a modeling route by which the shape, solidification history, and resulting stress profiles can be predicted.

Computational modeling techniques for reliability of electronic components on printed circuit boards

This paper describes modeling technology and its use in providing data governing the assembly and subsequent reliability of electronic chip components on printed circuit boards (PCBs). Products, such as mobile phones, camcorders, intelligent displays, etc., are changing at a tremendous rate where newer technologies are being applied to satisfy the demands for smaller products with increased functionality. At ever decreasing dimensions, and increasing number of input/output connections, the design of these components, in terms of dimensions and materials used, is playing a key role in determining the reliability of the final assembly. Multiphysics modeling techniques are being adopted to predict a range of interacting physics-based phenomena associated with the manufacturing process. For example, heat transfer, solidification, marangoni fluid flow, void movement, and thermal-stress. The modeling techniques used are based on finite volume methods that are conservative and take advantage of being able to represent the physical domain using an unstructured mesh. These techniques are also used to provide data on thermal induced fatigue which is then mapped into product lifetime predictions.

A computational modelling framework to predict macroscopic phenomena in solder joint formation

Abstract A computational model of solder joint formation and the subsequent cooling behaviour is described. Given the rapid changes in the technology of printed circuit boards, there is a requirement for comprehensive models of solder joint formation which permit detailed analysis of design and optimization options. Solder joint formation is complex, involving a range of interacting phenomena. This paper describes a model implementation (as part of a more comprehensive framework) to describe the shape formation (conditioned by surface tension), heat transfer, phase change and the development of elastoviscoplastic stress. The computational modelling framework is based upon mixed finite element and finite volume procedures, and has unstructured meshes enabling arbitrarily complex geometries to be analysed. Initial results for both through-hole and surface-mount geometries are presented.

Numerical modelling and validation of Marangoni and surface tension phenomena using the finite volume method

Surface tension induced flow is implemented into a numerical modelling framework and validated for a number of test cases. Finite volume unstructured mesh techniques are used to discretize the mass, momentum and energy conservation equations in three dimensions. An explicit approach is used to include the effect of surface tension forces on the flow profile and final shape of a liquid domain. Validation of this approach is made against both analytical and experimental data. Finally, the method is used to model the wetting balance test for solder alloy material, where model predictions are used to gain a greater insight into this process. Copyright

Numerical modelling of solder joint formation

Solder materials are used to provide a connection between electronic components and printed circuit boards (PCBs) using either the reflow or wave soldering process. As a board assembly passes through a reflow furnace the solder (initially in the form of solder paste) melts, reflows, then solidifies, and finally deforms between the chip and board. A number of defects may occur during this process such as flux entrapment, void formation, and cracking of the joint, chip or board. These defects are a serious concern to industry, especially with trends towards increasing component miniaturisation and smaller pitch sizes. This paper presents a modelling methodology for predicting solder joint shape, solidification, and deformation (stress) during the assembly process.

Using computer models to identify optimal conditions for flip‐chip assembly and reliability

Flip‐chip assembly, developed in the early 1960s, is now being positioned as a key joining technology to achieve high‐density mounting of electronic components on to printed circuit boards for high‐volume, low‐cost products. Computer models are now being used early within the product design stage to ensure that optimal process conditions are used. These models capture the governing physics taking place during the assembly process and they can also predict relevant defects that may occur. Describes the application of computational modelling techniques that have the ability to predict a range of interacting physical phenomena associated with the manufacturing process. For example, in the flip‐chip assembly process we have solder paste deposition, solder joint shape formation, heat transfer, solidification and thermal stress. Illustrates the application of modelling technology being used as part of a larger UK study aiming to establish a process route for high‐volume, low‐cost, sub‐100‐micron pitch flip‐chip assembly.

Solder paste reflow modeling for flip chip assembly

Solder paste printing and reflow can provide low cost techniques for producing the solder bumps on flip chips. Solder paste consists of a dense suspension of solder particles in a fluid medium (vehicle) that acts as an oxide reducing agent (flux) during reflow, cleaning the metal surfaces of oxides. This paper reports on optical observations of paste behaviour at the small length scales associated with flip chip solder joints, and attempts to model the process using computational fluid dynamics (CFD). Comparison of optical observations and CFD modelling show that the behaviour of the solder cannot be described simply by surface tension and viscous flow effects and it is deduced that oxides are still present on the solder surfaces during the early stages of reflow. The implications for the paste heating method and solder volume are discussed, and a preliminary CFD model (based on FIDAP) incorporating the effect of the oxide layers is presented.

An integrated modelling approach to solder joint formation

The attachment of electronic components to printed circuit boards is a complex process where solder material undergoes a number of physical processes. This paper presents a modelling methodology which aims to integrate the governing physics taking place during the reflow process. A multi-physics simulation tool, Physica, is described which has the ability to simulate fluid flow, heat transfer including solidification, and stress evolution in an integrated manner. Results using this code are presented, detailing the mechanical response of two solder materials as they solidify and cool during the reflow process. Surface Evolver is a program used to predict the shape that a specific solder volume takes during joint formation. Details are given on how this code has been coupled with the above computational mechanics code. This coupling now provides a modelling route by which the shape, solidification history, and resulting stress profiles can be simulated in an integrated manner. Some preliminary results from this modelling framework are shown.

Modelling technology to predict flip-chip assembly

This paper describes modelling technology and its use in providing data governing the assembly of flip-chip components. Details are given on the reflow and curing stages as well as the prediction of solder joint shapes. The reflow process involves the attachment of a die to a board via solder joints. After a reflow process, underfill material is placed between the die and the substrate where it is heated and cured. Upon cooling the thermal mismatch between the die, underfill, solder bumps, and substrate will result in a nonuniform deformation profile across the assembly and hence stress. Shape predictions then thermal solidification and stress prediction are undertaken on solder joints during the reflow process. Both thermal and stress calculations are undertaken to predict phenomena occurring during the curing of the underfill material. These stresses may result in delamination between the underfill and its surrounding materials leading to a subsequent reduction in component performance and lifetime. Comparisons between simulations and experiments for die curvature will be given for the reflow and curing process.

Predicting the movement of voids in solder bumps and subsequent reliability [flip chip assembly]

This paper describes modelling technology and its use in providing data governing the assembly and subsequent reliability of electronic chip components on printed circuit boards (PCBs). Products such as mobile phones, camcorders, intelligent displays, etc., are changing at a tremendous rate, where newer technologies are increasingly being applied to satisfy demands for smaller products with increased functionality. At ever decreasing dimensions, and increasing number of input/output connections, the design of these components, in terms of dimensions and materials used, is playing a key role in determining the final assembly reliability. Multiphysics modelling techniques are being adopted to predict a range of interacting physics-based phenomena associated with this manufacturing process. For example heat transfer, solidification, Marangoni fluid flow, void movement, and thermal stress. The modelling techniques used are based on finite volume methods that conserve the physics and take advantage of being able to represent the physical domain using an unstructured mesh. These techniques are also being used to provide data on thermally induced fatigue, which is then mapped into product lifetime predictions.