Super Ball Grid Array Research Articles

A Global–Local Approach for Mechanical Deformation and Fatigue Durability of Microelectronic Packaging Systems

Abstract This paper presents a global–local methodology for predicting mechanical deformation and fatigue durability of solder joints in electronic packaging systems subject to cyclic thermal loading. It involves a global deformation analysis, a local critical solder–joint analysis, and a fatigue life analysis. The global deformation analysis includes a new optimization formulation for determining an equivalent model. The methodology developed was applied to fine pitch ball grid array (fpBGA) and super ball grid array (SBGA) packages. Selective experimental efforts were also undertaken to evaluate the predicted deformation characteristics of the fpBGA package. A good agreement was obtained between the simulated deformation results and experimental observations. For the durability analysis, the total fatigue life predicted using the energy-based method is larger than 2500 cycles—a trend observed experimentally for both packages entailing widely different designs. Based on proposed modeling and simulation results and package designs studied, the SBGA package is more durable than the fpBGA package.

Mixed convection cooling of horizontally mounted printed circuit board

Mixed convection is a fundamentally significant heat transfer mechanism that occurs in a selection of industrial and technological applications. For example, it is of great interest to thermal designers in the field of electronic cooling. Given that its occurrence is frequent, there is a necessity to understand the physics of this fundamental mechanism at board level. Therefore, this paper is concerned with the development of optical measurement techniques for mixed convection, in order to try and improve the current understanding of why, where, and when mixed convection occurs. It is motivated both by a desire to gain an understanding the fundamental physics of mixed convection and to solve design problems in electronic cooling applications where mixed convection often occurs. It is focused on heated horizontal ribs, which are two dimensional approximation of a super ball grid array, mounted on a printed circuit board in cross-flow at a low Reynolds number, high Grashof numbers and at the Prandtl number for air. This paper describes two types of data from a series of experiments. Firstly, a physical description of the flow is discussed through interferometric images and Particle Image Velocimetry (PIV) streamlines with the objective of improving the understanding of mixed convection heat transfer from board mounted components. Secondly, explanation is given as to how the mean Nusselt number varies in terms of rib position and power input.

Thermomechanical reliability of underfilled BGA packages

The effect of underfill on various thermomechanical reliability issues in super ball grid array (SBGA) packages is studied in this paper. Nonlinear finite element models with underfill and no underfill are developed taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperature-dependent, while other materials are modeled temperature and direction-dependent, as appropriate. The stress/strain variations in the package due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, time-dependent creep strain and total inelastic strain in solder balls. The effect of copper core on the solder ball strains is presented. The possibility of delamination at the interposer-underfill interface as well as substrate-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA packages. The predicted number of thermal cycles to solder joint fatigue are compared with the existing experimental data on similar nonunderfilled BGA packages.