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Ball grid array solder joint reliability is known to be dependent on the shape of solder joints after reflow. To ensure good solder joint formation and prevent solder bridging, it is critical to understand the amount of paste volume needed during assembly and reflow. The final shape of the solder joint is a function of surface tension, wetting area, gravity, and applied forces. In this paper, a new methodology to simulate solder joint shape is presented. Large deformation viscoplastic finite element analysis is used to simulate incompressible fluid flow. A numerical model for surface tension is outlined and validated with closed-form solutions. The results of the numerical model are compared to other known solder joint shape prediction methods. The effects of package weight, coplanarity, warpage, paste volume, pad misregistration, and joint construction on solder joint shape are then analyzed. Recommendations are provided on ways to maximize standoff height and avoid bridging. Finally, the formation of leadless solder joints is studied and compared to experimental data.

A comprehensive study was undertaken to evaluate various board design parameters, assembly and reliability of 0201's as it relates to high reliability products. The design parameters considered included four pad designs, two trace width terminations, three orientations, three board thicknesses and four component to component spacing on a large PWB panel. The assembly parameter evaluation included, SnPb and Pb-free solder pastes for board assembly, which were built at four different board assembly locations. The reliability testing included ATC, shear and mechanical shock testing. The purpose of this study was to evaluate the design space that will result in high assembly process yield and high solder joint reliability. A 0402 control leg was included in the design for comparison.About 400,000 of the 0201 components were assembled for this study. The assembly and design parameters were evaluated on all three board thicknesses (62 mils, 93 and 126 mils). Among the four pad designs two of the designs were intended for under the BGA placement (via to via). Among the remaining four pad designs, two of them showed better assembly process yields than the rest. Within the design space considered, no significant trends in assembly yield were observed due to trace terminations, component to component spacing and board thicknesses.For the reliability testing the boards were temperature cycled between 0-100°C per IPC9701. Shear testing was performed prior to and after the temperature cycling testing to observe any deterioration in the solder joint reliability for any of the pad designs considered. Mechanical shock testing at 340G's 1.6ms input pulse was also conducted for the different design parameters.

Large Flip Chip BGA (FCBGA) packages are needed in high pin out applications (>1800), e.g., ASIC's and are typically used in high reliability and robustness applications. Hence understanding the package reliability and robustness becomes one of paramount importance for efficient product design. There are various aspects to the package that need to be understood, to ensure an effective design. The focus of this paper is to understand the BGA reliability of the package with particular reference to comparison of the surface finish, vis-a`-vis, between Electroless Nickel Immersion Gold (ENIG) and Solder On Pad (SOP) on the substrate side of the package, which are the typical solutions for large plastic FC-BGA packages. Tests, which include board level temperature cycling, monotonic bend and shock testing have been conducted to compare the two surface finish options. The results of these tests demonstrate that the mechanical strength of the interface exceeds by a factor of two for the SOP surface finish, while BGA design parameters play a key role in ensuring comparative temperature cycle reliability in comparison with ENIG packages.

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