This article describes the board-level drop reliability of thin-profile fine-pitch ball grid array (TFBGA) subjected to Joint Electron Device Engineering Council (JEDEC) drop test conditions featuring an impact pulse profile with a peak acceleration of 1500  G and a pulse duration of 0.5 ms. The solder ball is assumed to be an elastoplastic model and the other components linear elastic ones. Both the global/local finite element and the finite grid region methods are introduced to improve the accuracy and the convergence during the meshing process. Meanwhile, the contact impact process during the drop test is translated into the effective support excitation load on the printed circuit board (PCB) through the support excitation scheme to simplify the analysis. By means of optimal parameters of the Taguchi robust design, the average stress of the solder ball at the PCB side surface becomes 80.9 MPa, which shows a 57 per cent reduction compared to the original stress of 189.7 MPa. As a result, the impact reliability of the TFBGA package is significantly improved. Finally, the JEDEC drop test is conducted to verify the optimal results obtained by the Taguchi method.

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A global/local method with the modified sub-modeling approach of the two-parametric optimal equivalent volume solder balls is introduced to predict the deformation and reliability of the package. The equivalent solder balls as exhibit in this method can obviously reduce the required elements/nodes quantities to enhance computing efficiency. A package model of wire-bonded stacked chip ball grid array under cyclic thermal loading is used as a test vehicle to verify the influences of design factors by fatigue life indicator. Comparing the proposed method with the global fine mesh model, it is found that the difference in the accumulated strain energy density is merely 5.77%, but the optimal equivalent model has highly saved 90% finite element analysis required elements, which means the adopted method can effectively replace the global fine mesh model because both results are in accordance with each other. Using design of experiments to efficiently verify each factor influence with their crosscoupling effects, this paper adopts two kinds of response surface methods that confirm the fatigue life of the proposed approach can be improved by as much as 123.7% for the dual response surface method and 126.3% for the mixed response surface method when comparing with the baseline model. In addition, the optimization of generic algorithm for both response surface methods is demonstrated in this study. From the reviews of factor coupling effects, it is concluded that the response surface method is eligible to achieve the optimum design for package reliability improving.

SUMMARYIn recent years, the electronic industries tend to offer products with smaller scales, lower cost, larger storage space and integration of various functions. The development of redistributed chip package (RCP) technology is facilitated to reduce package size for three-dimensional integration and to enhance packaging capability for miniature requirements. Meanwhile, the logic unit is required to be combined with the memory unit to achieve miniaturization and system integration. The RCP with package on package (PoP) is constructed by stacking the RCP for the very fine pitch ball grid array at topside and the stack package ball grid array (SPBGA) at bottom side. The finite element software ANSYS is adopted in this study using the Global/Local modeling approach. The reliability of the RCP with PoP is subjected to a thermal cycle test of -40-125°C based on JEDEC specification. The Coffin-Manson strain-based model and the Morrow energy-based model are employed for prediction of package fatigue life, in which the SAC387 (95.5Sn3.8Ag0.7Cu) solder joints are treated as viscoplastic behavior according to the Anand constitutive model. The other materials are modeled as elastic behavior. To investigate the solder joint reliability for the stacked package design, a numerical experiment by means of the single factor analysis is first conducted for investigation. The material factors such as Young's Modulus and coefficient of thermal expansion, the geometric factors such as component thickness are evaluated. Accordingly, the significant factors are filtered and analyzed by the Taguchi method to obtain the optimal combination. As a result, the optimal design increases package fatigue life, which contributes a significant improvement by up to 86.5% when comparing with the original model

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