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Determination of a Meaningful Warpage Acceptance Criterion for Large PBGA Components Through the Correlation with Scattering in Material Properties

Warpage deformation under thermal loading has been a well-known problem in plastic ball grid array (PBGA) packages because of their overmold structure and thin substrate. The excessive warpage of PBGAs during the reflow of surface mount assembly may not only cause manufacturing yield problems but could also lead to solder joint failures due to residual stresses. Because of the mismatch in coefficients of thermal expansion (CTE) among packaging materials, it is impossible to eliminate thermal warpage. Nevertheless, if a certain warpage acceptance criterion can be established, screening could at least be performed before the board level assembly or even measures introduced for warpage compensation so that the threats to yield and reliability could be reduced. Therefore, during the mass production process, it would be desirable to identify a definite value of warpage of a whole batch of components. However, in reality, substantial scattering in the data of warpage measurement is quite common. Production engineers often feel very frustrated due to the fact that they are not able to determine a meaningful warpage acceptance criterion for the subsequent action. Thus, an effective methodology for the aforementioned issue is in strong demand. This study investigated the uncertainty in the data of warpage measurement and deduced that the root cause of uncertainty is mainly due to the scattering in packaging material properties. A systematic approach is presented to estimate the warpage value of large PBGA packages and its statistical range from the material property point of view. The finite element method is used to predict the PBGA warpage range. With a known mean value and range of material properties, a meaningful warpage acceptance criterion with its corresponding distribution range is determined. The accuracy of the proposed method is also assessed. From the comparison between the simulation and experimental results, the current approach appeared to be valid both qualitatively and quantitatively. It is believed that the proposed methodology should be useful not only for the board level assembly of large PBGA packages, but also for the emerging fan-out panel level packaging.