Methodology for Predicting C4 Non-Wets during the Chip Attach Process

Balancing the level of substrate warp at reflow with other sources contributing to C4 non-wets is an important problem. To address this, a methodology to predict the probability of non-wets during the chip attach process of an organic package has been developed. A technique for quantifying the convex or concave warp of a substrate in the form of a Shape Inversion (SI) plot is introduced. Geometrical factors that influence non-wets such as C4 height, the pad’s relative location, collapsed solder height etc. are described and their individual contributions to the non-wet conditions are computed. Combining these contributions onto the SI plot allows for a graphical representation of the non-wet probability. The technique is applied to a product substrate and the results compared with the actual yield observed during chip assembly.