Optimizing Flip Chip Substrate Layout for Assembly

Category: Flip Chip and Die Placement
Post Date: July 19, 2004
Author(s): Pericles Kondos, Peter Borgesen, Dan Blass, and Antonio Prats, Surface Mount Laboratory, Universal Instruments

Description
Programs have been developed to predict the expected yield of flip chip assemblies, based on substrate design and the statistics of actual manufactured boards (e.g. in pad sizes and locations, mask registration, etc.), as well as placement machine accuracy, variations in bump sizes, and possible substrate warpage. These predictions and the trends they reveal can be used to direct changes in design so that defect levels will fall below the acceptable limits. Shapes of joints are calculated analytically, or when this is not possible, numerically by means of a public domain program called Surface Evolver. The method is illustrated with an example involving the substrate for a flip chip BGA. It was found that the original design would lead to unacceptably high defect levels, but alternative designs significantly improved the yield without creating other significant problems like bridging.