Flip Chip on Board and Flip Chip in Package
The microscope photos of cross sectioned flip chip (top picture) assemblies illustrate one method of soldering flip chips with wafer-applied underfill to a printed circuit. Here the printed circuit copper pads are completely undefined by solder mask, allowing solder from the solder bumps to totally engulf the copper pads.
The cross section (bottom picture) illustrate another method. Here the copper pads are entirely covered with solder mask and laser ablation is used to selectively remove the mask from the exact locations desired. This defines the size and location of the solder mask opening for the soldering pad with great precision. In the most demanding high-reliability and finest pitch applications, such precise solder pad definition insures that every pad is optimally configured so as not to deplete solder from the solder bump excessively. In both cases, the photos illustrate flip chips fabricated using our wafer-level applied underfill encapsulation. This creates taller column-shaped solder joints rather than conventional short "barrel-shaped" solder joints. The bump pitch is 250 microns.