Industrial flip-chip process for CSP-3D from design to manufacturing

In order to respond to need for miniaturization, Valtronic has industrialized a new flip-chip process which is well suited to organic substrates. This paper describes industrial application of this flip-chip process using gold stud bump connections with a nonconductive paste (NCP); the resulting connections are not soldered to the PWB pads, but are based on purely mechanical contact. This technique does not require any further operation such as underfilling once the die is flipped, and is suitable for organic substrates where TCE mismatches are critical concerns. In addition, it can be applied to both production runs and prototyping. Die bumping can be easily processed on either a whole wafer or a single part, and does not require any pad preparation of the pads such as under-bump metallization (UBM) before bumping. As the bumps are crushed on the PWB electrodes, they adapt their shape to the surface where the contacts are located, allowing flip-chip on flex PWB where planarity is sometimes critical. The process was applied to electronic modules for hearing aids, where extreme miniaturization is essential, and a new CSP-3D package was developed. First, dice are connected to the top face of a flat flexible substrate, then miniature SMT components are soldered on the bottom face, and the third dimension is obtained by bending and/or rolling the substrate on itself, accomplishing a 3D package. Finally, the package is filled with epoxy if required. These techniques are being used for a wide range of applications, such as CSP and MCM.