Industrial approach of a Flip-Chip method using the stud-bumps with a non-conductive paste

As the use of lead in electronics won't be possible anymore in the coming years, research institutes and packaging industries are focussing on alternative materials for Flip-Chip assembly. Various conductive and non-conductive adhesives have been developed in recent years: today, they are mature enough and are excellent candidates to be implemented in a wide range of lead-free Flip-Chip applications. This report will describe an industrial approach of a Flip-Chip method using the gold Stud-Bumps connections with a Non-Conductive Paste (NCP) : the resulting connections are not soldered with the pads of the Printed Circuit Boards (PCB), but are based on a pure mechanical contact. This original and low-cost technique does not require any additional operation such as underfilling once the die is flipped, and is particularly suitable for organic substrates where TCE mismatches are critical concerns. In addition, it can be applied to large production runs as well as prototyping. Actually, the bumping of the dice can be processed on either a whole wafer or a single part, and the NCP is easily placed onto the substrate using a standard dispensing machine. A test vehicle was developed to evaluate the reliability of the packages : excellent performance was observed, especially with regard to thermal and humidity tests; these results will be presented in this report. Thanks to the fact that the bumps are crushed on the pads of the PCB, they adapt their shape perfectly to the surface where the contacts are located, allowing Flip-Chip on flex PCB where planarity is sometimes critical. The process was first applied to electronic modules for hearing aids, where extreme miniaturization is essential for that, a 3D package was developed using Flip-Chip on a flat flexible substrate that is rolled on itself at the final step of the production. Today, these techniques are being used for a wide range of applications, such as Chip-Scale Packages (CSP) and Multi-Chip Modules (MCM) including high i/o count dice.