Industrial approach of a flip-chip method using the stud-bumps with a non-conductive paste

As the use of lead in electronics will not be possible any more in the coming years, research institutes and packaging industries are focusing 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 paper describe an industrial approach to 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. 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 are presented in this paper. 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. 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.