Study of adhesive flip chip bonding process and failure mechanisms of ACA joints

Abstract

The flip chip bonding process using anisotropic conductive adhesives (ACA) and the consequent joint reliability were studied. The substrates used were rigid FR-4 boards, which are interesting due to their low cost and wide range of applications. The problems associated with the technique are discussed in this paper from the reliability point of view. Also, some aspects concerning production are introduced.The reliability of the joints was studied by accelerated environmental tests. A temperature cycling test was performed between temperatures −40 and +125 °C. Constant humidity testing was conducted at 85 °C and RH85%. In addition, reflow aging tests were performed using a conventional Sn/Pb reflow profile. For reducing the bonding cycle time, a two-stage curing process was used, which also utilizes the reflow process.The results show that the three bonding parameters, temperature, time, and pressure, all affect joint reliability. Most detrimental, however, seems to be reflow treatment performed after bonding. Most failures occurred only very briefly during the temperature cycling at the moment the temperature changed, while the joints were still conducting at both temperature extremes. However, a different failure mechanism caused a different kind of behavior during temperature cycling. The relationship between the failure modes and the failure mechanisms was studied using a scanning electron microscopy.