Purpose - Anisotropic conductive adhesive film (ACF) is used for very fine pitch applications in the microelectronics industry, such as flip chip (FC) technology. During the bonding process, bumps on the chip and pads on the substrate are first aligned and then heat and pressure are applied so as to apply thermal energy to the ACF for curing and to cause permanent plastic deformation of the conductive particles. Consequently, a permanent electrical and mechanical contact is formed between the bumps and the pads. The purpose of this study is to investigate the effect of the size and location of any voids in the ACF during subsequent solder reflow processes necessary for SMT component attachment. The paper also investigates the use of a protective aluminium cover during such reflow cycles, which reduces the temperature inside the ACF, and therefore, the stresses inside the ACF, especially when voids exist. Design/methodology/approach - In this study, the ACF is a temperature dependent material having various void sizes, entrapped within the ACF. A finite element method is used to analyse the stresses within a FC on flex assembly. Findings - The results indicate that the smaller the void, the larger the stress concentration. Also, the von Mises stress was found to be larger in the upper portion of the ACF, near the chip, than in the lower portion of the ACF nearer to the flexible substrate. This implies that the four corners of a void are seen to be the most likely site for crack initiation and propagation and therefore, for failure to occur. Moreover, the temperature profile of the reflow cycle and the locations of the voids have also been shown to affect the stress level within the ACF. Originality/value - The value of this paper is to show how the presence of voids may affect the reliability of a FC assembly. [ABSTRACT FROM AUTHOR]