Reliability of ACF in flip-chip with various bump heights

Anisotropic Conductive Film (ACF) is being widely used in flip-chip (FC) technology because of its fine pitch characteristic and can also serve as an environmentally friendly joining material. The bumps of the FC are usually made from nickel. Since pressure is applied to force the conductive particles to make a contact between the nickel bumps and the copper pads in printed circuit boards (PCB) or flexible circuits (flex), the deformation of the bump can affect the electrical conductivity. In particular, the height of the bumps can change the deformation characteristics, and hence the reliability of the interconnection in the FC assembly. In this study, FC assemblies on PCB and flex, with bump heights of 4, 20 and 40 μm, and with a constant copper pad height were considered. During the bonding process, a pressure of 4.5 kg/mm² and temperature of 180°C were applied to the top surface of the FC for 30 sec. Moreover, a thermal cycle from -40 to 125°C was also applied to the assemblies with various bump heights. The stress distributions of the assemblies were analyzed. It was found that the largest stress occurred when the height of bump is smallest. Also, the stresses in the ACF of the FC-on-flex assembly are generally larger than those in the FC-on-PCB assembly