A need for higher packaging density and functionality has increased the use of new packaging technologies, which has also caused demand for higher interconnect densities on printed circuit boards (PCBs). Sequential build-up (SBU) processes can be used to meet these demands. In the SBU process, additional dielectric and conductor layers are formed on a core board, which is typically made of FR-4. Microvias are formed on these layers to achieve an electrical connection between them and the core board. Resin-coated copper foil (RCC) is the most widely used dielectric layer in the SBU process. The effect of RCC on the reliability of flip chip joints with anisotropically conductive adhesive film (ACF) was studied. Two substrates were used. The difference between the substrates was RCC laminated on the other substrate. The reliability of the test samples was studied using a temperature cycling test and a constant humidity test. The reliability of the substrate with the RCC was found to be better in both tests. Failure mechanisms were studied after the tests, using optical and scanning electron microscopes. After the temperature cycling, several of the test samples made with two highest bonding pressures showed delamination, which has probably caused the failures. In addition, failures occurred during the changes in the test temperature. These were probably caused by warping of the flip chip package. No delamination was found in the test samples with the lowest pressure. The failures in these series were probably caused by relaxation of the adhesive matrix and by too low deformation of the conductive particles. Several cracks had formed on the FR-4 substrates without the RCC during the temperature cycling. In addition, air bubbles were found in the test samples with the FR-4 substrates without the RCC. Since RCC is a pure resin system, it has a high coefficient of thermal expansion, which may cause problems, especially when large components are attached to it. However, in this study, the RCC was found to increase the reliability of the flip chip joints made with ACF during both temperature cycling and constant humidity testing.