Especially for medical implantables applications size reduction of electronic packaging is coupled with a high functionality and high reliability. In this paper an implantable microelectronic system will be discussed where all single ohip packages are chip size packages (CSPs) mounted on a 3D rigid-flex MCM-L substrate. The project was devided in three parts: system and product design, component engineering and assembly process. This paper will focus on the technological aspects of CSP and its assembly technology. The entire chipset of four AS/Cs was converted to CSPs based on a waferievel thin film redistribution process (CSP-WL). The CSPs and the passive SMDs were mounted with an automatic pick and place assembly unit. All the process steps from the redistribution of the four dice to the assembly on board will be discussed in details.