Contactless Component Handling on PCB Using EWOD Principles

As the development of microelectronics is still driving towards further miniaturization, new materials, processes and technologies are crucial for the realization of future cost effective microsystems and components. Futures ICs and passives will also decrease in size, e.g. for RF-ID applications forecast die sizes are smaller than 250 ?m, thicknesses less than 50 ?m and pitches way below 100 ?m. Passives, if not directly integrated into the system carrier, will be even smaller. Touchless and self-assembly based procedures seem to be a promising method for handling miniaturized components not directly fabricated at the very place where they are needed. Based on the "electrowetting on dielectrics" effect (EWOD) - a contactless handling technology well known from lab-on-chip applications for liquid transport, sorting, mixing and splitting - is used as a basis for microelectronics assembly purposes on standard printed circuit boards. Handling shall be feasible for miniaturized components as duplets, smallest SMDs as well as for nano-scaled building blocks. The physical principle is a change in the droplet contact angle of a droplet when immersed into an electrical field, an effect that can be used for droplet movement and potentially for component transport. The process flow under evaluation starts with positioning of a droplet, containing a component, on a hydrophobic surface of the carrier substrate with rough accuracy. Using the mentioned electro wetting effect the droplet will be moved quickly until the desired position is reached. The precise placement of the droplet in ?m range takes place by means of field gradients and local manipulation of the carrier surface. The assembly is finished with the evaporation of the component containing droplets and the transfer of all components to the final substrate. The experimental work on EWOD described in this paper includes electrical layout, substrate manufacturing, hydrophobic surface modification and droplet handling in- combination with a process simulation. The electrowetting conveying system is simulated using the Multi Body Dissipative Particle Dynamics method (MDPD), where clusters of fluid molecules are represented by coarse grained particles. Wetting behavior is introduced by position-fixed wall particles: the force between a wall and a fluid particle is adapted such that the required contact angle emerges. The electrowetting model uses the Lippmann equation to find the influence of the applied voltage on the wetting behavior, i.e., the attractive forces between wall and fluid particles are modified to simulate the electrostatic forces on the contact line. The micro parts are also simulated by connected particles with special interaction forces for (almost) rigid body motion.