Void Reduction In Reflow Soldering Processes By Sweep Stimulation of PCB Substrate-Influence of Solder Paste
Author: Viktoria Rawinski Company: Ersa GmbH Date Published: 2/6/2017
Pan Pacific Symposium
Abstract: Due to the ongoing trend towards miniaturization of power components, the lossless thermal conductivity of solder joints in SMT-processes gains more and more importance. Therefore, the role of voidfree solder joints in power electronics becomes more central. Voids developing during soldering reduce the actual thermal transfer and can cause thermal damage of the power components up to their failure. For this reason, Ersa GmbH has developed a new technique to minimize the formation of those voids during the soldering process and has tested its practicability in industrial soldering processes as well as its influence on process time. The result of this development is a universal technique to reduce voids in the liquid solder between component and PCB by applying a mechanic sinusoidal actuation. Primarily the PCB is stimulated by a longitudinal wave with an amplitude of less than 10 µm on the PCB level. During this sinusoidal actuation of the PCB in a defined frequency range, the self-resonances of this area are stimulated regardless of the PCB layout. The low starting frequency of the sweep stimulation ensures a gentle, homogeneous propagation of the vibrations in the PCB, without damaging the molecule chains (f.e. in FR-4). The intensification of the frequency causes a stiffening of the PCB substrate, an increase in the elastic modulus and, because of the reduced damping factor, an improved energy transmission of the liquid solder. Thereby areas with a low density, so-called voids are moved out of the solder joint by the vibration. Since a sinusoidal actuation of the PCB in a defined frequency range is actuated over the complete spectrum of this range, all the self-resonances of the PCB in this frequency range are stimulated, too. By this, the liquid solder is stimulated repeatedly by the vibration propagation in a relative shearing motion leading to a reduction of voids in the solder joint. The sweep stimulation onto the components is absorbed mostly by the liquid solder, which protects the components from damage caused by vibration transfer. Positive side effects of the sweep stimulation are the centering of the components on the pad and an optimized spreading of the solder on the pad. The process of void minimization takes place within seconds, without causing any significant increase in cycle time. The insights of the feasibility study were the basis for the integration of the voidless technique into an industrial production process. The quality of the solder joint concerning the voidrate is strongly influenced by the duration of the voidless process. Therefore manifold tests were made, contrasting the influence of the process duration as well as the as the influence of the different types of sweeps. A further decisive factor for the void rate in the solder joint and its reduction is the selection of a suitable solder paste. Numerous types of solder pastes composed of diverse alloys as well as fluxes and additives are available on the market. Due to the varying composition of the solder pastes' components, the solder pastes do not only influence the solder joint's stability and elastic behavior but also the formation of gaseous inclusions during soldering. Within a further study different solder pastes were examined testing their influence on the void rate in solder joints using soldering processes with and without nitrogen atmosphere. The void reduction potential of the different solder pastes in an industrial soldering process by means of a sinusoidal sweep was also analyzed.