Journal of SMT Article

Void Reduction in Reflow Soldering Processes by Sweep Stimulation of PCB Substrate - Influence of Eigenmodes

Author: Viktoria Rawinski
Company: Kurtz Ersa
Date Published: 6/30/2018   Volume: 31-2

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 void-free 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 time1.

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 time2.

The decisive thing for the void minimization process is the stimulation of a sufficient number of self-resonances. For this, simulation models of the eigenmodes and numerous examinations on the actual circuit boards were held. Amongst others, the influence of increasing the e-module in each individual resonance frequency stimulated in the sweep and the transmission of energy of the sweep actuation resulting from this on the void minimization rate were analysed. Here, there is a direct connection between the number of eigenmodes and the void minimization result.

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