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Void Reduction in Reflow Soldering Processes by Sweep Stimulation



Void Reduction in Reflow Soldering Processes by Sweep Stimulation
The insights of the feasibility study were the basis for the integration of the voidless technique into an industrial production process.
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Authored By:


Viktoria Rawinski
Ersa GmbH
Wertheim, Germany

Summary


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 multifold tests were made, contrasting the influence of the process duration as well as the influence of the different types of sweeps.

Conclusions


It was shown that the influence of the oscillation on the PCB during the soldering process dependent on the test parameter caused a significant improvement in the void rate of the soldering joints. The research project showed positive results concerning the influence of a sinusoidal sweep actuation on the minimization of the void rate. In the project, 60 identical PCBs with three identical components were analyzed. The influence of different parameters like sweep duration, width, and the influence of the tension of the actuation could be analyzed. It was possible during the research project to reduce the rate of residual voids from 9.09% in the reference PCB to 1.86% using the right actuation parameters. This is a decrease in the void rate of about 83%. Further valuable findings during the research project: A shorter sweep duration causes the formation of smaller voids in the soldered area. A longer sweep duration causes the formation of few bigger voids that gather on the edge of the component. A larger sweep width causes a decrease in the void rate in the soldering joints. An increase in the tension of the actuator has a positive influence on the reduction of voids.

The results of the feasibility study were the basis of the integration of the voidless process into an industrial soldering process. The influence of the process time on the residual void rate of the liquid solder joint plays a major role for the integration into an industrial process. Equally important is the choice of the right sweep modulation. Components with a low weight and a relatively large soldering area the voidless process with a logarithmic sweep actuation has proven to be favorable. This kind of modulation reaches a sufficient relative movement of the liquid solder, which causes an effective reduction of the residual void rate.

Initially Published in the SMTA Proceedings

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