No abstract

The increasing demand for electronic devices associated with the increasing competitiveness between enterprises, pushes towards process automation to decrease production costs. The reflow soldering has proven to be effective in this regard. This is composed by a series of steps or processes, such as: (a) stencil printing, (b) component placement and (c) reflow oven soldering. Each process has its specific traits that contribute to the overall process efficiency. The present study is directed towards process (a), which includes the rolling of the solder paste over the stencil surface, followed by the subsequent filling of the stencil apertures. Several parameters influence the solder paste behaviour and thus the effectiveness of the rolling process. This work focuses on the solder paste non-Newtonian viscosity properties, with the solder paste presenting a thixotropic behaviour, necessary for the filling of the stencil apertures. Although the increase in the squeegee velocity causes extra shear in the solder paste and consequently lower viscosity, the excess of velocity may cause defects in the aperture filling process. In addition, during the rolling process, air may become entrapped in the solder paste. The complexity of this process is addressed by numerical simulation, in particular, using the work-package ANSYS to study the solder paste progress, during the rolling process, as well as the parameters influencing it. The fluid flow simulation is solved using the solver FLUENT®, a simplified 2D domain with real case dimensions, a transient prediction of the viscosity, which is a function of the solder paste solicitation, and finally by using the Volume of Fluid (VOF) method to track the solder-air interface boundary. Dynamic meshing methods are also employed to replicate the movement of the squeegee wall, in its task to push the solder paste tumble over the stencil. This study enlightens the role played by the printing velocity in the stencil aperture filling, a logarithm correlation can be found between them. It was found that lower print velocities provide better results than higher speeds. It was observed that the back tip of the squeegee blade causes a partial removal of the solder paste from the aperture, which is higher for faster print processes. An analysis of the filling process over time concluded that, independently of the printing velocity, 90% of the filling occurs in the first quarter of the process.

The exposure of miniaturized components to the environment leads to new failure analysis as a result of environmental conditions and constant innovation of the component materials and dimensions. Generally, these failures occur on the solder joints, which ensure the mechanical and electrical connection between the printed circuit board and the component. Conformal coating is a thin layer of a polymeric material that encapsulates the components and consequently their solder joints to protect against harsh environments. However, this application is not recommended in some components such as ball grid array and quad flat no-lead (QFN) packages, since it can reduce the reliability of the solder joints when exposed to temperature fluctuations. Therefore, by using a finite element analysis, a thermal cycle test was simulated with and without conformal coating. The simulation output was extrapolated to lifetime theoretical methods with the aim of predicting the number of cycles until the failure of the solder joints. Therefore, this study demonstrates that for both components without conformal coating, solder joint lifetime was a precise approximation. Coated solder joints reveal a drastic reduction in their reliability due to the influence of the conformal coating behavior and its thermomechanical properties.

The increased demand for smaller and more reliable electronic devices, pressures companies to tune and innovate the production methodologies, always aiming to decrease the production time while maintaining the products’ quality. In the manufacturing of a Printed Circuit Board - PCB, there are two main types of electronic components, namely, Surface-Mount-Devices and Through-Hole components. Both connections are achieved through soldering, an essential manufacturing process that greatly affects the quality of the final product, and may compromise the lifetime of the PCB. While SMD components are commonly soldered by a reflow process, TH components are generally soldered by a wave soldering process. However, a PCB is generally composed of both types of components. Since both processes are too different in nature, the manufacturing of the board requires two separated mounting lines which represent increased costs and production time. Taking this into account, the present study investigates the usage of the reflow solder printing to deposit the solder paste on through-hole apertures both experimentally and numerically. In general, the results were similar showing a tendency for the solder paste to deposit on the aperture’s right side. Nevertheless, the numerical model predicts a filling area greater (by 22%) than that verified experimentally.