# Transient Numerical Modelling of the Pin-in-Paste Technology
###### Abstract  
The pin-in-paste technology is an advancing soldering technology for assembling complex electronic products, which include both surface-mounted and through-hole components. A computational fluid dynamics model was established to investigate the stencil printing step of this technology, where the hole-filling by the solder pastes is the most critical factor for acquiring reliable solder joints. The geometry of the transient numeric model included the printing squeeze, the stencil, and the through-holes of a printed circuit board with different geometries and arrangements. A two-phase fluid model (solder paste + air) was applied, utilizing the Volume of Fluid method (VoF). The rheological properties of the solder paste were addressed by an exhaustive viscosity model. It was found that the set of through-holes affected the flow-field and yielded a decrease in the hole-filling if they were arranged in parallel with the travelling direction of the printing squeeze. Similar disturbance on the flow-field was found for oblong-shaped through-holes if they were arranged in parallel with the squeeze movement. The findings imply that the arrangement of a set of through-holes and the orientation of oblong-shaped through-holes should be optimized even in the early design phase of electronic products and during the set of assembly processes. The soldering failures in pin-in-paste technology can be reduced by these early design-phase considerations, and the first-pass yield of electronic soldering technologies can be enhanced. pin-in-paste technology; fluid dynamics; non-Newtonian fluid properties; transient numerical modelling; through-hole shape; reflow soldering +
Footnote †: journal: Applied Sci. The rapid spread of pin-in-paste technology requires exhaustive analyses of the hole-filling by solder paste and optimization of the stencil printing process. This is of particular importance due to previous researchers reporting that the improper setup of this process could cause 50-60% of the soldering failures [2]. These soldering failures include the formation of solder bridges or open joints, if the amount of deposited solder paste is larger or smaller than expected (defined by the volume of the stencil aperture). Optimizing the stencil printing process can improve both the quality and the reliability of solder joints [3]. During the optimization, parameters affecting the process shall be considered, such as the rheological properties of solder paste, the printing speed, and the geometry/orientation of apertures and plated holes for SM and TH components, respectively [4; 5]. Many research works have proven the correlation between the paste properties and the transfer efficiency of the printing [6; 7]. The transfer efficiency characterizes the amount of solder paste deposited onto the soldering pads and into the plated through-hole of the printed circuit board. Solder pastes have shear-thinning properties, aiding the flow into the plated through-holes. Solder pastes also have a thixotropic nature for recovery, which allows them to be fixed in the through-holes until the components have been inserted [8]. Many researchers developed numerical models for the stencil printing process to study the influence of the different parameters on the process. [PERSON] et al. created a 2D numerical model to identify optimal stencil printing parameters for flip-chip assembly [9]. A similar finite volume model was created by [PERSON] for the stencil printing process to investigate the flow of the solder paste and the pressure distribution along the stencil  
Figure 1: Schematic of the steps of pin-in-paste technology. during the printing [10]. [PERSON] et al. and [PERSON] et al. also analyzed the differences between using Newtonian and non-Newtonian fluid properties. They proved that the non-Newtonian behavior of solder pastes should be considered in the modelling [11; 12]. Though these works provided valuable insight into the process of stencil printing, their model used a 2D approach and applied steady-state analyses.