Correlation of solder paste rheology with computational simulations of the stencil printing process

Abstract

Soldering technologies continue to evolve to meet the demands of the continuous miniaturisation of electronic products, particularly in the area of solder paste formulations used in the reflow soldering of surface mount devices. Stencil printing continues to be a leading process used for the deposition of solder paste onto printed circuit boards (PCBs) in the volume production of electronic assemblies, despite problems in achieving a consistent print quality at an ultra‐fine pitch. In order to eliminate these defects a good understanding of the processes involved in printing is important. Computational simulations may complement experimental print trials and paste characterisation studies, and provide an extra dimension to the understanding of the process. The characteristics and flow properties of solder pastes depend primarily on their chemical and physical composition and good material property data is essential for meaningful results to be obtained by computational simulation.This paper describes paste characterisation and computational simulation studies that have been undertaken through the collaboration of the School of Aeronautical, Mechanical and Manufacturing Engineering at Salford University and the Centre for Numerical Modelling and Process Analysis at the University of Greenwich. The rheological profile of two different paste formulations (lead and lead‐free) for sub 100 micron flip‐chip devices are tested and applied to computational simulations of their flow behaviour during the printing process.

Evaluating the printability of solder paste from paste roll characteristics

“…In all three cases the steady state value of the face velocity was 1.5-2.0 cm/s. This value is close to that in the computer simulation by R. Durairaj, which also showed the free surface of the roll paste maintaining a uniform and constant velocity [12]. The steady state velocity reflects the behavior of the rheological agents, the solvents and spherical solder powder.…”

“…Though seemingly simple, the print process is complex and the corresponding procedures include: preprint paste treatment, paste roll, aperture filling, aperture emptying and paste deposit standing [12]. An appropriate evaluation of the solder paste printability involves all the characteristics needed to achieve print quality and meet further processing requirements.…”

Finite volume modelling of stencil printing process

“…al. created models for solder pastes with different alloys [11]. They analysed the effect of different printing speed on the pressure-, shear rate-and viscosity distribution along the surface of the stencil.…”

Differences in the sub-processes of ultra fine pitch stencil printing due to type-6 and type-7 Pb-free solder pastes used for flip chip

“…Whilst the stencil printing process is paramount to producing high volume, low cost joints it also accounts for almost 60% of assembly defects within the SMT process [7]. In light of these defect statistics, there is an increased focus to better understand the sub processes involved with stencil printing especially for ultra…”