A study of SnAgCu solder paste transfer efficiency and effects of optimal reflow profile on solder deposits

Abstract

The reliability of solder joints in electronic products are greatly enhanced by good stencil printing and quality reflow soldering. The stencil printing process is widely used in Surface Mount Technology (SMT) to deposit solder paste on the substrate and is a critical step in SMT assembly as it has been widely claimed that up to 50% of the defects found in the assembly of printed circuit boards (PCBs) are attributed to stencil printing. Solder paste release from stencil during printing is a key factor which affects the quality of solder prints. Thus, the efficient transfer of paste from stencil through aperture to pad is a fundamental concern in SMT production process. The recent trends on further miniaturisation of electronic products have introduced more process challenges in the SMT assembly. The assembly of surface mount packages such as flip chips, chip scale packages and fine pitch ball grid arrays are challenging the current stencil printing and reflow profiling capabilities. The effective mounting of these miniaturised packages requires good transfer efficiency of solder paste through small stencil aperture. As further electronic product miniaturisation culminates in progressive use of decreasing stencil aperture sizes, the in-depth understanding of the dependency of transfer efficiency of solder paste on diminishing stencil aperture areas has become vital to improving solder joint reliability. This study investigates the transfer efficiency of type 3, 96.5Sn3.0Ag0.5Cu solder paste through linearly decreasing rectangular stencil aperture sizes typically used in PCB assembly. In addition, the research examines the effects of optimal reflow profile (ORP) on the solder deposit volumes. The results from the study show a power law relationship between actual solder deposit volume (SDV) and aperture cavity. The observed effect of ORP on actual SDVs was a 46% volume change which was fairly constant across the pad geometries.

Our study shows that the transfer efficiency of solder paste decreases with decreasing stencil aperture size. A novel mathematical model ${\eta }_T=100e^{-|V_A-V_C|\ast |V_A-V_C+D_m+e^{{10}^{-5}}{D}_m^{-1}|}\text{for}{V}_C⩾0$ is proposed for computing the transfer efficiency of solder paste through rectangular stencil aperture sizes.

Introduction

Increasing global customer demand for miniaturised, hand-held and pocket electronic products has been a key driver in the design, development and wide application of high-density area array package format [1]. Electronics manufacturing industries have utilised advanced Integrated Circuits (ICs) packages such as Micro-Ball Grid Arrays (μBGA), Chip Scale Packages (CSP), Flip Chip (FC) technology and Surface Mount Resistors (SMR) to achieve the manufacture of smaller, lighter, faster and cheaper products. However, the assembly of these surfaces mounts packages and the reliability of the products are challenged by crucial manufacturing process steps. The use of ultra fine pitch packages makes the stencil printing process more critical to produce a reliable solder joint [2]. The most important step in the SMT production process besides paste printing is the reflow soldering process [3]. The solder reflow profile is one of the key variables in the electronics manufacturing process that significantly impacts product yields [4]. The manufacturing challenges associated with both solder paste printing and reflow soldering increases as electronic device size decreases. The increasing manufacturing difficulties are because at very small aperture sizes, the rheological dominancy of surface tension over paste viscosity impacts negatively on the printing process. The outcome may include overfill of stencil cavity with solder paste. The consequence of this outcome may lead to printing defects such as solder slumping and bridging. A vital key to achieving urgent customer satisfaction on further miniaturisation of electronics with good solder joint reliability is the in-depth understanding of the solder paste transfer efficiency through diminishing stencil aperture sizes and effect of ORP on SDVs. Solder paste transfer efficiency is identified in the work reported in this paper as an essential parameter which influences the reliability of solder joints in particular and electronic products in general.

Many studies [1], [2], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] have been conducted in this area of solder paste printing, transfer efficiency and reflow profiling but none appears to have really focused on the solder paste transfer efficiency through stencil aperture sizes and effects of ORP on SDVs. While Amalu et al. and Pietrikova et al. [1], [4] investigated optimal reflow profile of solder paste, only Aravamudhan et al. [2] studied paste release from small stencil apertures and references [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] researched on paste printing process and correlated paste performance during printing with paste rheological properties.

The objectives of the research presented here include the determination of the relationship between actual SDV and the theoretical solder volume/aperture cavity size. The work also investigates the effects of ORP on SDVs. This research work further studies the transfer efficiency of SnAgCu solder paste from stencil through diminishing stencil aperture sizes to substrate.

The significance of the study is that the findings would be useful for R&D personnel in designing and implementing newer applications with finer-pitch interconnects and in minimising the occurrence of printing and reflow soldering defects in SMT.