Robustness of solder paste stencils

Robustness of high tension, standard tension, and mesh mount solder paste stencils

Prithvi Kotian, MS., Rochester Institute of Technology, Rochester, NY, USA
Jeff Schake, ASMPT, Suwanee, GA, USA
Martin Anselm, Ph.D., Rochester Institute of Technology, Rochester, NY, USA

Abstract

The miniaturization of components has caused a corresponding reduction in pad sizes, which makes maintaining a well-defined solder paste deposit for fine pitch components like BGAs, LGAs, QFN, and passive devices a challenge. This study was initiated to analyze the impact of various parameters such as stencil tension, number of print cycles, squeegee, stencil wear, and solder paste volumetric transfer efficiencies on the quality of the solder deposit. Primarily, the work undertaken was designed to gain greater understanding about the influence of stencil foil tension on printing performance. Using three different stencil frame types, print quality results are investigated and correlated against accumulated squeegee strokes at 5,000, 10,000, 15,000 and 20,000 print cycles. Data was collected to determine which stencil type produced repeatable paste transfer by examining the volumetric transfer efficiencies at different aperture locations. Following 10,000 print cycles, the mesh mount and standard clamping tension stencils exhibited concerns such as solder bridges, insufficient paste deposits, scooping, and stencil damage in stressprone locations. This correlated to squeegee wipe induced stencil damage most notably observed on areas inside the dense array of perimeter shield enclosure apertures, affecting the printability of BGA and passives devices within. The corresponding SPI data revealed increased print volume standard deviation, raising paste transfer repeatability concern. However, the high clamping tension stencil provided significantly stable process capability throughout the print cycles, exhibiting consistently narrow standard deviation distributions, which resulted in uniform paste deposits for all the apertures investigated.

Key words: Fine pitch components, stencil tension, stencil wear, volumetric transfer efficiencies.