SLDA021B March   2014  – February 2020 AM3892 , AM3894

 

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3.2 Stencil Design

The optimum stencil design is 0.35-mm (0.014") circular apertures on 1250μm (0.005") foils. These dimensions produce a pad-to-wall Area Ratio of 0.70, and should consistently supply approximately 0.013 mm3 (770 mils3) of solder paste for each deposit. Smaller apertures or thicker foils may change the Area Ratio, the paste deposit volumes, and the repeatability of the print process.

The 0.35-mm (0.014") square apertures, as well as 0.30-mm (0.012") and 0.40-mm (0.016") diameter circular apertures have been tested and provided acceptable results. However, square apertures may introduce variation to the paste deposits’ print definition and volume repeatability. Additionally, smaller circular apertures may increase the risk of open joints or head-in-pillow defects, and larger circular apertures increase the risk of solder bridges and solder balls.

Stencil thicknesses other than 0.005" have not been tested. If PCB design mandates other foil thicknesses, the aperture sizes should be adjusted appropriately to maintain a area ratio of 0.66".