The long-held area ratio rule (the ratio of aperture size and stencil foil thickness) of 0.66 is under attack from all sides, it seems.
Writing this month in CIRCUITS ASSEMBLY, columnist Clive Ashmore explains that improvements to the shear thinning capability of the print stroke can reduce the area ratio to 0.40. This is an important development because tighter ratios offer greater latitude in stencil design. The rule today is that, since a larger component generally requires more paste volume than a smaller one, one or the other suffers: Optimize for the larger part and print quality suffers on the small aperture; optimize for the smaller ones and the larger parts are starved of paste. With the lower ratio however, as Ashmore notes, 0.3 mm CSPs could be placed beside large tantalum capacitors without penalizing one or the other.
Next week at SMTAI in Orlando, Rockwell’s Kevin Liticker will present his work evaluating several stencil technologies including “PhD,” fine grain stainless and nickel stencil, and aperture forming methods like pulsed YAG laser, fiber optic laser (with and without electropolish) and electroform as they relate to paste transfer efficiencies for small apertures. I’m not going to give away the store, but he found some evidence that the release characteristics of fine grain material may be superior as apertures shrink.
I strongly suggest checking out both engineers’ work.