Darrel,

When procuring a stencil I use the following guidelines.  As usual, there may be exceptions for each particular product and stencil type.

Global stencil thickness of 0.006" for products with
minimum aperture dimension of 0.009" or larger.
Global stencil thickness of 0.005" for products with
minimum aperture dimension of 0.0075" or larger.
Step etch from 0.006" or 0.005" for individual smaller
aperture sets down to:
0.004" for 0.006" and larger apertures and
0.003" for 0.0045" and larger apertures and
0.002" for 0.003" and larger apertures.

Of note, with electroform stencils, thickness can be specified in odd values (ex: 0.0034") and can be customized to your aperture dimensions.  The advantage here is maximum paste volume due to increased stencil thickness and improved paste release from nickel.

Step etch regions have a minimum clearance of 0.1" from the nearest small aperture to the edge of the step etch to avoid

any transition region printing effects.  Large pads are not as sensitive to the transition and related pad height variation

and should not be worried about if near the step edge.

My goal is to have the minimum aperture dimension be no less than 1.5 times the stencil thickness.  This number
(the aspect ratio) was first introduced due to limits in the chemical etching process but remains a good value for
ensuring complete paste release (no paste left in aperture).

I prefer a foil thickness of 0.005" or thicker to minimize imprinting of board features into the foil.  With 0.004"
foil and thinner, the tendency to deform and conform to the circuit surface increases dramatically.

Hope this helps,

John Vivari
Technical Service Engineer
EFD Inc.
Ph: 401-333-3800 x2204
Fx: 401-333-4954