Choice of Thickness

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What Thickness Would be Best?

The choice of foil thickness is probably the most important decision in ordering a stencil.

It affects everything from print release to solder joint reliability. Basic rule of thumb is to start with a .007 or .008 foil thickness and adjust thinner as dictated by the smallest aperture to be printed.

Thicker foils will of course be ideal in giving more volume for robust solder fillets especially for larger passives and leaded SOIC's and connectors. But thicker foils cannot be used to reliably print smaller openings. Since the smallest openings must be printable, they are what ultimately determine the foil thickness you choose.

No fine pitch
1.27mm SOIC and passives
0805 and larger.

Ultra fine pitch (0.4mm)
MicroBGA with .010" squares
and 0201 passives.

 

Here is a quick and easy chart for determining foil thickness.

Choose this
thickness for parts:

Leaded Ic's
with pitches of:

BGA's with
pitches of:

Chips / Passives
sizes of:

.007 / .008 31 (0.8mm) 50 (1.27mm) 0805
.006 25 (0.65mm) 40 (1.0mm) 0603
.005 20 (0.5mm) 25 (0.65mm) 0402
.004 16 (0.4mm) 20 (0.5mm) 0201
  12 (0.3mm) 16 (0.4mm) 01005
 

The above chart determines foil thickness based upon the types of parts on the assembly. It is based on our experience and is for recommendation only based on what we believe will yield the best results.

More specific printability formula are provided below. For small rectangular and square openings you will need an area ratio of 0.6, for circles a value of 0.66, and for longer rectangles an aspect ratio of 1.5. The formula changes depending upon the geometry of the stencil opening.

Squares and rectangles where
length < 2x width

Circular openings

 

Long rectangles

 

Length X Width
(2 x foil thickness) x (length + width)
Diameter
4 x foil thickness
width of rectangle
(2 x foil thickness) x (length + width)

 

Value must be equal or greater than
0.6 *

Value m u st be equal or greater than
0.66 *\

Value m u st be equal or greater than 1.5 *

Technical knowledge, Sensibly Applied

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