Dr. Ron, I know that the area ratio for circular and square stencil apertures is 4d/t. What is it for an elongated “D” aperture?
The area ratio of a stencil aperture is the area of the aperture opening divided by the area of the side walls. It is interesting, as Ismail points out, that the area ratio of a circular aperture is the same as that of a square aperture. A little 10th-grade geometry will point this fact out. It ends up that the area ratio of an elongated “D” is a little more complex. All of these aperture shapes and that for a rectangle aperture are shown in Figure 1. The area ratio formulas are at the bottom of the figure.
Figure 1. The Area Ratio for Several Shaped Apertures. The Elongated "D" aperture is the 3rd from the left.
A rule of thumb that still seems to hold is that the area ratio should be 0.66 or greater for the best printing result. It is possible to do somewhat better (i.e with an area ratio less than 0.66) with a superior solder paste and/or some of the new stencil nano-coatings.
The derivation of the area ratio for the elongated “D” is in Figure 2.
Figure 2. The Derivation of the Area Ratio for an Elongated D Shaped Aperture.