Issue link: https://iconnect007.uberflip.com/i/948150
42 SMT007 MAGAZINE I MARCH 2018 A detailed view of the step area shown in Figure 11 captures the arrangement of the M0201 apertures. The clear majority of PCB designs, including this one, are not config- ured optimally for step stencil printing. Ideally the step perimeter is rectangular and there is sufficient clearance or "keep out" space between the apertures inset the step and the step border. The dimensions labeled in red color indicate the step border is too close to the aperture according to IPC policy illustrated in Figure 12 [7] . The K1 (i.e., keep out) zone should be > 1.08mm. Four of the six step wall perimeters violate the K1 rule. The main reason to accommo- date the K1 margin is to permit enough space for a rigid squeegee to flex down into the step area and wipe the aperture top side surface clean. Any solder paste film or residue remaining on the stencil surface around aperture openings after the print stroke will destabilize transfer efficiency performance. Two identical laser-cut, 80µm base thickness, stainless steel stencils were manufactured. Both were step etched locally to 50µm as per the previous discussion. The type of stencils made were mesh-less format foils, whereby the metal sheet comprises nearly the entire stencil area and is fitted into a 23-inch square master frame which mechanically clamps and tensions the foil. The only difference between the two stencils is that one was produced with a polymer type nano - coating applied to its bottom side and the other stencil did not have any nano-coating. Printing performance reports based on sten- cil nano-coatings have been a popular publi- cation topic in recent years [8, 9, 10, 11, 12] . As the name implies, a nano-coating is a very thinly applied material adhering to the bottom side of a metal stencil containing flux repelling prop- erties (i.e., fluxophobicity). The original func- tion of a nano-coating was to assist in preserv- ing the cleanliness of the stencil bottom side by preventing premature flux and solder parti- cle smearing. There are also claims for nano- coated stencils to deliver higher print transfer efficiency enabled by reducing the adhesion and friction of paste on aperture walls. A sample of 16 apertures from each sten- cil were manually measured using a coordi- nate measuring machine tool. Top and bottom side stencil foil measurements were compared for the same apertures to determine average dimensions. Comparing averaged aperture size of aggregated measurements for each stencil against the GBX designed aperture size, it was verified the stencils were manufactured within 4.5µm of specification. Figure 11: M0201 step area borders and violations. Figure 12: Step stencil guidelines, IPC-7525B [6] .