ATLANTA -- The first revision of IPC-7525, “Stencil Design Guidelines” was circulated for vote in February, and could be published shortly. Among the proposed changes: Refined aperture configurations and area ratios, and new guidelines for printing adhesives.

More recently, the group decided to update a number of major concepts, the most obvious resulting from the advent of Pb-free. “We asked, How is that going to change guidelines?” said Coleman. He explained, “For SnPb, the standard recommendation is to take the paste you’re printing and undersize that a little.” But, for Pb-free, the solder paste doesn’t fit as well, so the task group didn’t want to recommend much reduction.

The group came up with various aperture configurations that would minimize solder balls. For this revision, it resolved that the c-shape worked better for Pb-free because it covered the outside pad, according to Coleman.

For the new revision, Coleman and his colleagues also emphasized area ratio, as opposed to aspect ratio, determining that the smaller the ratio (more wall), the harder for paste to release out of the aperture. “The area ratio had to be greater than 0.66; this covered laser stencils, but with new technology, you could go to 0.5,” explained Coleman. “Smaller components had smaller apertures with lower area ratios; this was a good thing.”

In light of this, the task group devised area ratio charts, plotting width and length. If anything fell below 0.5, the group recommended going back to redesign. “Between 0.5 and 0.66,” said Coleman, “you should be looking at electroform stencils. Above 0.66 and less than 0.9, you should use laser stencils. 0.9 area ratio is for all stencil technologies including chemically etched stencils."

Next, the group tackled printing adhesives, expanding the second document to include several designs dealing with glue printing. In addition, Coleman said the original document didn’t address too many QFN devices and LCCs, so they decided to add design guidelines in this area. “People were printing one to one on ground plane,” he said. “In the revision, we designed apertures for ground plane to reduce solder paste so it wouldn’t float.”

Finally, the revision addresses positional accuracy with respect to the Gerber file. “The Gerber file is used to create pads on PCBs and stencils. You want to match the stencil with pads on a board. This is particularly important for Pb-free because it doesn’t wet as well,” explained Coleman. “You have to get paste on the pad more.”

The new document could be ready by month’s end.