Stencil Technology and Design Guidelines for Print Performance

Circuits Assembly, March, 2001 by William E. Coleman

Three major performance issues exist for solder paste printing. First, the aperture size (width and length of the aperture) and stencil foil thickness determine the potential volume of solder paste applied to the printed circuit board (PCB) or substrate. The second issue is the ability of the solder paste to release from the stencil aperture walls. The third issue is positional accuracy of exactly where the solder brick is printed onto the PCB or substrate.
As the squeegee blade travels across the stencil during the print cycle, solder paste fills the stencil apertures. The paste then releases to the pads on the board during the board/stencil separation cycle. Ideally, 100 percent of the paste that filled the aperture during the print process releases from the aperture walls and attaches to the pads on the board, forming a complete solder brick. The ability of the paste to release from the inner aperture walls depends primarily on three major factors:
* the area ratio/aspect ratio for stencil design
* the aperture side wall geometry
* the aperture wall smoothness.
The first factor is aperture design-related while the other two factors are stencil technology-related. The area ratio is the area beneath the aperture opening divided by the area of the inside aperture wall; area ratio = [(LXW)/(2(L W)T)]. Historically, the aspect ratio is the width of the aperture divided by the thickness of the stencil; aspect ratio = W/T. The generally accepted design guideline for acceptable paste release is [greater than]0.66 for the area ratio and [greater than]1.5 for the aspect ratio.

The aspect ratio is really a one-dimensional simplification of the area ratio. When the length (L) is much larger than the width (W), the area ratio is the same as the aspect ratio. When the stencil separates from the substrate, paste release encounters a competing process: Will it transfer to the pad on the substrate or will it stick to the side aperture walls? When the area of the pad is greater than two-thirds of the area of the inside aperture wall, the paste will probably achieve 80 percent or better paste release.
A laser-cut stencil that is electropolished definitely has smoother inside aperture walls than a non-electropolished laser-cut stencil. Therefore, the former will release a higher percentage of paste than the latter at a given area ratio. Likewise, an electroformed stencil with mirror-type aperture wall finish will release even a higher percentage of paste at the same area ratio. For aspect ratios that approach 1.5 and area ratios that approach 0.66, some stencil technologies are better suited than others to achieve higher percentages of paste release.
The aspect ratio and the area ratio are important considerations when designing stencil apertures. For example, a 20-mil pitch quad flat pack (QFP) with an aperture design of 10 mil X 60 mil in a 5-mil stencil has an aspect ratio of 2.0 and an area ratio of 0.86. Good print performance can be expected with this design using a good quality laser stencil.
However, consider a 20-mil micro ball grid array (microBGA) with a 10-mil aperture in a 5-mil-thick stencil. Because the aperture is round or is a square with rounded corners, the area ratio is the deciding factor. In this case, the area ratio is 0.5, which is well below the recommended value of 0.66. The aperture design can be changed by reducing the stencil thickness or increasing the aperture size, or a stencil technology can be chosen that gives better paste release at this area ratio.
Stencil Technologies
Basically, five stencil technologies are being used in the industry: laser-cut, electroformed, chemical etched, plastic and hybrid. Hybrid is a combination of chemetch and laser-cut. Chem-etch is very useful for step stencils and hybrid stencils.
Laser-cut process
Laser-cut is a subtractive process. The Gerber data is translated into a CNC-type language that the laser understands. The aperture is cut out by moving the laser head only, moving the table holding the stencil only or a combination of each. The laser beam enters inside the aperture boundary and traverses to the perimeter where it completely cuts the aperture out of the metal, one aperture at a time. The smoothness of cut depends on many parameters, including cut speed, beam spot size, laser power and beam focus. The typical beam spot size is about 1.25 mils. The laser can cut very accurate aperture sizes over a wide range of size and shape requirements.
As with chem-etch, the laser-cut aperture size must be adjusted to the post-processing treatment employed because aperture size change will occur during this process. Figure 1 shows scanning electron microscope (SEM) pictures of laser-cut apertures with no electropolish, with electropolish and with electropolish followed with nickel plating.
Electroform process
Electroformed stencils are made by an additive process as opposed to the subtractive process used for chem-etch and laser-cut. A nickel bath containing nickel ions and a nickel hardening additive is used to electroplate onto a substrate called a mandrel. However, photoresist is first applied to the mandrel. The resist is exposed and developed, forming photoresist pillars anywhere an aperture must be in the stencil. Nickel is electroplated out of the bath one ion at a time until the desired foil thickness is achieved. Then, the nickel foil is removed from the mandrel, creating the completed stencil.

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