ATLANTA – Stencils are often overlooked or taken for granted, but offer ample opportunities for process improvement and even cost savings.

Those were the takeaways from Photo Stencil regional sales manager Skye McDaniel, who on Thursday offered up an informative and occasionally humorous presentation on stencil selection at the Atlanta SMTA chapter meeting.

For stencils, flatness and cleanliness are big issues among the solder paste printing process variables. Other issues include printer vibration, squeegees and the environment, including temperature, humidity, operator training and print cycle time.

Other foci: PCB cleanliness; solder mask alignment and thickness; stretch; warping and fiducial alignment. Boards sourced from multiple regions show differences as well, said McDaniel. “There are complaints of boards not being consistent when coming from outside the U.S.” – such as changes in the copper pad sizes and locations.

McDaniel said solder paste is a variable with regard to slump, adhesion, shear strength, stability and age.

He added, “You can make a stencil to match a board; data can be scaled. Boards are expensive, so this is something the industry can us as an option for variation in bare board fabrication.”

He said assemblers can adjust such printer variables as cleaning frequency, downstop distance, separation distance and speed; the squeegee’s speed and pressure: “The more pressure, the more smear action – instead of sheer action,” McDaniel noted.

Finally, with respect to the solder paste volume on a stencil, “You don’t want too much or too little, what you want is a predictable solder paste volume.”

McDaniel illustrated that there are many controllable variables by design. Examples include the stencil’s size, shape and thickness, of which 0.005" is common; and again, he emphasized the importance of the squeegee.

According to McDaniel, a good squeegee will help the stencil print better, while a bad one will have the opposite effect.

The end-user influences other controllable variables, such as board layout and design; stencil and aperture design, and type of stencil and surface finishes selected. The stencil manufacturer influences the type – nickel plating, chem-etch, laser cut, electroform – and the surface finishes.

He touted electroforming for its clean, smooth finish – available in 1/10 of a mil.-thick increments – verses laser stencils that are typically only fabricated in 1 mil. increments. This is a clear advantage for engineers trying to optimize their process. “The laser-cutting of nickel sheets by some, has been considered a low-cost alternative to electroform stencils,” he said.

Aperture fill variables start with blades, and McDaniel said that metal blades “are a hot topic right now,” and are needed for 100% aperture fill. However, he smiled, “There are still people who use polyurethane blades; they still have a good uses, as in printing glues, adhesive or inks.”

He recommended eight to 11 lbs. psi of squeegee blade pressure when printing, but there is a variable that has to be considered when determining pressure; that is the actual length of the blade being used.

McDaniel continued, “When you slow the squeegee blade print speed down, it helps with apertures perpendicular to the blade. Too low of pressure, you’ll leave paste on the stencil. Too high of pressure and you’ll damage the blade and the stencil.”

With regard to squeegee troubleshooting, he asked, is the problem below or above the stencil? “If it’s below, it’s a board support issue; if it’s above, it’s a squeegee issue.”

“The paste should roll across,” he stated. If the blade is bad, solder paste could stick to the blade. “You need as smooth a blade as possible,” he pointed out. “Blades are the forgotten item in the print process.”

He went on to discuss more formal blade types and their characteristics:
nickel teflon: 0.012" thick, with a surface finish of 3.0; steel-coated nickel: 0.012" thick, with a surface finish of 5.0; OEM: 0.012" thick, with a surface finish of 6.0; steel coated NiSn: 0.012" thick, with a surface finish of 5.0, and electroform: 0.011" thick, with a surface finish of 1.0.

Each of the blade types would require a different pressure and print speed to obtain a clean stencil wipe. Testing has shown the electroform blade requires the least amount of blade pressure, and would be less likely to damage the stencil.

McDaniel said that aspect ratio has become obsolete because of smaller components, so designers need to rely on area ratio, which is area of aperture opening divided by the area of aperture wall. He couldn’t say enough about the area ratio calculator now available because it takes the guesswork out of the determining which stencil technology should be used.

Chemical-etch is not outdated if used in the right application, such as 0.031" or 0.051", he said. But, McDaniel continued, “Thank God for laser cut!” which can be used down to 0.020" pitch. Finally, electroform can be used for 0.020" pitch and below or ceramic BGAs, and is strongly suggested on 0.016" pitch or less to effectively print with a high level of repeatability.

One potentially negative laser-cutting variable he mentioned was movement of the table, creating jaggedness of the aperture walls. Electropolishing and nickel-plating helps to effectively reduce the jaggedness of the wall and smooth the aperture wall.

Shape and smoothness of the aperture is affected by cut speed and the type of laser used. “Proper cut speed which is dependent upon the type of laser used, must be maintained to get a good consistent aperture wall,” he said. Things that also affect accuracy of the cut are laser power and focus of the beam.

With laser cutting, post-processing treatments such as electropolishing and nickel plating are highly recommended.

McDaniel saved the best for last: electroforming, which he stressed needs time. “It’s an additive process, rather than a subtractive one, such as chemical etched or laser cut stencils,” he said.

The process includes:  

  1. Plot phototool.
  2. Coat mandrel with photoresist.
  3. Expose photoresist using phototool.
  4. Develop resist.
  5. Electroplate nickel onto mandrel.
  6. Peel off nickel sheet from corner to corner.
  7. Inspect.
  8. Mount on frame with mesh.
Thicknesses on electroform stencils range from anywhere between 0.001" and 0.012" thick, and can be in 1/10 of a mil. increments, said McDaniel.

He said the advantages of electroform are that it has the strongest tensile strength; smooth trapezoidal walls; has a longer stencil life; precise aperture sizes; no nickel plating or electropolishing is needed; higher process yields; lower defect rates, and approximately 98% paste release, which is much higher than laser.
To be a full service stencil supplier, the supplier should have multiple stencil technologies available. He also stressed that stencil manufacturers can design and modify stencil apertures for them.

He concluded, “Stencil technology choice is largely dictated by aperture size and use of the area ratio calculator.”
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