Choosing the right stencil: plastic makes its mark, but for 0.5 mm pitch laser-cut or electroformed are best.

When discussing materials deposition technology, emphasis is often placed on the screen printer's capabilities and how it will affect final product yield (rightly so, in many cases). While the equipment's performance has a tremendous impact on the final product, the design, material and manufacture of the stencil cannot be overlooked as a critical parameter for high product yields, especially as we move to finer pitch devices. Good stencil design, high quality stencil material and the manufacturing method must all be considered when evaluating your screen-printing process.

First, let's address the price versus performance issue. The true cost of the stencil lies not in its price but in its performance. Many factors contribute to superior performance, including quality of materials, knowledge of design and the supply chain and service network of the stencil manufacturer. The stencil manufacturer's process knowledge is also an important factor. Given that a well designed and manufactured stencil will always give good process yield and repeatable results, taking the time at the beginning of the stencil design process to understand the product, process and desired result will deliver immeasurable value.

Stencil materials and manufacturing method are also critical when determining the right stencil for your particular application. A wide variety of materials and fabrication techniques permit stencil designs that meet the challenges of fine-pitch technology, miniature components and densely packed boards. The most common stencil materials are metals, primarily stainless steel and nickel. In the past few years, some plastics have also gained acceptance. Manufacturing methods include chemical etching, laser cutting and electroforming.

[ILLUSTRATION OMITTED]

Chemical etching has historically been the lowest cost fabrication method, but it is only suitable for large aperture applications and cannot satisfy the requirements of sub 0.5 mm pitch applications. To address decreasing pitches and increasing component densities, laser cutting has become a more widely used and accepted method. Electroforming is an additive process in which stencils are formed by electrodepositing a plating material (in most cases, nickel) onto a mandrel carrying a negative photoresist image of the aperture pattern. This method produces extremely precise, smooth-walled apertures, which makes electroformed stencils ideal for use in ultra-fine-pitch applications. Finally, there are plastic stencils, which have gained acceptance for adhesive printing over the past several years and provide the ability to produce stencils up to 8 mm thick. Some of the latest frame-mount stencil systems permit the interchange of stainless steel, electroform and plastic stencil foils within the same frame, which can further maximize resources.

Rigorous control of aperture quality is imperative, as the size and shape of stencil apertures determine the volume, uniformity and definition of the materials that are deposited onto the substrate. Fully understanding stencil design, available materials and various manufacturing methods will further ensure that you and your supplier select the correct stencil for your application. For applications with pitches below 0.5 mm, laser-cut or electroformed stencils are really the only choice. Both techniques can produce high-quality, accurate fine-pitch stencils and each has advantages and disadvantages such as time to manufacture, stock foil thicknesses available and any further aperture wall treatments that may be required.

For ultra high-density applications such as wafer bumping, where aperture counts are now more than 2 million, eletroforming is the preferred stencil fabrication technique. For these applications, correct stencil design and extreme manufacturing control are key. A wafer bumping stencil dictates that the fabrication technique be capable of producing thousands of small, closely spaced apertures to extremely tight dimensional and positional tolerances. Even very small deviations from the designed aperture size can result in large bump height variations and can produce open circuits in the assembled chip.

Finally, in addition to the advanced semiconductor applications, stencil material and manufacture have a tremendous impact on successful printing of today's lead-free paste materials. As discussed in this column in March, my company's research has shown that the new lead-free paste formulations do not have the same release characteristics as traditional tin-lead pastes and therefore may require different stencil material for maximum performance. In the lead-free paste evaluations that we have conducted, we have found that electroformed stencils and laser-cut nickel stencils deliver the best performance. The findings also indicate that stencil manufacturing method is critical, as each of the three laser-cut stainless steel stencils used in our evaluations produced different outcomes.

Stencil suppliers must address the constant technology challenges presented by the increasing use of a wide mix of components that require both large and small material deposits, the demands of finer-pitched applications and the new era of lead-free printing. And, as we move to pitches below 100 [micro]m, new stencil materials and fabrication techniques may need to be explored. Stay tuned!

Clive Ashmore is global applied process engineering manager at DEK (dek.com). His column appears semimonthly.