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The State of Stencil Technology
December 31, 1969 |Estimated reading time: 14 minutes
In surface mount assembly, the stencil is the gateway to accurate, repeatable solder paste deposition.
By Mark Whitmore
As solder paste is printed through the stencil apertures, it forms deposits that hold the components in place and, when reflowed, secures them to the substrate. The stencil design — its composition and thickness, the size and shape of its apertures — ultimately determines the size, shape and positioning of the deposits, which are critical to ensuring a high-yield process with minimal defects.
Today, a wide variety of materials and fabrication techniques enable suppliers to design stencils that meet the assembly challenges of fine-pitch technology, miniaturized components and densely packed boards.
In addition, stencil technology now serves a full range of mass imaging materials. As stencil designers have gained in-depth knowledge about how aperture size and shape affects deposition, new technologies extend the capabilities of printing platforms and stencils into applications as varied as adhesive deposition and wafer bumping.
The stencil serves two primary functions. The first is to ensure precise placement of a material, such as solder paste, flux or encapsulant, on a substrate. The second is to ensure the formation of properly sized and shaped deposits.
Stencil Materials and Manufacture
The most widely used stencil materials are metals, primarily stainless steel and nickel. In recent years, various plastics also have gained acceptance. Stencil manufacturing techniques include chemical etching, laser cutting and electroforming. A brief review of these materials and techniques indicates that manufacturers can select from a wide variety of stencil types to meet specific application needs.
Historically, the most commonly used, lowest cost stencil fabrication method has been chemical etching. This is a subtractive process that uses photolithographic techniques to define the aperture pattern, and then applies an etchant to form the apertures simultaneously from both sides of the stencil. To achieve apertures with trapezoidal walls, which improve solder paste release, the artwork can be designed to create slightly larger apertures on the side of the stencil that faces the substrate.
Double-sided etching can create knife-edge aperture profiles, as well as "under-etch" and "over-etch" conditions. It is possible to "electropolish" foils after they have been etched to remove knife edges and "smooth" the aperture walls. Chemical etching is suitable for large aperture/coarse pitch applications but cannot satisfy the requirements of sub-0.5 mm pitch applications.
In response to decreasing pitches and increasing component densities, laser cutting has become a more widely used stencil fabrication process. Laser-cut stencils are produced directly from Gerber data or other data formats, reducing the number of photolithographic steps and subsequently reducing the chance of image misregistration significantly. Computer numerically controlled (CNC) laser cutting, driven directly by Gerber data, produces highly accurate, repeatable stencil apertures. The precision of the technique means that aperture dimensional tolerances of ±5 µm can be produced, even over large print areas.
Figure 1. Laser-cut aperture showing characteristic striation markings.
By adjusting the intensity of the laser during manufacturing, high-contrast fiducials can be created on the stencil surface with no need for in-filling. This feature also aids the accuracy of the manufacturing process. The intrinsic properties of the process result in apertures with a trapezoidal cross-section, which enhance paste release. A possible concern is the characteristic "striated" finish of aperture walls left by a laser (Figure 1). The latest laser cutting technologies have helped reduce this, but when a specific application demands a smooth surface wall, it is possible to electropolish or even electroplate laser-cut stencil apertures following the cutting procedure.
In contrast to chemical etching and laser cutting, electroforming is an additive process. Electroformed stencils are "grown" by electro-depositing a plating material, usually nickel, onto a mandrel carrying a negative, photo-resist image of the aperture pattern. The process produces extremely precise, smooth-walled apertures with a natural taper that require no additional finishing processes. The extreme precision of the process targets electroformed stencils for use in ultra-fine pitch applications.
Figure 2. The range of adhesive dots that can be produced from a single-thickness plastic stencil.
While standard SMT stencils can be manufactured using polymer foils, plastic as a stencil material has gained real acceptance in adhesive printing over the past five years. The primary benefit of using plastic is that it is possible to create stencils up to at least 8 mm in thickness.
Figure 3. Bottom side of a plastic stencil, showing undercut features to sit over board components.
Such stencils are manufactured using standard CNC machining techniques. By drilling apertures of different sizes in the plastic, adhesive deposits of varying heights can be printed from a single thickness stencil with a single print stroke (Figure 2). The extreme thickness also allows the stencil underside to be undercut and routed so that it can accommodate previously placed components and clinched leads (Figure 3).
Design Rules and Capabilities
The size and shape of stencil apertures determine the volume, uniformity and definition of the material deposited onto substrates. Rigorous control of aperture quality therefore is critical to successful stencil design, particularly for fine and ultra-fine pitch applications where small amounts of material must be deposited with great precision. Measures such as area ratio (the area under the aperture opening divided by the surface area of the aperture wall) and aspect ratio (aperture width divided by stencil thickness) can be used to determine appropriate aperture sizes.
The general rule is that, for acceptable paste release, the area ratio should be greater than 0.66 and the aspect ratio greater than 1.5. When designing apertures that adhere to these rules, it is necessary to consider each stencil manufacturing technique on its own merits. For example, it is challenging for the chemical etching process to drop below a 1.5 aspect ratio while, with laser cutting and electroforming, apertures can be produced that have a 1:1 aspect ratio to the stencil thickness.
Of greater value to the stencil designer is the area ratio, which can be related directly to eventual solder paste release. During the printing process, when the stencil separates from the substrate, competing surface tension forces dictate whether the solder paste will transfer to the pad it has been printed on or remain adhered to the stencil aperture walls.
When the pad area is greater than 66 percent of the aperture wall surface area, the probability of achieving efficient paste transfer is increased. As the ratio decreases below 66 percent, paste transfer efficiency decreases and print quality becomes erratic. The finish of the aperture walls can have an impact at these levels. Laser-cut apertures that have been electropolished and/or electroplated during manufacture promote improved paste transfer efficiency. Similarly, the smooth aperture walls produced by electroforming also enhance paste release.
Component pitches and aperture density also can dictate the appropriate manufacturing technology to select. For applications with pitches below 0.5 mm, choices are limited to laser cutting or electroforming. Both techniques are capable of producing high-quality, accurate fine-pitch stencils, though each has its own advantages and disadvantages.
The laser process does not require artwork, reducing misregistration issues. Foil thicknesses of 50 to 500 µm can be cut, although stock foil thicknesses are limited. Conversely, the thickness of electroformed foils can be controlled in 2.5-µm increments, anywhere between a total thickness of 25 and 300 µm. Electroformed apertures reproduce the mandrel photo resist finish faithfully and do not require any further processing. However, laser-cut apertures might require further treatments to smooth aperture walls for ultra-fine-pitch applications.
Finally, laser cutting is a serial process, so time-to-manufacture increases as aperture count does, impacting the stencil's final cost. The features in an electroformed stencil are formed in one process so aperture count does not affect costs so dramatically. This makes electroforming the preferred technique for high-density applications such as wafer bumping, where aperture counts are now more than 2 million.
Expanding Capabilities
The ability to fabricate stencils with stepped apertures in the top surface and pockets routed out of the bottom surface has expanded the capabilities of the basic stencil printing process. Depending on the nature of the material — its viscosity and flow characteristics — different sizes and shapes of apertures can be created to produce different volumes of deposits.
A second advance that contributes to this expansion is the enclosed print head, which allows a wide variety of materials to be delivered through the stencil to the substrate. When the material is fully contained within the printing system, concerns about handling, drying out, moisture absorption or waste are eliminated.
These two advances allow the stencil printing process to be inserted into the assembly process wherever required, before or after placement of different types of components. As a result, virtually any material required in electronics assembly now can be deposited through a properly designed stencil onto bare or partially populated substrates, at a pace significantly higher than even the most advanced dispensing equipment. Since any number of apertures can be filled in a single traverse of the print head over the stencil, even the highest I/O counts will not create bottlenecks on a high-speed, high-volume assembly line.
The process also is inherently flexible, since the basic equipment platform remains the same. With a change of stencil and perhaps an adaptation to the enclosed print head, a stencil printer may be used as needed to deposit solder paste, adhesive, flux, encapsulants or thermal interface materials.
Wafer Bumping
In this process, a standard stencil printing technique is used to print solder paste directly onto silicon wafers (Figure 4). To achieve the required bump heights, the solder paste is over-printed onto the wafer bond pads. During subsequent reflow the solder pulls back onto the wettable pad surface to form a solid solder bump structure.
Figure 4. Solder paste deposits, printed directly onto a wafer using 110-µm square apertures on a 150-µm pitch. During subsequent reflow, the solder will pull back onto the wafer bond pads to form individual solder bumps.
Executing the process with the ultra-fine features involved requires extreme control. Correct stencil design is key. However, it is no trivial task to create well-designed stencil apertures that achieve defect-free results with a tight distribution of tall reflowed bumps, especially for full array pad arrangements with pitches of less than 250 µm.
For a wafer bumping stencil, the cutting technology must be capable of producing thousands of small, closely spaced apertures to extremely tight dimensional and positional tolerances. Small excursions from the optimally designed aperture size can lead to large bump height variations, which may, in extreme cases, produce open circuits in the assembled chips.
Figure 5. Transfer efficiency of a solder paste related to aperture area ratio and its effect on reflowed bump co-planarity.
It also is critical to maintain the accuracy of the aperture position as close to the computer-generated design as possible. Since the entire pad on the wafers must be overprinted to achieve an acceptable reflowed bump size, when overprinting is done on tight pad pitches, the apertures must be positioned with enough space between them so that bridging defects do not occur. As a rule of thumb, aperture webs should be no less than the stencil thickness to curtail bridging defects.
Sometimes aperture openings need to be offset so they are not centered directly over pads. Depending on the density of the apertures in a particular region of the stencil, it may be necessary to print on only a portion of the pad. Considering that the pad size on a wafer can be less than 100 µm, the stencil cutting technology must be able to position the aperture openings precisely, with offsets of only a few microns one way or the other (Figure 6).
Figure 6. The latest wafer bumping stencils for 300 mm can contain more than 2 million apertures.
When designing the stencil aperture itself, common practice is to work backwards from the bump specification. From this information the volume of solder paste required can be calculated, which can in turn be used to design the aperture dimensions. In an ideal situation, the largest aperture in the thinnest stencil with the maximum space between apertures provides the optimum process.
Based on extensive research, paste transfer efficiency curves have been generated to assist the stencil designer. Figure 5 shows the transfer efficiency of a solder paste with respect to aperture area ratio and the resultant effect on reflowed bump co-planarity. From this it becomes clear that apertures must be designed with an area ratio in excess of 0.6 for a repeatable process.
Figure 7. Closeup of the stencil shown in Figure 6, with 110-µm square apertures on a 150-µm pitch.
Designing a wafer bumping stencil is a highly skilled task. A number of parameters interact, such as stencil thickness, aperture size, shape and other factors, including image orientation and positioning, which impact the process. As the industry moves towards 300 mm wafers, with pitches pushed down towards 120 µm, the process is about to challenge stencil technology even further (Figures 6 and 7).
Conclusion
Stencil design and manufacturing technologies have evolved to meet the needs of both standard and fine-pitch SMT assembly, and will continue to do so. One challenge is the increasing use of a wider mix of components, requiring both large and small material deposits. Stencil suppliers need to explore ways to overcome current area ratio rules.
However, wafer bumping will pose the greatest challenge to stencil technology. While today's most advanced applications use 150 µm pitches on 300 mm wafers, the 2002 International Technology Roadmap for Semiconductors called for 80 µm pitches by 2007.
The issues involved in meeting this requirement include manufacturing capability, as well as image alignment accuracy and stability. Current materials may have reached their limits. In order to achieve pitches below 100 µm, forward-looking manufacturers may need to explore new stencil materials and manufacturing techniques.
Mark Whitmore, future technologies manager, may be contacted at DEK, mwhitmore@dek.com.
The Five Steps of Stencil Processing
By Phil AplinFrom the moment an order for a new laser-cut stencil arrives until the completed stencil leaves, a carefully plotted sequence of events ensures the accuracy and timely completion of each step in the process. At one company's* stencil manufacturing facilities, the process can be broken down into five steps.
1. Sales Order Processing
A Global Stencils Database controls workflow. To assist in the order entry stage, every customer has a profile set up, which automatically populates key information when each job arrives: customer and invoice addresses and contact details on the commercial side and, for production, frame type, size, front or center justified, text position, and type and fiducial type.
Once an order is taken, a unique image number is generated for each stencil, ensuring any repeat orders match the first one. The database leaves only variables such as turnaround and delivery service to be entered.
2. CAD Manipulation
The company operates two software packages to develop jobs. One** is used at the preview stage and for basic manipulation work, and the other*** for more complex orders. The latter package can save specific customer designs into a library so that repeat orders can have exactly the same modifications applied, eliminating potential errors. Gerber 274 X or ODB++ are the preferred formats for the data used to design the stencil.
First, the operator previews the data and order information, making sure all the correct information needed to design and produce the stencil has been supplied. This step is the key process in stencil flow because, if anything is specified incorrectly at this point, the stencil will be designed and produced incorrectly.
The data then is passed to the manipulator, who "teaches" the apertures and, if needed, carries out any modifications required by customer instructions or design specifications, as well as company recommendations. The manipulator then positions the stencil into the frame, adds the required text, and outputs the CNC file for the laser machine and the GBX file for the scanner.
3. Manufacturing
Stencils come in all shapes and sizes but generally can be categorized into two distinct types: framed or frameless.
The sequential nature of the laser cutting process accounts for the majority of time each order is in the system. The cycle time depends on the aperture shape and aperture count of each job. Typically, the company's lasers operate at 3,000 apertures per hour, adding any customer-requested fiducials onto the foil and either half-etching or cutting through any text requirements.
4. Quality Control
The finished stencil undergoes quality checks for aperture presence and size. Aperture presence consists of matching the GBX file generated for the scanner at the manipulation stage against a raster, or scanned image, of the stencil. The operator overlays the two images and the software highlights any missing or blocked apertures. The second check consists of measuring critical apertures on each job against the Gerber data measurement.
5. Dispatch
Once data checks have been successfully completed, the stencil is visually checked and cleaned, put into its protective packaging and delivered to the customer. The invoice usually is sent to the customer the following business day, detailing the contents of the order and any additional work performed. Three-quarters of all orders are completed within a four- to 24-hour period.
* DEK** GC Cam*** Valor
Phil Aplin, UK PSP operations manager, may be contacted at DEK, E-mail: paplin@dek.com.