Stencil Printing for Wafer Bumping

Jeffrey D. Schake, DEK Advanced Technologies Group, Flemington, N.J. -- Semiconductor International, 10/1/2000

Several techniques can be used to apply a solder alloy to the pad sites on a wafer, a process known as wafer bumping. While most of these require high-priced, specialized equipment, the use of stencil printing for wafer bumping offers an attractive, cost-saving alternative for high-volume production. In the few seconds it takes for the printhead to distribute solder paste across a stencil, hundreds of thousands of pads can be bumped simultaneously. This method does require the utilization of unique design guidelines tailored for each application. In addition, specific equipment, materials and process instructions need to be followed to establish a robust, high-yield wafer bumping print and reflow procedure.

Since the printing method for wafer bumping is designed to be compatible with the tools on a standard surface mount assembly line, no additional investment in upgrading or retrofitting existing capital equipment is required. It is suggested, however, that the stencil printer be equipped with precision optics for automatically locating and aligning fiducials. The printer alignment system should be able to recognize small, non-standard fiducials on the wafer; these include pad corners, distinctive emblems, insignia or other metalized features that have good contrast against the background covering.

To ensure that the delicate wafer is secured and protected during the printing process, a rigid pallet should be used to support the wafer. The pallet functions as a wafer carrier for safer overall handling and can be machined for easy drop-in use on the printer's existing conveyer system. The pallet also supports the implementation of a fully automated, hands-off wafer printing system with robotic wafer placement modules.

Water-soluble solder paste is customarily used for wafer bumping. Although the current trend is toward the use of no-clean solder pastes, it is essential not to use these formulations for wafer bumping due to the difficulty of cleaning and removing stubborn flux residues.

Despite the straightforward nature of stencil printing, numerous variables can severely degrade the quality or yield of wafers bumped by this method. The issue of known-good-die (KGD) is typically used to benchmark the performance of a wafer bumping process. With a solid understanding and respectful appreciation of the many design aspects that affect this process, sound design strategies can produce excellent results.

Flip-chip bond pad design

The stencil printing bumping method is probably most limited and hindered by the layout of the bond pads on the die. Pad size, shape and pitch need to be designed carefully to realistically achieve the required solder bump sizes. The pad bonding surface composition, known as the under-bump metal (UBM), also requires consideration.

The size of the pads on the chip has a direct influence on solder volume requirements. Smaller pads require less paste than larger pads to achieve the same target reflowed bump height. When the pad size is too small, however, less bonding area is available to support a relatively large volume of solder. Thus, bump and solder joint strength may be sacrificed for only slight gains in standoff distance. Solder bump sizes also are affected by the shape of the wettable bonding area of the chip pads. This shape is not necessarily dependent on the geometry of the pads, but is chiefly determined by the passivation opening overlying the pads (Fig. 1).

In Figure 1, the shape of the actual bonding site, as defined by the passivation window, is octagonal on a square pad, to reduce the stress concentrations that may occur at the corners of the solder ball base. These stress points can potentially weaken the bond strength and deteriorate solder joint reliability. Circular passivation openings are ideal to minimize this.

A general rule of thumb is to optimize the sizes of the pad bonding sites to be no larger than half the pitch distance, but larger than 65 µm in diameter.¹ To calculate the size of the reflowed bump as a function of pad size and geometry, the following equation is used. This equation models a reflowed bump as a truncated sphere geometry and is a helpful stencil design tool for determining the appropriate aperture size that will deliver a required volume of paste.

where:

V = solder bump volume

A = pad area

H = bump height

The ability to produce large solder bumps on wafers by stencil printing depends strongly on pad configuration and density. High-density layouts can be better designed to accommodate stencil printing of larger paste deposits by staggering adjacent pads apart from one another (Fig. 2). The current state of the art for stencil printing on fine pitch seems to be hovering around the 6-mil threshold, although these limits are continually being pushed further. It is possible to print on smaller pitches, but the quantity of paste that can be deposited without defects is restrictive enough to raise concerns about bump size and issues of standoff, underfilling capability, solder joint strength and overall reliability.

Concerning the location of the pads, the corner pads of each die are typically the most difficult locations on the wafer to achieve uniform, defect-free printed solder paste volumes.² The use of non-standard aperture shapes is sometimes necessary in these places, due to the high density of aperture openings that may consume a significant portion of stencil area. It is difficult to predict the nature of the transfer efficiency that can be achieved through uniquely shaped apertures; thus, the bump distribution across these die may not be favorable. By eliminating the corner pads altogether, this potential problem can be avoided to give more leniency in the design of the stencil apertures.

There are several under-bump metals that can be used and many techniques of depositing them. With the high cost associated with the UBM deposition process, there is much competition to find cost-effective materials and processing methods. Hence, much of the information regarding UBMs is proprietary and withheld from public eye. One commonly known method that has proven a reliable, low-cost option involves plating the aluminum pad through electroless methods with nickel, then immersing it in gold.³ In this process, only those non-passivated metal features (i.e., the aluminum bond pads) will be blanketed with the Ni/Au UBM layer. The Ni/Au system has been found to be compatible with eutectic Sn/Pb solder alloy, exhibiting excellent reliability characteristics.⁴ To maintain favorable wetting characteristics, it is important to keep the UBM as clean and oxide-free as possible. Storing wafers in an insulated, nitrogen-regulated storage cabinet is ideal to retard buildup of undesirable oxide residues.

Pallet design

Since wafers are very delicate objects, handling is a critical issue. A pallet made of durable material that is at least 1.5 in. larger on all sides than the wafer should be used to hold the wafer during stencil printing. The reason for oversizing the pallet is to allow a large enough surface for the printhead to prime and circulate the solder paste before it is emptied into the apertures.

Another critical element of a properly designed pallet is the recessed pocket area that secures the wafer in place. The pocket should be deep enough so the wafer's top surface is level with the surrounding plane of the pallet. The pocket opening should be located in the center of the pallet so the stencil artwork can be aligned to it properly. The pocket boundary should outline the contour of the wafer with a tight fit, as accurately as possible.

3. Vacuum channels help hold the wafer firmly on the pallet.

Wafers often have a flat edge or v-notch, which also should be designed into the pocket for accurate and repeatable wafer positioning. To hold the wafer down firmly on the pallet, vacuum channels are essential. Suction may be supplied through the printer or from a portable external pump. A picture of a wafer bumping pallet is found in Figure 3.

Stencil design

The stencil is probably the most important tool that must be designed correctly to achieve good bumping results with high yields. Issues that affect stencil-cutting technology for wafer bumping include stencil thickness, aperture size, shape, orientation and position.

For a wafer-bumping stencil, the cutting technology must be capable of producing thousands of very small, closely spaced apertures to extremely tight dimensional and positional tolerances. It is critical to maintain the accuracy of the aperture position as close to the computer-generated design as possible. Small excursions from the optimally designed aperture size can lead to large bump height variations, which may, in extreme cases, produce open circuits for assembled and underfilled chips. Generally, the minimum space between apertures is determined by the stencil thickness. On occasion, aperture openings need to be offset so they are not centered directly over pads. Considering that the pad size on a wafer can be less than 4 mils, it is critical that the stencil-cutting technology be able to position the aperture openings precisely, with tolerances on the order of a few microns one way or the other.

Of the available technologies, only laser-cut and electroformed stencils are suitable for replicating computer-generated artwork to meet the requirements of printing effectively on wafers. The laser cutting process leaves the aperture walls with a rough surface morphology and jagged edges. This characteristic can be remedied with special polishing and electroplating techniques. Another attribute of the laser cutting process is that the aperture openings have tapered walls with the opening size on one side of the stencil slightly larger than on the opposite side.

Some suggest tapered apertures actually enhance paste release from the stencil, provided the larger opening is on the stencil's bottom side. The downfall of the laser cutting process is the high cost of producing stencils with numerous small apertures. As the number of apertures increases, so does the cost of making the laser-cut stencil. Alternatively, the cost of producing the artwork on an electroformed stencil does not depend on the number of apertures in the design. Electroformed stencils are just as accurate in terms of aperture size and position as the laser-cut stencils, and the aperture walls are relatively smooth, although they are not naturally tapered.

The thickness of the stencil metal foil is significant in determining the size of the opening needed to achieve the required volume of paste to produce a target reflowed bump height. Thicker stencils are attractive because the openings do not need be as large to achieve the same aperture volume requirements as they do with thinner foils. This can be helpful when designing for challenging applications with tight pitches. Thus, as a general rule, it is best to use the thickest foil that delivers good paste transfer efficiency. However, paste transfer efficiency suffers with thicker stencils because more paste adheres to the aperture walls. The most common thickness for pad pitches in the range of 8-12 mils is 2.5-4 mils, with paste transfer efficiencies usually exceeding 70%. Wafer-bumping research studies recommend selecting a stencil thickness for which the ratio of the wall to opening area (area ratio) of the smallest aperture on the stencil is less than 1.75.⁵ There is a strong interrelationship between stencil thickness and aperture size in the proper design of a stencil that complies to the area ratio recommendation of less than 1.75, as shown in Figure 4. While thinner stencils require larger aperture opening sizes, thicker foils need smaller apertures to satisfy equivalent solder volume requirements. With tightly spaced pads, it can sometimes be a challenge to find the right combination of aperture size and thickness to produce an acceptable transfer efficiency that satisfies paste volume requirements while not crowding apertures too closely together. Often the greatest stencil design challenge lies at the corners of the die, where this situation is common. Apertures should be designed for these troublesome areas first, to identify any congestion issues that may need to be addressed. Adjacent apertures should always be designed to be spaced at least as far apart as the stencil foil thickness to avoid bridging defects.

To achieve more consistent and better filling quality, apertures with rounded corners offer better results than apertures with sharp corners. It is easier for paste to surround and encompass gentle turning boundaries than to occupy abrupt intersections fully. Ovals and circles have a much better filling characteristic than triangles and diamonds.⁶ The drawback of using rounded corners on apertures lies with the release process. Rounded edges, as opposed to sharp edges, contain more surface area and rob volume capacity from the aperture. In comparing a circle with a square, it is evident that, for equivalent area ratios, the square aperture has more volume capacity.

Nevertheless, it often is difficult to use either square or circular apertures on wafer-bumping stencils due to pad density constraints that can lead to severe aperture congestion and bridging defects. To deal with this, longer and narrower aperture shapes, in the form of either rectangles or oblongs, are preferred. Oblong apertures are ideal to satisfy wafer bumping paste volume deposit requirements because they can be made narrow to accommodate tight pitch designs. Since the lengthwise boundaries of these apertures are rounded, this feature provides for better filling characteristics.

Circular apertures do not have an orientation issue, since any perspective always shows the same object. Squares, however, become diamonds at a 45° angle. Oblong and rectangular apertures suffer more serious consequences because they appear different when oriented east-west, north-south or at any intermediate angle. The way in which stencil-printed solder paste fills the apertures can be different for dissimilarly oriented apertures of the same size and geometry. To achieve uniform filling and release, it is ideal for all apertures to be positioned in the same orientation relative to the direction of the print stroke; however, this is not always possible without design tricks.

One technique is to have all apertures on the stencil be orthogonal to one another, similar to the orientation strategy of apertures used for a standard quad flat pack (QFP) component. At a 45° rotation, these orthogonal apertures all become diagonal, as shown in Figure 5. Often, stencil vendors may simply be able to rotate an entire pattern of orthogonally oriented apertures at a 45° angle. The wafer also needs to be rotated under the stencil to comply.

The overprinting of pads is essential in stencil printing wafers to get enough paste volume for a distribution of sufficiently tall reflowed solder bumps. In very tight spaces, it may not be possible to place apertures centrally and symmetrically over pads and leave enough space between the adjacent apertures to prevent bridging defects. In these cases, it is suggested that at least 50% of the pad be covered with paste for proper pullback of molten solder to coalesce completely onto the pad. Also, for good pullback behavior, it is far better to have the center of the aperture offset from the pad than to have the end of the aperture offset from the pad (Fig. 6).

For tightly spaced pads in a single peripheral row arrangement, it can potentially be a problem to accommodate large deposits without bridging. In such cases, the apertures can be made long and narrow, in oblong or rectangular geometries, and arranged in a staggered formation to separate the adjacent solder deposits as much as possible (Fig. 7). Given enough space, this staggering scheme also may be applied to multiple row pad layouts. For pads located near the dicing lanes, printed paste deposits can be allowed to occupy these areas as long as they are clear of any non-passivated metalization. Problems also may arise in finding enough room to fit apertures at die corner pad locations. Since the most serious constraints on aperture attributes occur at these places, it is suggested to begin the stencil design process here to avoid potentially disastrous problems downstream.

Process parameters for wafer bumping

Once the design of the stencil and pallet are completed, there is little that can be done to change or re-work them. Because of this, design rules have been more completely developed for these materials. Due to the staggering number of combinations that are possible, there is a much greater diversity of opinion concerning the process parameters appropriate for stencil printing wafers. Some of the most important issues to be considered for successful wafer printing include solder paste, the printing environment, printer machine settings, the reflow profile and cleaning options.

As mentioned earlier, water-soluble solder paste is required in this wafer bumping process. Another important attribute of the solder paste is particle size and distribution. This depends mostly on the volume requirements for the solder deposits and the size of the aperture openings on the stencil. It is generally agreed that the particle distribution of a paste should allow at least three of its largest solder sphere diameters to fit into the width of the smallest aperture on the stencil.⁷ It is best to use the finest solder powders in the stencil printing of wafers; the Sn63/Pb37 alloy in a Type V particle distribution with 90% metal loading has been reported to be effective for bumping pads on a 10 mil pitch to achieve 5 mil tall bumps.⁸ With such small particle sizes, more alloy surface area is exposed to the elements of oxidation, and the constraints of exposure time and useful lifetime are shorter than for typical SMT pastes. To maximize the lifetime of the paste, it should be removed from the stencil whenever the printer is idle. One of the key benefits of using an enclosed printhead is that the paste is always enclosed in the conditioning chamber or isolated in the cassette from direct exposure to the ambient.

Environmental variables in the assembly facility are not always well controlled. The primary consequence of environment on the assembly process is moisture absorption into components, substrates and, especially, solder paste.

Compared to no-clean pastes, water-soluble wafer-bumping pastes are more sensitive to environmental changes. Relative humidity levels exceeding 60% induce moisture loading in water-soluble paste; and, on the opposite end of the scale, relative humidity lower than 40% causes premature paste dryout. At either extreme, the rheological character and printing behavior of the paste is altered.⁹ As a result, the stencil printing process will deviate significantly and be the most affected part of the assembly procedure.

Since the integrity of this wafer bumping method depends on a robust stencil printing process, regulation and close observation of the environmental conditions are paramount to achieving stable yields. It is recommended that temperature be kept between 21ºC and 25ºC and relative humidity levels be between 40% and 60%, with as little deviation as possible from the set points.¹⁰ It also is highly recommended that the paste vendor's product data reports be checked to select optimum climate control settings.

Hard polyurethane or metal squeegee blades have been used successfully for a stencil-printing wafer bumping process. It has been found that polyurethane squeegee blades with a hardness below 90 durometer on the Shore A scale do not provide optimal solder paste transfer efficiencies and may exhibit adverse print definition characteristics.¹¹ The deformation of soft squeegees will pump paste through thin apertures, allowing it to leak under the stencil and potentially bridge with adjacent deposits. Squeegee deformation can sometimes be reduced by using lower print pressures to improve the print definition. It is recommended to use the lowest pressure that will wipe the paste cleanly across the apertures during a printing stroke. Some consequences of excessive squeegee pressure are premature stencil wear (especially on thin metal foils), paste bleeding and squeegee blade flexing. Blade flexing under pressure can reduce the squeegee angle of attack below 45°, possibly restricting paste rolling motion. Squeegee angles between 45° and 60° are appropriate for this paste deposition method. As far as printing speeds are concerned, 7 mm/sec to 25 mm/sec is an appropriate range.

The introduction of the enclosed printhead has proven to be a great benefit for bumping wafers. The acceptable process window is much wider for such a paste deposition system, allowing a much broader range of print parameter settings that provide for better overall environmental control of the printing medium to deliver superior results. Higher-metal-content pastes can be used with these systems, and fuller paste deposit profiles can be achieved by the direct injection of solder alloy material into the apertures via an independent application force. Generally, the amount of direct pressure applied to the paste should be strong enough to form well-defined deposits without excessive flux bleedout. The wiper pressure needs just enough force to maintain a clean stencil surface throughout the printing procedure. Finally, print speed can be increased beyond the limits of printing with squeegees, if necessary, and still provide good print definition.

The amount of solder paste applied to the wafer bond pads can be affected significantly by the printing gap or snapoff, which controls the distance between the bottom of the stencil and the substrate. Wafer bumping in off-contact mode can be troublesome and difficult to control. The thin stencil foil is vulnerable to stretching, and the stresses induced by the traveling printhead can lead to permanent deformation in this print mode. Furthermore, in off-contact printing the paste definition is more susceptible to smearing and paste bleeding, which often leads to bridging and scatter in the distribution of deposited volumes. With on-contact stencil printing, the most common practice, the release rate of the substrate from the stencil is an additional factor that can be controlled by the printing machine. To achieve the most uniform printed deposit distribution, it is best to use a slow separation speed to gain the highest transfer efficiency and obtain finer print definition.

The cleanliness of the stencil is critical to the success of this bumping process. The combination of tacky solder paste with small particles and tiny stencil apertures can reduce the transfer ratio of paste extruding out of the apertures. Even after a single printing stroke, the stencil apertures can accumulate a significant lining of paste residue, which may dry quickly and contaminate subsequent printing strokes. For this reason, thorough cleaning of the stencil between each print is recommended. Manual wiping using lint-free cloths with de-ionized water or isopropyl alcohol works well. The development of a fully automatic top and bottom stencil cleaning device will enhance this application; promising results have been achieved recently with a prototype system.¹²The use of a standard surface mount assembly line reflow oven is appropriate for melting the stencil-printed solder material to form the solid spherical bumps on wafers. The industry considers that forced-convection reflow ovens with both top- and bottomside heating in nitrogen environments and low oxygen PPM levels give optimal performance. Conduction reflow ovens also are ideal to use with wafers.

It is standard practice to use a reflow profile recommended by the solder paste vendor. This generally consists of a preheat ramp to a flux activation temperature plateau to prepare the alloy and pads for bonding, followed by a sharp spike above the alloy's liquidous temperature. A rapid cooling period completes the profile. The time the wafer spends in each temperature zone in the reflow process is quite important, and the temperature ramping/cooling rates should be controlled precisely, according to paste vendor specification. Characterization of the reflow process occurs through the use of a special temperature recording device that monitors the temperature of different locations on the substrate during reflow as a function of time.

It is highly recommended that the solder bumps on a die ready for flip-chip assembly be as clean as possible, to bond more effectively with substrate bond sites and to allow encapsulant to flow freely between adjacent solder joints through the physics of capillary action.

After the printed wafer exits the reflow oven, the water-soluble flux residue must be washed from the bumps immediately. The preferred cleaning method is to immerse a batch of wafers in a cleaning tank holding warm, de-ionized water (agitation may improve cleaning efficiency). Automated wafer cleaning systems are available, and for high-volume bumping this approach is likely to operate more quickly and without issues associated with manual handling. Once washed, the wafer is dried with clean forced air or nitrogen, followed by a final 125ºC bake for up to two hours to drive out any absorbed moisture.

Conclusion

The technique of bumping wafers by stencil printing is a production-ready, low-cost alternative to standard techniques such as evaporation or electroplating. However, as I/O counts increase and pitches continue to decrease, the method of stencil printing solder paste onto those sites becomes more challenging, and the need for more compliant materials (i.e. pastes, stencils, etc.) and process design guidelines grows. Hence, studies are continuing to better understand these details and enhance the process of bumping wafers by stencil printing. 

Jeffrey D. Schake is an advanced technologies engineer in DEK's Advanced Technologies Group and assists with wafer bumping research in Universal Instruments' Consortia efforts. Jeff holds a B.A. in physics, B.S. in mechanical engineering and M.S. in industrial engineering from State University of New York at Binghamton.
Phone: 1-607-779-4384
Fax: 1-607-779-4646
e-mail: jschake@dek.com .

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REFERENCES

1. Adriance, J.H., "Process Summary & Guidelines For Bumping Wafers by Stencil Printing of Solder Paste," DCA/CSP Consortium, Universal Instruments Corp., Binghamton, N.Y., December 1998, p. 29.
   
2. Schake, J.D., "Investigation of Bump Distributions from the Stencil Printing Wafer Bumping Process," Master's Thesis, Binghamton University, Binghamton, N.Y., December 1998, p. 371.
   
3. Nicewarner, E., "Interconnect Resistance Characteristics of Several Flip-Chip Bumping and Assembly Techniques," Microelectronics Reliability, Vol. 39, 1999, p. 113.
   
4. Bogdanski, J., and Patterson, D., "Flip Chip Solder Deposition Processes," NEPCON West, Anaheim, Calif., 1997, p. 606.
   
5. McPhail, D., "Screen Printing is a Science, Not an Art," Soldering and Surface Mount Technology, No. 23, June 1996, p. 25.
   
6. Schake, p. 368.
   
7. Schake, p. 103.
   
8. Adriance, J.H., and Whitmore, M.A., "Wafer Printing With Pressurized Print Heads," HDI, Vol. 2, No. 6, June 1999, pp. 22-23.
   
9. Anglin, C., "Solder Paste and Soldering Defects," Stencil Print and Touch-Up Training Presentation, DOVatron Manufacturing, Binghamton, N.Y., December 1997.
   
10. Adriance, p. 20.
    
11. Adriance, p. 17.
    
12. Adriance, J.H. , Whitmore, M.A., and Schake, J.D., "Bumping of Silicon Wafers by Stencil Printing," SEMICON Southwest, Austin, Texas, Oct. 18, 1999, p. 4.
    

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