Using stencil printing to bump wafers: convergence in action
02/01/2000
Integration of assembly and fabrication techniques saves time and money
JEFFREY D. SCHAKE
The traditional division of the electronics industry into two segments - "back end" component packaging and "front end" product assembly, each with its own distinct production techniques - was rarely questioned as the industry has evolved. However, recent advances in both packaging and assembly technologies have raised the issue of convergence: Is there a potential for overlap between these two distinct areas of the industry? In a number of instances, each of the two segments already has production methods and expertise that can benefit the overall process of creating electronic packages and assembling them into end products.
A case in point is the process of using stencil printing for wafer bumping. Stencil printing has long been a standard surface mount technology (SMT) assembly process. In its simplest terms, solder paste is pushed through stencil apertures to form a pattern of pads on a substrate. Components are placed on the pads and then reflowed for attachment. Over the years, as the SMT industry has progressed - developing smaller components, more densely populated boards and different substrates - so has stencil printing. It has become a highly sophisticated and repeatable manufacturing process, capable of extreme accuracy in depositing complex, ultra-fine pitch solder paste patterns for advanced applications.
In the course of developing these advanced techniques, an equipment manufacturer can amass a significant, in-depth understanding of the numerous process elements that relate to stencil printing. When this knowledge is applied to resolving the various issues involved in using stencil printing for wafer bumping, the result is an integrated solution for cost-efficient, high-volume wafer bumping.
Stencil Printing Benefits
Stencil printing has been used for wafer bumping for some time, with a variety of time and cost-saving benefits. The process is fast, because the solder paste is deposited across the entire surface of the silicon wafer simultaneously, rather than sequentially, and higher I/O-per-wafer counts do not lengthen the cycle time, as sequential bumping does. Fewer process steps are involved, because there is no need for masking, and productivity is high.
Stencil printing is also economical. Even the most advanced, programmable stencil printing systems are significantly lower in cost than conventional substrate bumping systems. This is critical for all fabricators, particularly the rapidly growing and highly competitive contract segment of the packaging industry. With the use of flip chip technology projected to increase significantly over the next decade, a growing need for high-volume, low-cost wafer bumping is expected to follow.
There are three categories of issues that have previously limited the use of stencil printing for wafer bumping to low-volume applications: wafer handling, environmental material control and stencil cleaning. The lack of automated handling systems, which has required that wafers be loaded and unloaded manually, is not conducive to high-volume operations. When a traditional squeegee is used to propel solder paste through a stencil, the paste is exposed to the atmosphere and can dry out, absorb humidity and contaminate the work area. The need to remove residual solder paste from stencil surfaces and apertures, by cleaning both the top and bottom of the stencil manually after each cycle, limits throughput. However, it is possible to have a clean, automated and production-ready solution for high-volume wafer bumping.
Automating Wafer Handling
Automated wafer handling is critical to transform a manual, batch mode operation into a high-volume bumping process. Traditionally, wafers were unloaded from a cassette, inserted into the stencil printer for processing, and then removed and reloaded into a cassette to be taken to the next process step.
An integrated system can bring automated handling into the process by converging the manufacturers of stencil printing equipment and the type of robotic equipment typically used in a fabrication facility. Equipment, as well as standards and process requirements associated with both packaging and assembly operations, were coordinated to work in concert. The robotic systems, designed to comply with SEMI standards, were adapted to meet the SMEMA standards of the stencil printer.
For this example, a stencil printer is flanked by two clean room-compatible wafer handling modules. The presentation module has entry points for two cassettes; once these are loaded into the unit, the rest of the operation proceeds automatically. A robot extracts a wafer from one of the cassettes and pre-aligns it for delivery to the stencil printer (Figure 1). Each wafer is rotated so that the stencil apertures are presented at a 45° angle (Figure 2); such a rotation can provide an equal angle of attack for solder paste transfer and an equal volume transfer. The wafer is placed on a specially designed pallet with a recessed pocket that holds the wafer flush with the top of the pallet. As the pallet moves through the stencil printer for processing, vacuum assistance prevents the wafer from adhering to the bottom of the stencil.
The bumped wafer is then transferred from the printer into the pass module, where a second robot unloads it from the printer (Figure 3). The pallet remains inside the printer and returns automatically to the machine`s entry point to receive the next wafer. The finished wafer can be placed into a cassette for removal, or directly onto a conveyor for reflow processing. The entire system can be operated in batch mode or as a fully automated, pass-through procedure.
Automating Material Control
Within the stencil printer, the issue of environmental material control can be resolved by replacing conventional squeegees with a fully enclosed, pressurized print head (Figure 4). Because the solder paste is fully contained within a cassette that is inserted into the print head, the material is not exposed to the atmosphere until it is extruded through the stencil apertures and onto the surface of the wafer. The paste is maintained continuously in optimal condition, minimizing waste because of environmental degradation.
A pressurized print head also addresses some of the issues related to stencil cleanliness and process automation. The print head scoops up residual paste as it moves over the stencil, minimizing the amount of material remaining on the top surface. This renders the entire process to be clean and "hands-free," because it eliminates the need for the operator to handle the paste. Another study also supports the capability of this technique to bump wafers without cleaning the stencil surface between prints, thus reducing the amount of manual intervention required for the process.1
Enhancing Automated Cleaning
Currently, manual cleaning is often used to keep the apertures clear and, thus, ensure high yields. However, automated cleaning methods are being addressed by comparing both squeegees and a pressurized print head when bumping a series of wafers with a variety of stencil aperture cleaning methods: no cleaning (apertures wetted with solder paste), manual cleaning and a prototype of an automatic cleaning system.2
A prototype system consisting of foam strips contained in carrier units that were located on either surface of the stencil, directly opposite but slightly offset from each other, was studied. The leading edge foam strip in each unit was saturated with a cleaning solution. As the two carriers moved in unison across the stencil, vacuum was applied from the bottom, suctioning the cleaning solution and any residual paste or flux through the stencil apertures and into the bottom foam layer.
Comparisons of yields and bump heights indicated that clean stencil apertures, whether achieved manually or automatically, produce a repeatable process with higher bumping yields (Figure 5). The prototype system showed promise on two fronts: aperture cleaning efficacy and the ability to increase the level of automation in the overall process. The development of this system is continuing.
Conclusion
For high-volume wafer bumping, the integration of assembly and fabrication techniques - advanced stencil printing with a pressurized print head and robotic wafer handling systems - can enhance the speed, material control and level of automation involved in a process. While this combination can improve process cleanliness, further innovations are introducing a greater degree of automated stencil aperture cleaning, as well. The result may be considered process convergence, as new solutions draw on previously disparate technologies to automate the manufacture of advanced packages entering the marketplace.
References
1. James H. Adriance and Mark A. Whitmore, "Wafer Bumping with Pressurized Print Heads," HDI, June 1999.
2. James H. Adriance, Mark A. Whitmore and Jeffrey D. Schake, "Bumping of Silicon Wafers by Stencil Printing," Semicon Southwest, October 1999.
JEFFREY D. SCHAKE, advanced technologies engineer, can be contacted at DEK USA, 8 Bartles Corner Road, Flemington, NJ 08822; 908-782-4140; Fax: 908-782-4774; E-mail: [email protected].
 |
Figure 1. Wafer extraction from a cassette is the first step in automated bumping.
 |
Figure 2. Wafers rotated to optimize the squeegee angle of attack.
 |
Figure 3. Automatic wafer unloading completes the integrated bumping process.
 |
Figure 4. A pressurized print head can speed the wafer bumping process and enhance material control.
 |
Figure 5. Maintaining stencil aperture cleanliness can ensure high bumping yields.