Solder Jet Printing: Wafer Bumping and CSP Applications

A solder micro-deposition technology, based on inkjet printing, has been developed for use in microelectronics manufacturing.

By Dr. Donald J. Hayes and Dr. David B WallaceMicroFab Technologies, Inc., Plano, Texas

Flip-chip bonding and chip-scale packages are rapidly becoming critical elements in today's changing microelectronics world. The marketplace in which these two package types are emerging is much more complex today, because the roles of semiconductor front-end, back-end and microelectronics assembly have converged.

Multi-chip packages and modules are replacing what used to be assembly work. Bumped die are going directly to assembly, bypassing the packaging area. Wafer-scale packaging is being discussed and implemented, and companies that used to make packaging equipment are bumping wafers.

These changes are being driven by performance, which is measured by the number of I/Os and operating speed and by package size, which is application-specific. Figure 1 illustrates how the need for higher performance, coupled with the IC manufacturer's ability to deliver higher performance, is driving area-array interconnect technology.

Figure 1. Key market dynamics

The array technology selected today is generally influenced by package size requirements and cost.

Solder jet technology™ is an emerging technology that addresses the packaging needs listed on the right side of Figure 1. The solder jet method is based on piezoelectric demand-mode, inkjet printing technology and is capable of producing and placing molten solder droplets, 25-125 µm in diameter, at rates up to 1,000 per second.

Solder jet-based deposition is low cost (no tooling required), non-contact, flexible and data-driven. No masks or screens are required because the printing information is created directly from CAD information and stored digitally. It is also an environmentally friendly, additive process with no chemical waste.

Ink Jet Technology

In demand-mode inkjet printing systems, a volumetric change in the fluid is induced either by the displacement of a piezoelectric material that is coupled to the fluid, or by the formation of a vapor bubble in the ink, created by heating a resistive element. This volumetric change causes pressure/velocity transients to occur in the fluid. These are then directed to produce a drop that issues from an orifice.1,2 A droplet is created only when it is desired in demand-mode systems.

Demand-mode inkjet printing systems produce droplets that are approximately equal to the orifice diameter of the droplet generator. Figure 2 shows a schematic of a demand-mode inkjet system and Figure 3 shows a single channel MicroFab demand-mode inkjet device generating 50 µm diameter drops of ethylene glycol from a device with a 50 µm orifice at 2,000 per second.

Figure 2. Schematic of a demand-mode inkjet printing system

Figure 3.Demand-mode inkjet device generating 50 µm-diameter drops at 2 kHz

Operation of piezoelectric demand-mode inkjet devices at temperatures above 200°C has been one of the principle challenges in developing solder jet technology. In addition to selecting materials, designs, and assembly processes that are compatible with these operating temperatures, MicroFab has developed patented drive waveforms for piezoelectrics at elevated temperatures.3 Formation of solder spheres with 25-125 µm diameters, on-demand, at rates up to1,000 per second have been demonstrated.4 Figure 4 shows a pattern of 60 µm diameter bumps deposited using solder jet technology, illustrating the data-driven nature of inkjet printing technology.

Figure 4. 60 µ-diameter drops of solder were desposited into silicon using solder jet technology. (Curtesy of Motorola.

Liquid metals, once they are above their liquidi are well behaved (i.e., Newtonian), low-viscosity fluids.5 A number of solders with liquidi under 190°C have been dispensed using solder jet technology. This was initially demonstrated with Indalloy-58, a low-temperature (70°C liquidus) eutectic solder. Most of our efforts have focused on Sn63/Pb37, due to its wide usage in elec- tronics assembly. Other solders that have been successfully jetted include indium, In52Sn48 and Sn63/Pb37 with various dopants. Our current efforts to achieve temperatures in excess of 300°C are discussed below.

Drop-Size Modulation

Solder jet technology is inherently flexible because each droplet is dispensed under digital control. To increase the flexibility of the system, we have developed a novel drive waveform technology that allows the drop size to be modulated over (approximately) a 2:1 diameter (8:1 volume) range. Figure 5 shows a solder-jet device producing 62 µm diameter droplets at a rate of 120 Hz. The image on the left shows the droplet being formed while it is still attached to the orifice of the dispensing device; the image on the right shows the drop approximately 1 ms later, after it has broken free from the dispenser.

Figure 6 shows the same device operating moments later, again at 120 Hz. In this figure, a drive waveform that extended the drop formation process over a significantly longer time period was used. By extending the drop formation process, a considerably larger droplet is produced. In this case, the diameter is increased to 106 µm. The volume modulation using this method is continuous over the entire range of achievable volumes. This capability could be used to allow bump size to be changed under software control, either for product changeover or for the application of variable-sized bumps onto a single substrate.

Figure 5. The drop formation process for a solder jet device is shown at two times actual size during the process. The drop rate is 120 Hz and drop size is 62 µm

Figure 6. The drop formation process for the same solder jet device shown in the previous figure, but using a different drive waveform is illustrated. the drop rate is 120 Hz and drop size is 106µm.

Prototype Platform and Printhead

The piezoelectric, demand-mode droplet generator, described above, was incorporated into a printhead design suitable for integration into a printing platform. Key features of the printhead include a heated inert environment localized to the tip of the droplet generator and impact area of the substrate, separate heaters for the solder reservoir and droplet generator and vertical dispensing capability. The solder jet printhead is shown in Figure 7 mounted on the MPM feasibility platform (Figure 8).

Figure 7. Solder jet printhead mounted on MPM feasibility platform.

Figure 8. MPM solder jet feasibility demonstration platform.

Some of the features incorporated in the platform include:

• Printhead setup, maintenance, and visualization station
• Substrate temperature control
• Vision system alignment of the dispensing site to fiducials on the substrate
• Substrate pad data file input
• Automated dispensing onto the pad locations with an arbitrary number of droplets onto each pad
• Print-on-the-fly for high-throughput operation
• Vision system assessment of solder droplet placement accuracy

Bump Creation

Given droplet creation rates up to 1000 per second, the printing platform must be able to deposit droplets onto pads while the printhead is in motion. To accomplish this task, the printing platform must integrate and synchronize the image data (pad locations), drop-generator controller and substrate motion controller.

As mentioned, the MPM feasibility platform has print-on-the-fly capability and has been used to evaluate throughput related performance issues. Figures 9 and 10 show results from the MPM feasibility platform for printing onto an 18x18 test coupon with 100 µm diameter pads on 250-µm centers. The deposited solder volume is equivalent to a drop diameter of 100-µm.

Note that the drop shape shown in Figure 9 is a consequence of rapid (<100 µs) solidification.6 The instantaneous droplet rate for these tests was 400 droplets per second, and the pattern was printed by rastering the substrate in the horizontal direction of the figures. An average placement error of 10 µm was achieved in these tests. This error is close to the accuracy limitations imposed by the positioning and alignment systems on the MPM feasibility platform.

Figure 9. 100 µ-diameter solder bumps were placed onto 100 µm pads on 250 µm centers at 400 per second on the MPM solder jet feasibility platform.

Figure 10. Another view of figure 9

Although operation at 600 bumps per second has been demonstrated on the feasibility platform, routine testing at this and higher bump rates will require the more accurate positioning and alignment systems that are being included on MPM's commercial platform, discussed below.

Wafer Bumping

To illustrate the use of solder jet technology in a wafer bumping application, the locations of the pads of an integrated circuit test vehicle with more than 1,400 pads were programmed into MicroFab's solder jet research platform. Droplets of Sn63 Pb37, 70 µm in diameter, were deposited onto several of these test vehicles. Figure 11 shows the results from one test vehicle. Figure 12 is a detail from the same image. The solder bump was deposited onto a nickel pad metalization covered by a flash of gold, which promotes adhesion during the droplet impact and freezing process.7

Figure 11. IC test vechicle with 1,440 pads, bumped with 63/37 using solder jet technology. The ball size is 70 µm

Figure 12. Detail of Figure 11

Chip-Scale Packaging

A microelectronic package must satisfy various functional requirements. It must 1) support the die; 2)protect it from the environment; 3) provide direct electrical interconnect and form compliant interconnects to allow for thermal expansion mismatch; and 4)enable easy assembly to PC boards.

Figure 13. CSP manufacturing method

Figure 14. Concept for construction of CSP's on a wafer using inkjet type dispensing for both solder and polymer dialetric

A direct-write, wafer-level CSP concept satisfying these requirements is presented in this section. The key elements have been demonstrated, but the total concept has not yet been proven.

Figure 13 illustrates the three major steps in one version of the proposed direct write, wafer-level, chip-scale package assembly process. First, solder columns with an aspect ratio of 2 or greater (and approximately the same width as the pads) are printed onto each pad using one solder jet device. Second, a dielectric polymer coating is printed onto the die surface and cured (UV or thermal). Third, solder spheres 0.25-0.30 mm in diameter (the size of spheres used today in µBGA packages and in other state-of-the-art CSPs), are printed for interconnect to the substrate pads using a second solder jet device. The solder for the bumps would have a lower melting point than that of the columns so that the columns would not reflow when the CSP is attached to a substrate

This type of CSP has electrical interconnects that are of minimum length, and the leads can extend to more than 500 µm above the die surface to allow for thermal expansion mismatch between the IC surface and the PC board. Figure 14 illustrates this process when both the solder and dielectric polymer coating are printed onto a wafer using ink-jet-type dispensing.

Figure 15. Printed solder interconnect concepts

In volume manufacturing, three printheads (solder columns, polymer and solder balls) can be mounted onto a single machine, or the wafers can be processed in series through three separate machines dedicated to a single process.

Figure 15 illustrates how the concept of Figure 13 can be extended to a three-dimensional interconnect structure for redistributing leads. The first layer can be printed as discussed above. Horizontal and vertical solder interconnects are printed in the next process step, followed by another polymer layer. This process can then be repeated.

The basic components of the process described above, printing of solder columns, dielectric polymers, and solder spheres, have been demonstrated. Figure 16 shows 25-µm diameter 63/37 solder columns, 250-µm high, printed on 50-µm centers. Figure 17 shows 100-µm polymer waveguides printed into a splitter.

Epoxies, UV-curable adhesives and thermoplastics have all been demonstrated with drop-on-demand jetting technology. Using multiple drops per bump, 325-µm bumps have been printed using demand mode solder jet technology.

MPM has demonstrated printing solder spheres this size with one drop per spot using the company's Continuous Metal Jet Technology.8 MicroFab is currently developing solder (electrical interconnect) and polymer (micro-lenses) dispensing processes for use in vertical cavity semiconductor emitting laser (VCSEL) fabrication and assembly.

Developments

Figure 16. 25 µm-diameter towers on 50 µm centers of 63/37 created using solder jet technology

Figure 17. 100 µm waveguides of an optical polymer created using inkjet type dispensing

Currently, we are working to expand the operating temperature range of solder jet technology. The systems used in the results described here have a maximum operating temperature of 240°C, with 220°C operation producing longer lifetimes. The next generation dispensing device is designed to operate at temperatures up to 250°C over extended periods. Initial test data has been obtained with this design at temperatures up to 300°C.

The follow-on design is targeted to operate at temperatures up to 325°C. These two operating temperatures will allow solders with higher melting temperatures to be dispensed using solder jet technology. Solders under consideration for use in the intermediate (250°C) device include 96.5 Sn / 3.5 Ag, 55 In / 45 Pb, and proprietary non-lead solders. Solders under consideration for use in the high temperature device include 20 Sn / 80 Au, 10 Sn / 90 Pb and 5 Sn / 95 Pb.

High deposition rate experiments will be conducted in the near future on MPM's solder-jet-based commercial wafer platform, shown in Figure 18. In addition to material handling, automated set-up/maintenance and many other features not available on the feasibility platform discussed above, this platform will have a significantly more accurate positioning system that will allow for evaluation of print-on-the-fly at 500 bumps per second, and higher, with the goal 1,000 bumps per second.

Conclusion

The feasibility of bumping wafers and die using solder jet technology has been demonstrated at rates up to 600 bumps per second. Assembled packages employing technology have been verified.9 The key elements to a new approach to wafer-level, chip-scale packaging have been demonstrated using solder jet and polymer jet technologies. The next step is to demonstrate chip-scale packages printed at the wafer level.

Acknowledgements

This research was funded in part by DARPA and the MPM Corp., and is based substantially on earlier work funded in part by a Department of Commerce Advanced Technology Program award. Most of the experimental work described in this paper was performed at MicroFab by Michael Boldman, Roger Self, Scott Ayers and Virang Shah. A team of engineers at MPM headed by Bill Johnson and Skip Ashton were responsible for development of the feasibility and commercial platforms. Financial and technical assistance was provided by AMP, Delco, Kodak, Motorola, Philips and Texas Instruments.

Figure 18. MPM Corp's solder jet based commercial wafer bumping platform.

References

1. D.B. Bogy and F.E. Talke, "Experimental and Theoretical Study of Wave Propagation Phenomena in Drop-On-Demand Ink Jet Devices," IBM Journ. Res. Develop., Vol. 29, 1984, pp. 314-321.
2. J.F. Dijksman, "Hydrodynamics of Small Tubular Pumps," Journ. Fluid Mech., Vol. 139, 1984, pp. 173-191.
3. D.B. Wallace, "Method and Apparatus for Forming Microdroplets of Liquids at Elevated Temperatures," U.S. Patent 5,415,679, May 16, 1995.
4. D.J. Hayes, D.B. Wallace, M.T. Boldman, and R.M. Marusak, "Picoliter Solder Droplet Dispensing," Microcircuits and Electronic Packaging, 16:173-180, 1993.
5. S. Beer, editor, "Liquid Metals, Chemistry and Physics," Marcel Dekker, Inc., New York, 1972.
6. D.J. Hayes, D.B. Wallace, and M.T. Boldman, "Solder Jet for Low Cost Wafer Bumping," Proceedings, ISHM '96, October 1996.
7. J.M. Waldvogel, D. Poulikakos, D.B. Wallace, and R.M. Marusak, "Transport Phenomena in Picoliter Size Solder Droplet Dispensing on a Composite Substrate," ASME Journal of Heat Transfer, Vol. 118, February 1996, pp. 148-156.
8. G. Pham-Vam-Diep, R. Smith, and R. Godin, "An Investigation of Precision, Continuous Solder Jet Printing for CSP Solder Ball Deposition," Proceeding, NEPCON West '97, February 23-27, 1997, pp. 842-858.
9. L.T. Nguyen, "Advanced Packaging Needs for the Year 2000," Gorham/Intertech's Microelectronic Packaging into the 21st Century, December 9-11, 1996.

Dr. Hayes is President of MicroFab Technologies Inc. He earned bachelor's and master's degrees in physics from Louisiana State University and a Ph.D. in materials science from Rice University. Contact him at dhayes@metronet.com or by phone at 972.423.2438.

Dr. Wallace, a Registered Professional Engineer, is Vice President for Technology Development at MicroFab. He received undergraduate degrees from Southern Methodist University and was awarded a Ph.D. in aerospace engineering from the University of Texas (Arlington). Readers may contact him at dwallace@metronet.com, by phone at 972.578.8076, fax 972.423.2438.