Solder Jet Technology Update
 

David B. Wallace & Donald J. Hayes

Abstract
Solder Jet Technology (e.g., piezoelectric demand-mode ink-jet printing technology used to dispense molten solder droplets) has demonstrated the ability to place 25- 125µm diameter bumps onto metallized wafers, circuit boards, and other substrates. Recent developments are discussed, including test vehicle printing, drop size modulation, microbump printing, and print-on-the-fly.

Key Words: solder bumps, wafer bumping, chip-scale-packages, flip-chip

Introduction
The formation of microdroplets of solder using ink-jet printing technology (Solder Jet technology) has been demonstrated, and MicroFab's Solder Jet development efforts have been described in detail in previous papers and patents. (1) (2)(3) (4) (5)(6) Recent research efforts have been directed toward demonstrating Solder Jet Technology as a robust wafer bumping method for high volume manufacturing. This paper describes recent results in the areas of test vehicle printing, drop size modulation, microbump printing, and print-on-the-fly (i.e., dispensing droplets without stopping the translation stages).

Background
In demand mode ink-jet printing systems, a volumetric change in the fluid is induced either by the displacement of a piezoelectric material that is coupled to the fluid,(7) or by the formation of a vapor bubble in the ink, caused by heating a resistive element.(8) This volumetric change causes pressure/velocity transients to occur in the fluid and these are directed so as to produce a drop that issues from an orifice. (9) (10) A droplet is created only when it is desired in demand mode systems. Demand mode ink-jet printing systems produce droplets that are approximately equal to the orifice diameter of the droplet generator.

The goal of our research is the development of advanced solder deposition equipment for the electronics manufacturing industry. Solder Jet Technology is based on piezoelectric demand-mode ink-jet printing technology and is capable of producing and placing molten solder droplets, 25-125µm in diameter, at rates up to 2,000 per second. Solder Jet-based deposition will be low cost (no tooling required), noncontact, flexible & data-driven (no masks or screens are required because the printing information is created directly from CAD information and stored digitally), and environmentally friendly (it is an additive process with no chemical waste).

Test Vehicle Printing
The locations of the pads of an integrated circuit test vehicle with over 1400 pads were programmed into our initial Solder Jet research platform [5]. This platform is a modified Universal Instruments Corporation hot-bar bonder with the Solder Jet printhead replacing the hot-bar mounted onto the moving gantry. Because this platform moves to each bump location and stops before printing, the net throughput is 4-5 bumps/second.
 

Figure 1: IC test vehicle with 1440 pads, bumped with 63/37 using Solder Jet Technology. Ball size is 70µm.
 For these tests, droplets of Sn63 / Pb37, 70µm in diameter, were deposited onto several of these test vehicles. Figure 1 shows the results from one test vehicles, and Figure 2 is a detail from the same image. The solder bump was deposited onto a nickel pad metallization, covered by a flash of gold, which promotes adhesion during the droplet impact and freezing process.(11)

Figure 2: Detail of previous figure. A scanning electron micrograph (SEM) was obtained of a bump on the substrate and is shown in Figure 3. This bump was imaged in the as deposited state (i.e., not reflowed), and the tin and lead rich phases are clearly visible. An SEM was also obtained from a Free Ion Beam (FIB) cross-section of a bump, and is shown in Figure 4. This bump was also imaged in the as deposited state, and, again, the lead rich and tin rich phases are apparent. The voids seen in the cross-section are most likely an artifact of the cross-sectioning process, because they are not apparent at the surface of the original bump.
 

Figure 3: SEM image of the top of a 60µm diameter 63/37 bump deposited onto the IC test vehicle.
 Figure 4: Cross-section of bump shown in previous figure. 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 recently developed novel drive waveform technology that allows the drop size to be modulated over an approximately 2:1 diameter (8:1 volume) range. Figure 5 shows a Solder Jet device producing 62µm diameter droplets at a rate of 120 Hz. The image on the left in this figure shows the droplet being formed while it is still attached to the orifice of the dispensing device, and the image on the right shows the drop approximately 1ms later, after it has broken free from dispenser. The drop velocity is approximately 1.5 m/s.
 

Figure 5: The drop formation process for a Solder Jet device is shown at two times (t ~ 1ms) during the process. The drop rate is 120 Hz and drop size is 62µm.


 Figure 6 shows the same device operating moments later, again at 120 Hz. In this figure, a drive waveform that extends the drop formation process over a significantly longer time period is being used. By doing this, a considerably larger droplet is produced. The magnitude of the drive voltage has been altered (increased) to keep the drop velocity at approximately 1.5 m/s. In this case, the diameter is increased to 106µm.Volume modulation using this method is continuous over the entire range of achievable volumes, as illustrated by the results presented in Figure 7.

Figure 6: The drop formation process for the same Solder Jet device shown in the previous figure, but using a different drive waveform. The drop rate is 120 Hz and drop size is 106µm. This drop volume modulation capability could be used to allow the bump size to be changed under software control, either for product change over, or for the application of variable sized bumps onto a single substrate (wafer or die). With sophisticated drive electronics, volume modulation can be accomplished on a drop-to-drop basis, as has been demonstrated for conventional ink-jet printing at rates up to 3,000 drops per second.(12) This real-time drop volume modulation capability does not currently exist for Solder Jet printheads. Because the drive waveform used to obtain the larger drop sizes increases the duty cycle of the waveform, drop formation is limited to rates of up to 1,000 per second.
 

Figure 7: Drop size modulation results.
 Microbump Printing

Solder bumps currently used in flip-chip processes are typically in the 100-125µm diameter range, although some companies are currently evaluating 75µm diameter and smaller bumps. As higher circuit densities and/or greater I/O counts are achieved in integrated circuit devices, there is likely to be a need for smaller bumps for flip-chip processes. Initial experiments were conducted to evaluate the suitability of Solder Jet technology for smaller bump sizes. The same printhead and dispensing device design were used for these tests, but the dispensing device diameter was deceased to ~20µm. Figure 8 shows small section of an array of 25µm 63Sn / Pb37 bumps deposited on a 35µm pitch onto a silicon wafer. Because of the high surface tension associated with solder (and all liquid metals), it is likely that a 15-20µm diameter is the practical lower size limit for ink-jet based solder bump deposition.
 

Figure 8: 25µm bumps of 63/37 deposited on 35µm pitch using Solder Jet Technology.


 High Rate Deposition

The ability to deposit bumps onto substrate at rates of greater than 200 Hz is critical to the commercial viability of Solder Jet technology. The ability to form liquid metal droplets at this rate, and higher rates, was demonstrated several years ago, but platform limitations have prevented us from demonstrating bump rates this high. Two research platforms have been completed that have the ability to deposit bumps, using Solder Jet Technology, while the substrate and/or the Solder Jet printhead are moving. This operating mode is referred to as "print-on-the-fly."

Because the solder droplet travels at a rate of 1m/s or greater and the velocity of the printhead or substrate would be 5cm/s to place 200 bumps per second on a 250µm pitch , the oblique impact of a drop in print-on-the-fly is not a major concern. However, the locally inert environment near the dispensing device [5] is potentially far more sensitive to translation velocity. Therefore, maintenance of the low oxygen environment was the focus of initial print-on-the-fly experiments. The effectiveness of the inert environment control was measured by an oxygen meter sampling through an empty (of solder) dispensing device and reservoir while the printhead was in printing position above a moving substrate. With substrate velocities up to 10cm/s, oxygen levels of less than 50 ppm were maintained using nitrogen flow rates of less than 0.14 standard cubic meters per hour (5 scfh).

In addition to measuring oxygen concentration near the dispensing device, initial experiments were conducted with the printhead dispensing at a constant rate over an unpatterned substrate at a rate of 200 drops per second. Bumps were placed using substrate velocities of 2-10mm/s. The low oxygen level was evidenced by the roundness of the deposited bumps: drops that have significant oxide formation during flight and impact are teardrop shape due to the oblique impact. In addition, microscopic inspection indicated no visible oxide formation.
 

Figure 9: Section of 39x39 array of ~100µm diameter bumps of 63/37 printed on-the-fly on 304.8µm spacings at 12.7 cm/sec (417 bumps/sec) onto a copper substrate. Printing was bidirectional, and the direction of travel was horizontal in the photograph.
 MPM Corporation has fabricated a Solder Jet research platform that allows for print-on-the-fly operation at rates up to 600 bumps/second. This platform uses a fixed printhead and substrates mount on an x-y stage. In addition to print-on-the-fly capability, some of the features of this platform are: printhead setup, maintenance, and visualization station; substrate temperature control; vision system alignment of the dispensing site to fiducials on the substrate; and substrate pad data file input. Initial print-on-the-fly experiments were conducted on this platform by printing 39x39 arrays on unpatterned copper substrates. Figure 9 shows an example of the results from these experiments. Operating condition for this experiment were as follows: substrate translation velocity = 12.7 cm/s (5 ips); 304.8µm ( 12 mil) spacings between bumps; 417 Hz bump rate; bump size ~ 100µm; and bidirectional printing.

Print-on-the-fly performance was quantitatively assessed by measuring the orthogonal distance between successive bumps in both directions. The distance between bumps in the direction of travel (horizontal in the figure) reflects stage, droplet velocity, and straightness errors, while the distance between bumps normal to the direction of travel (vertical in the figure) is indicative of stage and straightness errors. The standard deviation of drop-to-drop distance was 0.005µm for the vertical direction and 0.007µm for the horizontal direction. Both are on the order of the stage accuracy.
 

Figure 10: Section of 18x18 array of ~90µm diameter bumps of 63/37 printed on-the-fly onto 100µm pads on 250µm spacings at 100 bumps/sec. Substrate is silicon and pads are copper.
 Work is currently in progress to assess the accuracy of printing onto pad locations in print-on-the-fly mode. These experiments are also being conducted on the MPM research platform described above. Figures 10 and 11 shows a section of an 18x18 array of ~90µm diameter 63/37 bumps printed on-the-fly onto 100µm pads on 250µm centers at 100 bumps/second. Substrate is silicon and pads are copper. Quantitative assessment of bump-to-pad placement accuracy will be completed in the near future.
 
Figure 11: Side view of array shown in Figure 10.
 Summary

The ability to accurately place solder balls, using Solder Jet technology, over a wide range of ball sizes (25-125µm), and at high rates (over 400 bumps/second), onto patterned and unpatterned substrates has been demonstrated. Incorporation of Solder Jet technology into commercial wafer bumping platforms by MPM Corp. is underway.

Acknowledgments

This research was funded in part by Cooperative with DARPA and is based substantially on 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, and Virang Shah.

References

1. D.J. Hayes, D.B. Wallace, and M.T. Boldman, "Picoliter Solder Droplet Dispensing," Proceedings, ISHM '92, October 1992.

2. D.J. Hayes, D.B. Wallace and M.T. Boldman, "Method and Apparatus for Dispensing Spherical-Shaped Quantities of Liquid Solder," U.S. Patent 5,229,016, July 20, 1993.

3. D.B. Wallace, "Capillary Instability of a Jet of Liquid Metal," Journal of Fluids Engineering, September, 1993.

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. D.J. Hayes, D.B. Wallace, and M.T. Boldman, "Solder Jet for Low Cost Wafer Bumping," Proceedings, ISHM '96, October 1996.

6. D.B. Wallace, "Method and Apparatus for Forming Microdroplets of Liquids at Elevated Temperatures," U.S. Patent 5,415,679, May 16, 1995.

7. D.B. Wallace, "A Method of Characteristics Model of a Drop-On-Demand Ink-Jet Device Using an Integral Method Drop Formation Model," ASME publication 89-WA/FE-4, December 1989.

8. J.S. Aden J.H. Bohorquez, D.M. Collins, M.D. Crook, A. Garcia, and U.E. Hess, "The Third Generation HP Thermal InkJet Printhead," Hewlett-Packard Journal, Vol. 45, No. 1, Feb. 1994.

9. 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, pp. 314-321, 1984.

10. J.F. Dijksman, "Hydrodynamics of Small Tubular Pumps," Journ. Fluid Mech., Vol. 139, pp. 173-191, 1984.

11. J.M. Waldvogel, D. Poulikakos, D.B. Wallace, and R.M. Marusak, "Transport Phenonmena in Picoliter Size Solder Droplet Dispensing on a Composite Substrate," ASME Journal of Heat Transfer, Vol. 118, pp. 148-156, Febuary, 1996.

12. D.B. Wallace, V. Shah, D.J. Hayes, M.E. Grove, "Photo-Realistic Ink-Jet Printing Through Dynamic Spot Size Control," Journal of Imaging Science and Technology, Vol. 40, No. 5, pp. 390-395, 1996.