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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.
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.
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.
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.
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.
Prototype Platform and Printhead
Some of the features incorporated in the platform include:
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.
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.
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
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.
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.
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.
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.
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.
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.
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 email@example.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 firstname.lastname@example.org, by phone at 972.578.8076, fax 972.423.2438.
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