Selective flux jetting
11/01/2000
Precision techniques to optimize process results for advanced packaging applications.
BY FABIO OKADA
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During the past few years, many new production process technologies have been driven by the migration of advanced packaging technologies, such as flip chip and chip scale package (CSP), into high-volume electronics applications. For example, the effective use of underfill has become a key factor in achieving acceptable flip chip and CSP results. By completely filling the area under the die with encapsulant, the chip is bonded to the substrate, thereby reducing stress on chip bumps. A well-controlled underfill process can materially reduce the risk of damage from thermal cycling and other stresses, thereby improving overall reliability of final assemblies.
Achieving consistent results from the underfill process has also necessitated the development of greater precision and more controllable flux-application techniques. Generally speaking, traditional techniques for flux application can fall short of these requirements, especially considering today's ever-smaller and denser chip-level designs. The use of too much flux, the presence of flux residue and/or the inconsistent application of flux can degrade the ability of the underfill encapsulant to adhere to all surfaces under the chip, eroding the underfill's effectiveness.
Using appropriate flux-jetting techniques can improve the ability to control the consistency of flux application while also expanding the robustness and flexibility of the fluxing process.
Why Consistency is Key
Precision application of flux within the die's site area is a critical factor in facilitating solder flow and reducing oxides to facilitate the formation of reliable solder joints. However, given the ultra-small dimensions used in advanced die-attach processes, too much flux can cause the die to float or move, resulting in misalignment with the pads. In addition, the presence of flux residues under the chip during subsequent underfill processes can cause the encapsulant to interact with the flux, preventing the underfill from properly adhering to the substrate, die and solder joints. Too much flux around solder balls can also impede the underfill from making contact with the bumped surface and cause voids. These effects can degrade the underfill's capacity to absorb and diminish stresses within the final assembly.
Contact vs. Non-contact Methodologies
Flux patterns for die-attach applications often require a thickness of less than 1 mil with consistently sharp edge definitions and minimal overspray. These requirements already exceed the limits of contact-based fluxing methods, including dipping or screen printing. Many applications currently use a dipping process, known as "doctor blade," in which an in-line placement machine applies flux. Manufacturers can also consider using non-contact selective fluxing.
 Figure 1. Pulsed air-assist technique. |
Some of the first non-contact methods can now be limited in their use. For instance, air spray can require secondary masking operations to avoid over-spray contamination, thereby reducing overall process efficiency and throughput. In contrast, new-generation jetting systems combine non-contact benefits with high precision and good throughput. Such flux jetting systems operate by selectively firing a high-speed series of micro-droplets onto a substrate, which allows for a consistent delivery of a variety of flux patterns while still maintaining good edge definition. A jetting head moves along an X-Y plane to dispense pre-programmed patterns, without Z-axis motion or height-sensing requirements. In addition, eliminating physical contact with the substrate further speeds the process while avoiding contamination risks.
 Figure 2. Different flux patterns. |
By augmenting the jet fluxing process with the use of "pulsed air assist" techniques, precision control can be achieved (Figure 1). In essence, this process emits a quick pulse of air after each micro-droplet or line is jetted, which helps break natural surface tension and smoothly spread the material onto the substrate. Unlike a traditional spraying process, in which droplets are randomly atomized in the air, pulsed air-assist jetting can deposit uniform lines in exact locations and then control their flow-out. When dispensing lines, sustained throughput rates as high as 4,000 units per hour are possible. Pulsed air-assisted jetting techniques also allow flux to be deposited on the substrate in continuous lines with extremely thin film thickness. By programming a system to dispense micro-droplets or lines, process engineers can define the overall shape and thickness to create required flux patterns on the substrate. One disadvantage of this method is that it is not possible to dispense a thin film build with very-high- viscosity fluxes; however, in high-viscosity situations, contacting or dipping would be appropriate.
Leveraging Process Flexibility
Selective flux jetting also allows process engineers greater latitude for effectively handling mixed technologies, such as the use of flip chip and CSP devices on the same substrate (Figure 2). These mixed-technology designs have become key to implementing highly integrated miniaturized products. Mixed technologies, however, pose particular problems with traditional dipping techniques. For example, because flip chips use 3-mil solder balls and CSPs use 12-mil balls, the optimal flux thickness differs from one technology to the next. Dipping processes are not adaptable for applying different levels of flux thickness on the same substrate. On the other hand, a flux-jetting process can be tailored to deliver a variety of thicknesses on the same substrate by modifying the software program to apply the appropriate amount of flux.
Re-evaluating the Tradeoffs of Tackiness
The "tackiness" of flux has become an interesting and sometimes controversial factor that entails some legitimate trade-offs. Traditionally, high-viscosity tacky fluxes have been considered necessary because of the widespread belief that extra tackiness was needed to help keep components in place during subsequent assembly steps before reflow. More recent experience has shown that flip chip designs typically have so little mass and inertia that they do not require a significant level of flux tackiness to keep them in place.
 Figure 3. Comparison of tacky and liquid fluxes. |
Analysis of flux residue indicates that tackier fluxes have a higher potential for leaving unwanted flux residues after reflow. Tacky fluxes have evolved from solder paste flux formulations and typically include a rosin or synthetic resin base compound that accounts for the majority of the content, with a solvent used to moderate the overall viscosity. In contrast, liquid fluxes, such as those used in jetting applications, have evolved from wave-solder fluxes and consist primarily of solvent as the suspension agent for the active ingredients. Practical experience has shown that tacky fluxes can leave a residual residue of 25 to 45 percent. In contrast, liquid flux formulations have demonstrated lower residual residue levels between 1 to 5 percent (Figure 3).
The inherent tackiness of fluxes used in dipping applications can also lead to some difficult process-control challenges, especially with small-size, low-mass components. For example, for small die, such as 3 x 3 mm, the force of the flux adhering to the bottom of the die after dipping can be stronger than the force exerted by the vacuum pickup head used to remove the die from the dipping disk. As a result, small die can sometimes be left sitting in the disk, causing disruptions in the process flow and affecting overall throughput rates.
Material Control, Clean-up and Environmental Issues
Of major concern with any flux dispensing process is maintaining adequate control over issues like material pot life or contamination, as well as the management of clean-up and environmental factors. Contact-based technologies that expose the flux to air can lead to evaporation of flux solvents, causing both environmental concerns and process issues. As the solvent evaporates, the flux can thicken, resulting in the need to replace the unused flux and frequently clean the system. Additionally, the die is more susceptible to contamination during the dipping and transfer processes.
In contrast, a closed-loop selective fluxing system eliminates environmental and materials management issues associated with open-air fluid systems. The liquid flux within the closed reservoir and jetting head is maintained under closely managed pressure, temperature and atmospheric conditions until the moment that it is jetted onto the substrate. In addition, the risk of contamination is reduced through the avoidance of any physical contact between the dispensing system and the components being assembled.
Optimizing Machine Utilization and ROI
For high-volume production environments, overall equipment utilization and efficiency are among the primary considerations that are currently driving migration of the fluxing operation to dedicated jet-dispensing systems. Because dipping operations are generally handled by component-placement equipment, the negative return on investment (ROI) impact of an inefficiently managed process can be significant.
Dipping processes can also necessitate frequent workflow stoppages for issues such as failed pickups of small die, replacing prematurely thickened material, dealing with contaminated parts, cleaning the system, and/or tweaking the process to maintain acceptable consistency. In addition, mixed technology applications can require multiple process set-ups for different levels of flux thickness or result in sub-optimal settings.
 Figure 4. New-generation jetting systems combine non-contact benefits with precision and throughput. |
The bottom line is that the primary purpose of sophisticated placement machines is to provide rapid and accurate component placement. These expensive machines are simply not designed to readily handle fluxing operations. Not only does the slow, contact-based nature of a dipping process run at cross-purposes to the high-speed placement activities that form the machine's primary task; the introduction of flux into a precision placement machine can result in a maintenance nightmare. Instead, handling the entire fluxing process within a dedicated closed-loop jet- dispensing system virtually eliminates the cleaning, maintenance and process-tweaking issues that can otherwise lower the efficiency of a placement machine.
Experience to-date has shown that overall production efficiency can be increased by as much as 20 to 30 percent by offloading the fluxing process to a dedicated selective flux jet dispensing system. The improvement on ROI can offset the cost of a dedicated fluxing system. This is an advantage, coupled with the benefits of flexibility, adaptability and process capability in product introduction cycles and quality levels.
References
- Brent Bacher, "Flip Chip Process Guidelines and Considerations," presented at SEMICON West, San Jose, Calif., July 1999.
- Raj Master, Ajit Dubey, Maria Guardado and O.T. Ong, "Novel Jet Fluxing Application for Advanced Flip Chip and BGA/CGA Packages," presented at the Electronic Components and Technology Conference, Las Vegas, Nev., May 2000.
FABIO OKADA, production specialist, can be contacted at Nordson Corp., Electronics Systems Group, 300 Nordson Drive, Amherst, OH 44001; 440-985-5200; Fax: 440-985-1122; E-mail: [email protected].