By Mario Agustin, Dexter Añonuevo and Brian Lynch, Amkor Technology Inc., and Glen Gibbs Sr., Asymtek Asia
Over the past decade, driven by global opportunities and escalating competitive pressures, the electronics industry has increasingly turned to more sophisticated die packaging methods for improving circuit densities, increasing chip reliability and lowering production costs. BGA packaging is a prime example of how a technology that furthers the above objectives is rapidly moving from the initial experimental stages to become a widely used production technology. In addition, chip-scale packaging and MCM-BGA techniques can provide even greater density opportunities than conventional packages by combining multiple die on a shared substrate. BGA packaging is now a critical factor in the success of many new component designs, and CSP techniques offer attractive new opportunities. These formats, however, also require extremely tight process controls to optimize production efficiency and to yield consistent quality. Intricate Challenges As the world's largest IC packaging foundry, Amkor has faced the intricate challenges of optimizing BGA and chip-scale packaging techniques for use within an inherently dynamic, high-mix production environment. (Figure 1 shows the encapsulation of a single BGA package, while Figure 2 illustrates dam and fill on multiple BGA packages.)
By partnering with a major supplier of automated fluid dispensing systems, Amkor has applied leading-edge technology solutions to higher levels of production efficiency. This article describes the specific issues involved with optimizing the dam-and-fill dispensing capabilities that are required to support a wide range of different package formats. We will also examine how Amkor accomplished its dam-and-fill goals, while minimizing initial setup and production changeover times. Dam-and-Fill Objectives Dam-and-fill dispensing is intended to provide a quick and effective mechanism for fully encapsulating an IC after the semiconductor die (or multiple die for an MCM-BGA), has been attached and wire-bonded to the substrate. In high-mix environments, dam-and-fill techniques can offer significant flexibility over traditional transfer molding methods. However, to maximize these advantages, it is essential to deploy a fast and robust dispensing process that can handle a wide range of dispensing. To compete with transfer molding's high throughput, the dam-and-fill process must be capable of flexibility in delivering precise amounts of both the dam and the fill materials at sustainable throughput levels. The Process Dam and fill is a two-step process. First, a dam is dispensed around the device with the cavity inside the dam then precisely filled to encapsulate the device, wire bonds and underlying substrate completely, as illustrated in Figures 3 and 4. Compared to other encapsulation methods, such as glob-top, dam and fill can provide distinct advantages by providing a uniformly flat surface for vacuum pickup and placement, as well as for marking and labeling.
In addition, dam and fill's two-step process permits the use of lower viscosity fluids inside the cavity, which ensures more complete fill under and around the wire bonds. Critical Factors for Optimal Results For both the dam and the fill processes, tight control over the amount of material and the speed of dispensing are critical factors for consistency. Additionally, because fluid characteristics vary between the dam and the fill, the dispensing process must be carefully tailored for each substep. For more than six years, Amkor has worked in close partnership with its dispensing vendor to deploy leading-edge platforms, develop real-world dispensing techniques and refine process control methods. Because dam fluids have to be thixo-tropic to maintain the required height-to-width relationship (aspect ratio), they are best dispensed using an auger screw pump, where the shearing action helps reduce fluid viscosity. Once dispensed, the fluid then quickly returns to its natural viscosity and the dam takes on a height and width determined by the amount of material dispensed per unit of length (line weight). Aspect Ratio for Fluids Varies The aspect ratio for different fluids ranges from about 1.5:1 to 4:1. However, for a given fluid formulation, the dam's aspect ratio remains consistent and predictable over a range of line weights. This consistency makes it possible to achieve uniform and highly repeatable dams by precisely controlling the weight of fluid per unit of length. Consistent dispensing flow rates and smooth dispensing head movements are critical to achieving acceptable throughput rates as high as 4.5 cm/sec while maintaining consistent line weight and line quality. For fill dispensing, the primary challenges are two-fold: achieving sustained high flow rates and assuring complete fill of the cavities to the specified height. In this operation, linear positive displacement pumps offer the best results because of their ability to deliver very fast fill times with consistently precise fill volumes. Consistent Volumes Linear positive displacement pumps employ servo-driven pistons to displace exact amounts of material from within the valve's fluid chamber, thereby repeatedly dispensing consistent volumes across variable conditions. Linear positive displacement pumps are also able to handle a wide range of fluid viscosity characteristics and suspended particle sizes. These capabilities allow the selected fluid formulations to be closely tailored to process requirements, ensuring a complete and voidless flow. As part of the close working relationship with its dispensing partner, Amkor provided continual, real-world-applications feedback leading to improved linear positive displacement pump designs. These pumps can dispense shot sizes from as little as 1 microliter to 2 cc, with a highly repeatable accuracy of 1-2 percent over a wide viscosity range from 1 to 1,000,000 centipoise. For high-mix environments, the ability to dispense a variety of fill patterns accurately also becomes a key factor for achieving required throughput and quality levels. Serpentine Patterns Traditionally, serpentine patterns have been used, starting either at the center and working out to the dam, or starting from the dam and working into the center of the die. However, as packages have become more complex (e.g., multiple die with different heights), simple serpentine patterns are no longer always sufficient to achieve efficient encapsulant flow without any voids. For example, using a serpentine pattern for tight, wire-bonded packages may cause fluid to web across the wirebonds, creating trapped air pockets. In this instance, it is often effective to fill the area with a series of dot-and-line patterns. Integrated in-process temperature control can also be a significant factor in achieving acceptable throughput levels. Encapsulant Flow For example, by heating the substrate prior to and during the fill operation, fluid viscosity can be reduced to help the encapsulant flow through wirebonds and adhere better to the dam, die and substrate. The viscosity, typically, can be cut by more than 50 percent when the temperature of the fluid is raised from ambient to 55°C. The integration of needle heaters can also be used to lower the viscosity of the fluid in the needle, which helps eliminate stringing at the needle tip and facilitates a clean fluid break at the end of the dispense cycle.
Boosting Throughput with Two Heads As Amkor's BGA business has expanded, the need to keep up with customer requirements has brought a continuing migration to higher throughput and more flexible dispensing platforms. Amkor's original installed base of dispensing systems could be flexibly equipped with either auger valves for dam dispensing or linear positive displacement pumps for fill (Figure 5). However, they could only be used for one type of dispensing at a time. By dedicating specific machines to damming and others to filling, Amkor process engineers were able to achieve sustained throughput rates of 100-150 UPH for BGA production. However, customer demand continued to drive the need for even higher rates. Two years ago, Amkor adopted dispensing platforms with dual heads, capable of dispensing both dam and fill materials on the same machine. By allowing continuous production with less need to transport parts between machines, the dual head systems have enabled Amkor to boost BGA throughput rates up to the 350 UPH range. With dozens of first- and second-generation cable-driven dispensing platforms operating in high-volume manufacturing lines in semiconductor packaging facilities in Korea and the Philippines, Amkor has developed a solid foundation of both equipment and experience for handling virtually any dam-and-fill challenge. In addition, Amkor has a multifactory expansion program underway to add several additional dispensing platforms with up to 2G acceleration and a linear speed of 1 m/s. Based upon preliminary production testing, the new generation dispensing systems will allow for doubling the current UPH throughput rates while maintaining or improving current quality. High-Mix Dispensing Challenges Other key factors that come into play for effective dam and fill in a high-mix environment include high-level dispensing programming tools and simplified setup and quick changeover mechanisms. The ability to develop new dispensing programs quickly and easily is another area in which Amkor has seen significant evolutionary advancements. For example, the dispensing system vendor's patented mass flow control technology has been integrated into Windows NT software for newer generation machines. This integration enables Amkor programmers to program, test and debug individually customized dam-and-fill dispensing patterns for each particular packaging design more rapidly. This programming can be quite important, because virtually every customer-specific design requires a unique dispensing program to achieve optimal results. For instance, while multiple customers might all require a similar 23 mm2 package size, such details as die area and height or wirebond configuration may dictate significant differences in dam height, volume of cavity fill and appropriate dispensing patterns. In addition, with multichip packaging scenarios, the size, shape and dimensions of the dam-and-fill configurations can vary significantly. In most cases, by using these new software tools, the initial dispensing program can now be developed and moved to a target production machine within a one- to two-hour timeframe. Changeover Capabilities Amkor's customer production schedules may routinely require a different dispensing machine setup for new parts three to four times during one production shift. Because any particular BGA package size may require many different dam heights and/or fill patterns, some of the current dispensing systems may have as many as 300 to 400 individual stored programs that can be quickly loaded and run within just a few minutes of setup time. Conclusion The ability to achieve and maintain a high level of process control for dam-and-fill dispensing is paramount for the successful encapsulation of a wide range of BGA devices. In addition, for high-mix contract packaging environments where many different customer devices are being manufactured simultaneously, dam-and-fill dispensing methods must be readily adaptable to a variety of different device-specific requirements.
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