Piston Pumps To meet the challenge of precision dispensing for chip packaging applications such as underfill and encapsulation, the industry has evolved toward piston technology. A piston pump is capable of achieving better than +/-1 percent volume repeatability over a wide operating range. Different designs of single piston pumps are available but all use the same principle. A chamber is filled with material and then expelled with a piston. The stroke of the piston determines the volume dispensed. Even though the repeatability and accuracy are excellent, a recharge step is required when the chamber is empty. It may take up to 10 seconds to recharge a 2 cc chamber, which may result in a throughput loss. To eliminate the recharge step inherent in the single piston pump design, technology has evolved into the multiple piston pump design, shown in Figure 2. In much the same way that the pistons and cam of an automobile engine are synchronized to deliver a steady stream of power to the drive train, the multipiston pump produces a steady flow of material. While one piston is discharging material, the other pistons are at varying points in their recharge cycle.
Dispensing Platforms Basic requirements of a dispense platform are accurate positioning of the needle over the target zone, automatic height adjustment and precise control of the dispensing pump. The amount of material deposited, and its shape, are determined by a number of factors, such as needle height, pump dispense rate, material supply pressure, temperature and the rate at which the pump is retracted after dispense. In many ways, dispensing is a complex process requiring more machine control than other methods.(5) Each subsystem is critical and its design must be balanced and well-integrated. The basic equipment subsystems are: 1. Machine frame or platform 2. X-Y-Z positioning gantry (Figure 3) 3. Vision system (pattern registration with substrate, shown in Figure 4) 4. Control head (pneumatic and electrical) 5. Fluid handling system including process monitor and pump 6. Substrate handling 7. System control unit
Cost of Ownership The growth in the semiconductor packaging industry, fueled by the ever increasing demand for faster, smaller, lighter and less costly personal electronic devices and telecommunications tools is forcing components manufacturers and their subcontractors to produce more components and achieve yields of 99.9 percent. Subcontractors are typically paid by the pin count and therefore cannot tolerate any scrap. They often compete against each other to win contracts and rely on the machine supplier to achieve greater performance. The cost of ownership is also a big factor in choosing a supplier. True cost of ownership is not a simple matter of comparing initial equipment costs; it requires a far more complicated analysis. According to Dr. Ronald Lasky, Consulting Director for Cookson Performance Solutions, "Our proprietary cost-estimating software has demonstrated some surprising things about the real cost of capital equipment. "For example, take a dispenser that costs $15,000 more, but gives an incremental amount of production time of, say, 30 additional minutes per week. Is this more expensive dispenser a good investment? You bet! It pays for its additional cost in less than 4 months."
Recent Developments Multiple-Head Dispensers To overcome the bottleneck of a traditional single-head dispenser, multiple gantry machines are available that minimize the footprint, but still achieve greater throughput in units per hour. Innovative designs using two Y-axis gantries allow a four-head system to occupy a floor space of only one-and-a-half times the size of standard dispenser, while delivering 3-4 times greater UPH, depending on the application.6 Modular Dispensers Today, compact, modular dispensing systems designed to allow two, three or four machines to be installed in-line, all controlled from a single PC-based controller and user interface are available. These provide assemblers with benchmark dispense speeds up to 120,000 dots per hour, depending on application and production requirements. Vacuum Assisted Underfill The Tessera µBGA is a popular CSP that employs a special low-modulus, silicone-based encapsulant. This material flows around the nubbin spacer columns that separate the silicon die and the flexible circuit tape. This material has a high viscosity and although it is easy to dispense, it will not flow by capillary action and must be forced under the die.7 A process of vacuum-assisted injection allows complete coverage underneath the die. Summary and Conclusion Many different tools have been developed to facilitate the circuit assembly process. No one method is universal. Choosing the best assembly equipment for each phase of the operation is more complicated than simply purchasing on price. As a method, needle dispensing liquid materials has been an effective means of assembling components on surface mount boards and IC packages for decades. Recent advances offer higher throughput, greater accuracy and improved reliability to high-volume manufacturers and contract assemblers. As a data-driven process, dispensing offers quicker changeover, higher flexibility, and lower cost of ownership than other types of deposition equipment in applications requiring high density or complex board assembly.
References 1. Prismark Partners, Global Market Analysis for Dispense Equipment, August 1999, Cold Spring Harbor, N.Y. 2. Frost and Sullivan, World SMT Equipment Market, 1998, Menlo Park, Calif. 3. P. Gibbs, Getting Started In Dispensing, Speedline U.K., 1998. 4. C. Lee, "Ultra-High-Speed Dispensing of Surface Mount Materials to Enhance Production Through-put," Proc. APEX, March 2000, Long Beach, Calif. 5. C. Lee and C. Lazinsky, "Designing Dispensing Equipment for Very High Speed," Proc. SMTA, Chicago, Sept 2000. 6. B. Miquel, C. Lee, "Advanced Process Control for BGA Encapsulation and Flip-Chip Underfill," Proc. SEMICON Taiwan, September 2000. 7. "Fluid Dispensing Terms and Definitions," Standard 7.0, Surface Mount Equipment Manufacturers Association. Acknowledgements The authors acknowledge the assistance of Dr. Ronald Lasky, Consulting Director, and Mick Swanwick of Cookson Performance Solutions.
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