Today's traditional ball attach line normally consists of five individual machines: a magazine loader, a ball mounter with an integrated inspection system, a reflow oven, a cleaner and a magazine unloader. The equipment is usually organized in successive fashion, placing each machine end-to-end, forming a continuous, straight process flow path (Figure 1).
Cleaning and Reflow - Separate but Equal? Initially, SMT reflow and cleaning systems were adopted for use in the semiconductor ball attach process, using ovens and cleaners designed for handling large, wide PC boards. The SMT reflow oven's length is optimized for providing uniform flux volatilization and solder reflow on a populated board of variable mass, materials and surface finish. SMT cleaners, on the other hand, incorporate long, multistage modules designed to remove contaminants resulting from excessive handling and multiple thermal excursions, typical of the SMT assembly process. High-volume and high-velocity impingement of the wash solution is required to extract residues from beneath low-profile components and around highly populated areas. Equipment Refinements Today, long reflow ovens and high-horsepower cleaners are somewhat out of place in the new ball attach process configuration. Recent equipment refinements have provided SMT reflow ovens and cleaners with the means to manufacture BGA packages; however, their ability to furnish optimum capacity and lowest cost of ownership is compromised by their misapplication. SMT equipment is limited due mainly to size and power consumption. The combined footprint of a large SMT reflow oven and cleaner installed in a ball attach line is 162 square feet. Compared to a typical ball attach footprint of 20 square feet, the SMT reflow and cleaner, together, are 800 percent larger. This ratio does not represent an efficient process line. As equipment suppliers recognized the opportunity presented by the ball attach market, new process-specific systems were developed to provide higher returns. The first process improvement was realized in cleaning equipment, where the open-face configuration of BGA semiconductor packages exposes solder balls and substrate surfaces to all sides. Without the need to penetrate under a component, impingement wash, rinse and dry pressures can be reduced. Additionally, the majority of ball attach lines have transitioned to OA fluxes, eliminating the need for chemical isolation modules. Reductions in dwell time and cleaning complexity allow for smaller footprint BGA cleaners, and new BGA cleaners are being reduced in size for that specific need. Typically, BGA cleaners occupy 30 percent of the area required for SMT cleaners, resulting in a dramatic reduction in cost of ownership. With such big savings from cleaners, the focus then shifted to reflow equipment manufacturers, pressuring them to offer proportionally sized ovens. The difference between the SMT reflow process and the BGA reflow process does not offer a significant avenue for creating smaller BGA reflow ovens and a catalyst is needed to provide an alternative. Solder performance is identical for BGA and SMT applications, and tight delta Ts must be maintained for the sake of uniformity. Vertical ovens offer the ability to take advantage of high ceiling manufacturing floors. Maintenance, however, is difficult and safety is a concern. A Logical Partnership To date, all BGA carriers (strips, boats and trays) are rectangular. This unique quality of BGA products allows for a proportionately sized reflow oven, compared to the rest of the ball attach line. Current ball attach process lines transport the BGA carrier along its length rather than its width, which is typically one-fourth the length. When calculating cycle times, the entire length (plus spacing) of the BGA must pass by for one count. If the BGA carrier processes along the width, rather than the length, cycle times are reduced (often by as much as 4x) by a factor equal to the width-to-length ratio. As ball placement systems evolve toward reduced cycle times, concurrent SMT reflow cycle times are being decreased by increasing the oven's length and conveyor speed. This practice of stretching the process line to accommodate faster cycle times cannot keep pace with increased ball placement speeds and is not pragmatic. Processing BGAs along their width, however, changes the relationship by incorporating the product geometry advantage into the reflow process, creating slower conveyor speeds and shorter ovens while effectively producing faster cycle times. To leverage the BGA carrier's product geometry advantage effectively, certain guidelines must be followed:
Leveraging the product geometry advantage is achieved by integrating the reflow, cleaning and handling processes into one small-footprint system that also provides additional opportunities to optimize both processes. This optimization allows for greater cost of ownership savings. Product Geometry New combined reflow and cleaning systems, like the MicroPro, developed by Speedline ACCEL, capitalize on the advantages of inherent product geometry and create a reduced 27 square foot footprint (Figures 2 and 3.). Reflow and cleaning systems within this novel configuration are integrated into one synergistic system (Figure 4), positioned perpendicular to the product flow path. A handling system orchestrates the product flow in a seamless fashion.
Manufacturing Efficiency Through this advanced line integration, new levels of manufacturing efficiency and production can be achieved. BGAs are passed, end-to-end, immediately from a typical ball attach system. The input conveyor gently shuttles the BGAs to the front of the reflow oven, which is positioned perpendicular to the BGA's line of travel. The BGAs are then smoothly swept along their width onto the reflow conveyor, which runs from front to back. The orientation of the BGA has not changed. Emerging from the reflow tunnel, the BGAs are then directed across the back to the cleaning conveyor where they are processed along their width, from rear to front, through the cleaning system. Emerging from the cleaner, the BGAs are realigned with the entry conveyor and resume a linear end-to-end travel direction.
The reflow and cleaning processes are brought together, in concert, to achieve optimal cleaning, cycle times and floor space requirements. BGAs can now transfer from reflow into cleaning in one machine, allowing the two processes to effectively merge as one continuous process. For example, BGAs are cooled from reflow to a temperature equal to the wash temperature. The wash water is no longer responsible for elevating the temperature of the BGA and its residual fluxes, dedicating the water's energy to the removal of flux residues. In addition, as BGAs emerge from the rinse section, they are higher in temperature than with conventional cleaners, allowing for rapid drying of the devices. Reflow takes place in an air or nitrogen-forced convection tunnel utilizing independent blower control, five reflow zones and bottom-side dominant cooling to provide shiny solder spheres of uniform size and strength. Cleaning occurs with two offset impingement wash manifolds and a DI-rinse manifold. Heated forced air manifolds, an IR panel and crossflow dryers ensure total drying. Integrating reflow and cleaning equipment into a single platform affords the lowest possible cost of ownership for processes using water soluble flux chemistries. In addition to the floor space savings, cost factors, such as operation and maintenance, are reduced. This single solution divides the number of operating systems, maintenance schedules and facility connections normally required for independent reflow and cleaning machines in half. In addition, complexities associated with purchasing and receiving technical support from multiple suppliers are eliminated. Every cost of ownership model factors in the fixed floor space cost of equipment. The conservation of space realized by using an integrated reflow and cleaning system results in a lower product cost (for product produced on the specific line) and the ability to add capacity without incurring expansion costs. In many instances, moreover, it is possible to install two complete ball attach lines in the same space formerly required for a single line. Implications The implications of such cost reductions were reviewed in the August 1996 issue of The Red Herring by Prudential Bache analyst Mark Edelstone, who noted, "One way companies have been able to reach favorable price points is through dramatic improvements in process technology. "These improvements have lowered costs by 20-30 percent per function. Ideally, a company should be able to price its final products to give a 50 percent-plus gross margin. Prices at this level let companies profitably innovate and bring newer, more feature-rich products to market." Conclusion
Innovations in capital equipment technology are enabling manufacturing improvements and capacity expansions. The benefits of adding an integrated reflow and cleaning system to the ball attach line are the result of its smaller footprint and improved performance measures. The bottom line, of course, is total cost of ownership. As the marketplace evolves and competition increases, it is these measures-greater capacity, more flexibility, higher yields and optimal total cost of ownership-that will provide the advantage of faster time-to-market for manufacturers and their customers.
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