By Vada W. Dean and Kim Tan, Intercon Technology Inc., Morgan Hill, Calif., and Dr. Michael Gadd, Jetsis International Pte., Singapore
The classic singulation approach, which employs saw blades to cut straight lines through device arrays, cannot always be applied to small semiconductor devices and curvilinear photonic devices. Many of these devices require curvilinear cut paths, which saw blades do not readily accommodate. (By definition, the cut path of a rotating blade must be the straight line defined by the intersection of the blade plane and the device plane.) This article discusses the feasibility of singulating devices with a newly developed, proprietary method: fine beam abrasive water jet technology. With cut widths from 0.050 mm to 0.500 mm, this technology provides a cost-effective cutting process for fine geometry devices with both straight line and curvilinear edges. In addition, the water jet cutting process is material non-specific; therefore, laminates and coated devices with both ductile and brittle material can be cut in a single pass. The flexibility of the water jet method enables the low-cost singulation of both QFN semiconductor packages and arrayed wave grating photonic devices. Background Understanding the importance of new singulation methods, such as fine-beam abrasive water jet, requires some knowledge of current saw technololgy and its limitations. There is continuing pressure in the semiconductor packaging industry to pack more features into minimal space. One result has been the growth of CSP and near-CSP packages, offering high performance electrical characteristics, enhanced thermal management, and small volume displacements. While initially cost prohibitive, these packages have been cost-enabled by the advent of matrix array processing. These arrays address the two largest cost drivers of high performance packaging: Substrate and capital equipment costs. The reduced pitch between devices minimizes substrate waste and maximizes the device density; therefore, fewer substrates are consumed, and material costs can be amortized over more devices. Similarly, the increased number of devices per substrate allows manufacturers to produce more parts per manufacturing cycle and amortize the cost of capital over more devices. Benefit-to-Cost Ratio Due to a high benefit-to-cost ratio, the industry migration to matrix array processing has been swift. Over the past three years, many QFP designs have been supplanted by matrix array FBGA. Now SOIC devices are shifting to QFN. While laminate substrates continue to ramp, the copper leadframe-based QFN package has been stifled by the inability of saw singulation to deliver cost-effective results. New photonic devices have also tested the limitations of saw singulation. Specifically, singulation must overcome four challenges: a) small device singulation, b) low process throughput, c) high consumable cost, and d) curvilinear cut paths. Singulation Challenges Small device singulation is particularly problematic for saw singulation. When device dimensions are less than 3mm x 3mm, vacuum fixtures are unable to retain the small devices during sawing, with consistency. As a saw blade passes through a device, it is both rotating and translating relative to the device under process. The resulting force vectors have both vertical and shear components. As the shear component overwhelms the holding force of the vacuum fixture, the singulation yield drops due to non-conforming geometries, damage, or lost parts. As feed rates increase, the magnitude of the shear component increases commensurately and magnifies the device retention problem. Therefore, feed rates are minimized to protect yields. The result, however, is lower throughput. QFN singulation requires specially formulated blades that must constantly expose new diamonds to the cut interface. As the diamonds remove material, they are "dulled" by the ductile copper from the leadframe and must be sloughed-off as the blade wears at a higher-than-normal rate. The balance between blade wear and cut quality is a delicate trade-off requiring costly technology to extend blade life while minimizing burrs and chips. The final challenge is relatively new to semiconductor manufacturers: Photonic devices and packages must conform to the physics of light rather than electricity. The resulting design rules can create precise curved boundaries rather than straight edges (Figure 1). Since saw singulation relies on the intersection of two planes, the blade and the substrate, the cut path must be a straight line. Therefore, saw singulation does not lend itself to photonic device needs. Unrealized Potential Water jet machining has been available for decades; however, its potential has never been realized in semiconductor manufacturing. |
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