HDI-buildup technology and microvias: no longer a mystery
You've been hearing for years that HDI technology will revolutionize the pc-board industry. Now, it's finally happening.
By Shawn Larson, Zuken -- EDN, March 6, 2003
Consumers' escalating demand for more features in their small and mobile electronics products, such as PDAs and cell phones, is driving a need for smaller feature sizes, process geometries, and pc boards. For engineers dealing with these desires, the need for HDI (high-density-interconnect) technology has become a reality. Happy Holden, manager of advanced technologies for Westwood Associates, describes HDI technology as a process that lets you produce a pc-board with through-hole, blind, or buried vias of less than 0.006 in. in diameter without using conventional drilling technology. Users of HDI technology must be able not only to assess and implement next-generation technology, but also to understand its boundaries in terms of layer stackups, via and microvia formation, feature size, and the primary differences between it and traditional pc-board technologies.
Layer stackup is a key differentiator of HDI-buildup technology. Engineers manufacture an HDI layer stack by depositing additional mirovia layers on traditional pc-board cores. The industry uses HDI construction types to describe the available layer stackups. Currently, three popular HDI-construction types are in use (Figure 1). Type I construction comprises a conventional rigid or flexible pc-board core with any number of layers using through-hole vias and a single microvia layer fabricated on one or both sides of the core. Type II construction is similar, but you construct the vias in the core before adding the buildup microvia layers. Type III has at least two microvia layers on at least one of the core's surfaces.
Several other construction types are available. Type IV construction comprises a "passive" core that can function as a nonelectrical shield or a thermal buffer. "Coreless" construction, which comprises a pair substrates laminated together, is Type V construction. Type VI construction, or colamination, occurs when you simultaneously form the interconnect and mechanical structure.
The multitude of layer stackups that engineers can derive by combining HDI-construction types and a varying number of layers has driven the need for a simple designation scheme to identify them. The identification method is straightforward. For example, a designation of "2 (C4) 2" indicates a layer-stack construction comprising a four-layer pc-board core (C4) with two HDI (buildup) layers on the top and two on the bottom. A designation of "2 (P) 2" indicates a Type IV construction with a passive core, two HDI layers on the top, and two HDI layers on the bottom.
Microvias and their effect on HDI
A microvia is formed, not drilled like a traditional via. Engineers currently use several processes to produce microvias. Laser drilling, the most common technique, employs a focused laser beam to form the hole. Wet/dry etching is a mass-production process that creates all vias at the same time, regardless of the number or diameter of the holes. Photo imaging coats the base substrate with a dielectric layer. Engineers can also use conductive ink in microvia formation. In such a process, you form the microvias by laser drilling, photo imaging, or insulation displacement. You can also form microvias mechanically, using piercing, punching, abrasive blasting, or simple drilling. Each process produces different microvia hole shapes, such as cups, positive tapers, negative tapers, and straight walls (Figure 2).
The advent of HDI technology and microvias has also led to a new vocabulary for via structures. The HDI Design Subcommittee of the IPC defines microvias as "formed blind and buried vias" that measure less than or equal to 0.15 mm and have pad diameters that measure less than or equal to 0.35 mm. In addition, designers use terms such as "capture land" (the area where the microvia originates) and "target land" (the area where the microvia ends) to describe the microvia pad sizes. A landless via has a land diameter that is the same size or smaller than the via diameter.
Currently, the size of microvias limits their current-carrying capability. Designers typically overcome this limitation by nesting several microvias in one large area called a plural via. Microvias that directly connect nonadjacent HDI-buildup layers are called skip vias. A variable-depth microvia is a microvia formed in one operation that penetrates two or more HDI-dielectric layers and terminates at one or more layers. Laser vias, conformal vias, filled vias, photo vias, and stud vias are microvias that derive their names from the processes used to form them.
Each HDI construction type allows the use of different combinations of "standard" vias and microvia structures. Type I construction lets you use blind, one-layer-deep microvias and a standard through-hole via. The standard via spans all layers in the stack including the HDI-buildup layers. Type II construction is similar to Type I but adds a buried via that spans all the layers of the pc-board core. Type III adds yet more complexity to the via structures, allowing the use of buried, stacked, staggered, and variable-depth microvias. These many via structures can add a significant level of complexity to the layout of HDI-buildup designs.
Size matters
HDI-buildup technology offers much smaller feature sizes than standard pc-board technologies. Table 1 compares the two technologies using relatively conservative numbers. HDI also offers significant space reduction in component placement. HDI buildup uses microvias that you can place inside the component's surface pad. They are blind and traverse only one or two layers in the stack, allowing for coincident placement of parts on the top and the bottom surfaces (Figure 3). Typical pc-board technology uses through-hole vias. The via pad, or at least the drill, exists on every layer of the board. HDI uses blind and buried microvias; they occupy only the layers that they traverse (Figure 4). These feature-size benefits add considerable space for routing, typically resulting in a 40% reduction in design size. Standard pc-board technology uses breakout patterns and vias with stringers to get from a surface-mount pad to the inner routing layers of a design. The breakout-pattern technique greatly increases the amount of space that the component occupies. The breakout vias are typically through-hole and occupy space on all layers of the design. These vias also interfere with the potential locations of parts on the opposite side of the board.
Impact of HDI
Today's designers must deal with the complexities of new layer stackups and microvia structures and the fact that HDI capabilities differ among fabrication plants, based on their processes. CAD librarians are shouldering the task of creating these HDI technologies along with the emerging advanced packaging technologies, such as chips on board and chip-scale packages.
By using HDI-buildup technology, designers gain the benefits of reduced costs; higher densities; improved performance of smaller, lighter, and denser boards; improved electrical performance; lower RFI and EMI; increased reliability; and access to advanced packaging technologies.
HDI-technology advancements have put CAD tools and the layout designers that use them in a precarious position. Although most EDA tools offer many of the necessary features for HDI designs, they do not offer them all. Zuken delivers a cost-efficient and advanced pc-board tool that enables the use of HDI-buildup technology by providing such capabilities as 3-D-design rendering and editing. Designers considering a transition to top-of-the-line HDI-buildup-technology tools should consider the following points:
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The tool must be able to deal with variable-depth, skip, landless, and stacked vias and complex staggered, staircase, and spiral microvia structures.
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The tool must be able to automatically create "microvia-in-pad" patterns for surface-mount parts.
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The autorouter must be able to effectively handle microvias. It must be able to efficiently select among all the various microvias and through-hole vias available in a given HDI technology. Traditional tools factor in the cost of each via, but the amount changes with buildup technology, so one via can cost the same as 10,000 (for example, when you use plasma-etching).
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The tool should be able to distinguish buildup vias from drilled vias in the core. The router should have a preference for microvias, rather than the more expensive, larger, drilled vias.
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The tool must offer "free-angle" autorouting for improved use of space.
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The tool should allow the creation of embedded components, such as rectangular resistors, donut resistors, capacitors, inductors, and, potentially, even silicon.
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The tool must be able to import standard HDI-technology rule sets from HDI-fabrication suppliers.
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The tool must support a way to look at the design in 3-D. A flat, 2-D working environment is no longer sufficient; the complexity of microvia structures requires a 3-D rendering for clear comprehension.
| Acknowledgments | ||
| Thanks to the HDI Subcommittee of the IPC for many of the terms and definitions used in this article. Thanks also to Happy Holden, manager of advanced technologies at Westwood Associates, for his ever-present knowledge and support of HDI technologies. | ||
Acknowledgments
Thanks to the HDI Subcommittee of the IPC for many of the terms and definitions used in this article. Thanks also to Happy Holden, manager of advanced technologies at Westwood Associates, for his ever-present knowledge and support of HDI technologies.
