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By DAVID WIENS, Mentor Graphics Corp.
When thinking about design and fabrication technologies deployed for integrated circuit (IC) and printed circuit board (PCB) products, a common axiom is that “technology flows downhill.” Basically, this implies that technologies commonly develop in the silicon world before they find their way to the printed circuit world. This is generally due to the cutting-edge nature of IC design, and high investment in the fabrication process.
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Historically, technology has flowed from silicon chip manufacturing to hybrids and their close cousins, multi-chip modules (MCMs), and then to PCBs. For example, additive metal layers started with ICs, found their way to MCMs in the form of thick- or thin-film deposition, and showed up on PCBs as high-density interconnects (HDIs) – also known as microvias. While the application and underlying substrate changed with each evolution, the fundamental manufacturing process remained the same. The driving force was also the same – higher circuit density in a smaller form factor.
Printed components made a similar transition from MCMs and hybrids to today’s PCBs as embedded passives. Along the way they’ve been adapted to current fabrication techniques; for example, the inclusion of a resistive material layer within a core laminate structure, and the creation of series termination resistors directly underneath micro-ball grid array (BGA) packages to improve circuit performance. Active devices have taken the same path – bare IC die are now embedded within cavities and wire bonded to core PCB structures.
The development of a manufacturing infrastructure that could cost-effectively produce these advanced technologies on a PCB is the primary facilitator for all of these transitions. One thing has remained consistent – silicon production is still exorbitantly expensive. While MCMs are less expensive than silicon, they are still more expensive than PCBs. Over time, application of new fabrication and assembly technologies to the PCB domain have only been widely adopted when the cost dropped sufficiently.
Along with the migration of manufacturing technologies are instances of design technology migrating from ICs to PCBs. The complexity and density of ICs drove the hierarchical approach to logic and layout design. This approach is now widely used within PCB schematics and within layouts (as reuse blocks) where the use of hardware description languages (HDLs) instead of schematics is another example of technology migration. This same reason drove the use of multiple parallel CPUs to design and verify circuits and has also found its way into the PCB domain – primarily for auto routing applications.
However, there are examples of technology flowing uphill. Due to slower signal propagation speeds of standard PCB materials, simulation for signal integrity and timing became critical to ensure optimal circuit performance. It wasn’t until much later that interconnect modeling became critical to IC design due to the much smaller physical trace dimensions and faster operating speeds. The same has been true in design for manufacturability (DFM) – again driven into the silicon world because of small physical dimensions.
The worlds of ICs and PCBs have found a fairly new area of collaboration – advanced packages. Driven by an increasing drive for performance, designers have begun optimizing the IC, package and PCB at the same time (in parallel). This process has enabled shortened design cycle times and increased product performance. It has also enabled trade-offs between the high-cost, high-NRE world of integrated silicon system-on-a-chip (SoC) and multi-chip packaging on PCBs. These advancements signal trends for future design and manufacturing approaches. In the future, many technologies will develop in both environments simultaneously due to the increased similarities in fabrication that have developed over time, and the continued drive to design complete systems, not just isolated components within the system.
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DAVID WIENS, director of business development, Systems Design Division, may be contacted at Mentor Graphics Corp., 1811 Pike Rd., Suite 2F, Longmont, CO 80501; 720/494-1086; E-mail: [email protected].