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Experts At The Table: Exploring the relationship between board-level design and 3D, and stacked, dies

By Sara Verbruggen

SemiMD discussed what board level design can tell us about chip-level (three-dimensional) 3D and stacked dies with Sesh Ramaswami, Applied Materials’ Managing Director, TSV and Advanced Packaging, Advanced Product Technology Development, and Kevin Rinebold, Cadence’s Senior Product Marketing Manager. What follows are excerpts of that conversation.

SemiMD: What key, or major, challenge does the transition to 3D and stacked dies – and increasingly ‘advanced packaging’ – present when it comes to board-level design?

Ramaswami: The three-layer system architecture comprising the printed circuit board (PCB) system board, organic packaging substrate and silicon die offers the greatest integration flexibility. From a design perspective, this configuration places the most intensive co-design challenges on the die and substrate layers. On the substrate, the primary challenges are dielectric material, copper (Cu) line spacing and via scaling. However, when the packaged die attaches to the PCB through the ball grid array (BGA), surface-mount packaging, used for used for integrated circuits for devices such as microprocessors, the design challenges are more considerable. For example, they include limitations on chip size (I/O density), warpage and worries about co-efficient of thermal expansion mismatch between the materials.

Rinebold: Any advanced ‘BGA style’ package, regardless if it is three-dimensional (3D) or flat can have a significant impact on PCB layer count, route complexity, as well as cost. Efficient package ball pad net assignment and patterning of power and ground pins can make the difference between a four-layer and a six-layer PCB. Arriving at the optimal ball pad assignment necessitates coordinated planning across the entire interconnect chain from chip level macros to board level components. This planning requires new tools and flows capable of delivering a multi-fabric view of the system hierarchy while providing access to domain specific data like macro placement, I/O pad ring devices, bump patterns, ball pad assignments, and placement of critical PCB components and connectors.

SemiMD: 3D chip stacking and stacked die chip-scale packaging is favoured by the consumer electronics industry to enable better performing mobile electronics – in terms of faster performance, less power hungry devices, and so forth – but how do PCB design and testing tools need to adapt?

Rinebold: One benefit of these package formats is that they entail moving most of the high-performance interconnect and components off the PCB onto their own dedicated substrate. With increasing data rates and lower voltages there is little margin for error across the entire system placing a premium on signal quality and power delivery between the board and package.

In addition to high-speed constraints and checking, design tools must provide innovative functionality to assist the designer in implementing high-performance interconnect. In some situations complete automation (like auto-routing) cannot provide satisfactory results and still enforce the number of diverse and sometime ambiguous constraints. Designers will require auto-interactive tools that enable them to apply their experience and intuition supported by semi-automatic route engines for efficient implementation of constraints and interconnect. Example of such tools include the ability to plan and implement break-out on two ends of an interface connecting to high pin count BGAs to reduce route time and via counts. Without such tools the time to route high pin count BGAs can increase significantly.

Methodologies must adapt to incorporate electrical performance assessment (EPA) into the design process. EPA enables designers to evaluate electrical quality and performance throughout the design process helping avoid the backend analysis crunch – possibly jeopardizing product delivery. It utilizes extraction technology in a manner that provides actionable feedback to the designer helping identify and avoid issues related to impedance discontinuities, timing, coupling, or direct current (DC) current density.

SemiMD: More specifically, what impact will this trend towards greater compactness – i.e. smaller PCB footprint, but with more stacked dies and complex packaging – have on interconnection technologies?

Ramaswami: The trend towards better quality, higher-component density PCBs capable of supporting a wide range of die has significant implications for interconnect design. An additional challenge, is attaching complex chips on both sides of a board. Furthermore, with PCBs going thinner to fit the thin form factor requirements for mobile devices, dimensional stability and warpage must be addressed.

Rinebold: In some regards stacked applications simplify board level layout by moving high-bandwidth interconnect off the PCB and consolidating it on smaller, high density advanced package substrates. However, decreasing package ball pad pitch and increased pin density will drive use of build-up substrate technology for the PCB. This high density interconnect (HDI) enables smaller feature sizes and manufacturing accuracy necessary to support the fan-out routing requirements of these advanced package formats. Design tools must support HDI constraints and rules to ensure manufacturability along with functionality to define and manipulate the associated structures like microvias.

SemiMD: How will PCB manufacturing processes, tools and materials need to change to address this challenge?

Ramaswami: To manufacture a more robust integrated 3D stack, I think several fundamental innovations are needed. These include improving defect density and developing new materials such as low warpage laminates and less hygroscopic dielectrics. Another essential requirement is supporting finer copper line/spacing. Important considerations here are maintaining good adhesion while watching out for corrosion. Finally, for creating the necessary smaller vias, the industry needs new etching techniques to replace mechanical drilling techniques.

SemiMD: So as 3D chip stacking and stacked dies become more mainstream technologies, how will board level design need to develop, in the years to come?

Rinebold: One challenge will be visibility and consideration of the PCB during chip-level floor-planning and awareness of how decisions made early on impact downstream performance and cost. New tools that deliver a multi-fabric view of the system hierarchy while providing access to domain specific data will facilitate the necessary visibility for coordinated decision making. However these planning tools are just one component of an integrated flow encompassing logic definition, implementation, analysis, and sign-off for the chip, package, and PCB.

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