IC substrate interposer

How Interposers Are Used in IC Substrates

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Interposers are sophisticated substrates that serve as intermediate layers, facilitating electrical connections between multiple chips or dies within a package. They are normally placed on an IC substrate in specific packaging types, such as 2.5D and 3D packaging. In 2.5D packaging, chips are placed side-by-side on an interposer to provide interconnections between the chips. In 3D packaging, groups of chips (e.g., memory) can be stacked vertically on an interposer. The interposer then sits on a substrate, which then connects to a PCB in a BGA land pattern.

Most commonly used is the silicon interposer with an organic or ceramic substrate material, but there are other options used in IC packaging. Today’s UHDI approach in IC/PCB design has mostly implemented layout and routing on an organic substrate with silicon interposers, but electromechanical and thermal demands of advanced chips may drive changes in interposer material selection in the future.

What Are Interposers?

In heterogeneous integration, an interposer is an intermediate layer in an IC package that sits between an IC substrate and two or more chips. This component provides signal routing, power distribution, and even thermal management for the IC dice in the package. The image below shows an example of a 2.5D that uses an interposer for chip-to-chip communication.

 

IC substrate interposer

 

In 2.5D packaging, chips are placed side-by-side on an interposer, which is most commonly an Si interposer with horizontal interconnects. Conversely, 3D packaging involves vertical stacking of chips with through-silicon vias (TSVs), facilitating vertical communication links between dice in a stack. This configuration has been responsible for enabling many of the advanced package designs used in cutting-edge products, most often in specialty chips that are fully custom designed. Many system-on-chip (SoC) components will use this packaging approach as it enables the implementation of diverse features into a single package, but without designing all features into a single die.

Modern application areas that leverage interposers in packaging include:

  • High-performance computing (HPC): Packages that use silicon interposers are favored for their high I/O density and proven technology platform for use in high-end processors. Multi-core devices favor these packaging approaches, as well as newer specialized processors for fully-custom embedded systems.
  • Consumer electronics: In consumer electronics, and more specifically in smartphones and tablets, 2.5D packaging with an interposer provides the feature integration needed to keep device sizes small without sacrificing compute.
  • Mil-aero and military embedded systems: Interposers provide the same advantages as in HPC, but these packages can facilitate the integration of sensors, processors, and communication modules as custom chiplets on an interposer in a 2.5D package.

Materials Used to Design Interposers

Interposers are constructed from various materials, each offering unique advantages and facing specific challenges. The primary materials used in interposer design include silicon, glass, and organic substrates. Each material's properties make it suitable for different applications in IC packaging.

Silicon Interposers

Silicon interposers are commonly used due to their high input/output (I/O) density and established manufacturing processes. They are effective in 2.5D and 3D IC packaging, where they integrate logic and memory devices within the same platform. Silicon interposers can utilize TSVs in 3D packaging to achieve high feature and I/O density. However, silicon interposers come with certain drawbacks, such as reliability challenges related to TSVs and thermal management. The cost is also a significant factor, as silicon interposers add substantial expense to the overall package.

Organic Interposers

Organic interposers are known for their cost-effectiveness due to a well-established supply chain and traditional subtractive manufacturing processes like wet etching. They are more flexible than silicon and glass interposers, making them suitable for certain applications such as logic-memory integration, large CPUs, GPUs, and specific types of ASICs. However, organic interposers have lower I/O density and face mechanical limitations due to their flexibility. Despite these challenges, they have been demonstrated in high-performance RF applications, and research continues to explore their potential for next-generation high-performance applications.

Glass Interposers

Glass interposers present a promising alternative to silicon due to their potential for higher interconnect density at a lower cost. They are advantageous in applications requiring ultra-high I/O pitch density, such as high-bandwidth memory (HBM), high-performance computing, and optoelectronics-based computing. Glass interposers are cost-effective when produced in panel form, which can demonstrate high yield. However, they face manufacturing challenges such as surface defects, lower thermal conductivity compared to silicon, and limitations in effective diameter for through-glass vias (TGVs). Research is focused on improving etching techniques, utilizing polymers and metallization, and developing panel-based approaches for cost-effective production.

Future trends in interposer technology include the development of active interposers, which would incorporate logic and power management circuits. Active interposers could potentially address some of the long-distance communication issues by incorporating buffers or signal repeaters, or they can address certain power integrity issues through implementation of VRMs for certain rails.

In addition to active interposers, the usage of interposers in packaging spans beyond the traditional 2.5D approaches with groups of stacked chiplets. Package-on-package approaches, top-side and bottom-side bonding to interposers, embedded dice, and much more are possible with this technology. However, without a properly designed IC substrate, signals and power will never make it into the chips inside the package. IC substrate design goes hand-in-hand with advanced components that use interposers, and both components are needed to continue advancing new electronics applications.

 

Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your design is fully manufacturable at scale. Contact NWES for a consultation.

 



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