2.5D / 3D Packaging

Once upon a time, packaging was primarily done with wire bonding and surface-mount technology. Today, flip chip is still the gold standard. The progression of flip-chip, integrated circuit (IC) packaging is important. In its most basic form, IC packaging facilitates fanout of the semiconductor device interconnects to meet the technology restrictions of the board that it will be placed on. Manufacturing constraints of the system level board prevent IC’s from being mounted directly for a variety of reasons, such as interconnect density limitations, product reliability concerns, and rework restrictions.

Single or multiple chips are therefore placed on the package substrate. This minimizes electrical performance loss, maintaining signal and power integrity, while also improving overall reliability. Reliability is also enhanced by pairing the silicon chips with specifically designed materials to address coefficient of thermal expansion (CTE) because mismatch stresses can lead to early system-level failure.

Since the advent of flip-chip technology, packages have evolved to reduce cost, size, weight and power (CSWaP). Overall, a smaller footprint—we can do more in less space with less power at a lower cost. This is of particular interest in military and aerospace deployments, which required stringent reliability standards.

As silicon interconnect densities have increased, so too have the interconnect densities of high-density-build-up (HDBU) style package substrates. At least to a point. It’s anticipated that minimum interconnect densities will bottom out around 125um pitch, perhaps ranging slightly lower. This presents an issue in the context of CSWaP, since die interconnect densities of 5um pitch are possible today.

Historically, when you see advancements in semiconductor performance, it was frequently done through the miniaturization of die through front end manufacturing improvements and shrinking process nodes. But semiconductors are starting to reach the limit of improvements in front end manufacturing. Cost is the primary limitation today, but the physics of fitting more transistors in the space available is starting to show diminishing returns.

Beginning with single-chip package architecture, increased semiconductor die sizes were the first evolution with monolithic silicon chips. The aim was do more on one chip in less space. However, larger silicon and more delicate active layers led to reduced product reliability. It also led to increased difficulty characterizing the product using existing packaging methods. Silicon turns are also very costly, making this progression unappealing for die sizes approaching 20mm.

The second architecture iteration was the beginning of what the industry is calling “disaggregation” or splitting out of various functions into multiple die. The concept of multi-chip modules is not new but is being revisited in support of CSWaP objectives. This package style is common in industry today and facilitates more effective product optimization during development and flexibility down the line. We have several multi-chip module (MCM) style products from external customers in active production today.

The third iteration, titled 2.5D heterogeneous integration, leverages semiconductor technology and disaggregation to do more in less space by adding another interposer—silicon, glass, silicon carbide—between the substrate and high density silicon devices or “chiplets.” 2.5D is becoming very important in the industry, and is something that I could talk about in-length, but maybe that’s better saved for its own interview.

Process assembly methodology, while more challenging compared to legacy package designs, is largely understood today. New challenges exist with product design methods, like software tools and design flow. There are also challenges with thermal solutions, needing to address increasing power densities, and electrical test capability. As products become more advanced and disaggregated from a functionality point of view, the concept of “known-good-die” is much more difficult to quantify. Chip-to-chip interactions can influence end performance in unpredictable ways, making wafer test a necessity but also a challenging cost tradeoff. End item product testing is an absolute necessity, but development work is also needed to ensure testing at various stages of assembly are deployed judiciously to ensure proper end item yield. Design for test is also an increasing area of focus for these architectures.

The more we miniaturize packaging and not just the process node, the better the performance we’ll see from the assembled device. The use of interposers is the next evolution in this arena. Shorter electrical connections provide higher bandwidth and lower loss solutions. Disaggregation, or the use of “chiplets” as it’s called, can facilitate better product characterization and customization. 2.5D architectures can significantly bolster CSWaP initiatives. So, as I mentioned, 2.5D packaging is a big focus in the industry today, and a significant priority for our team at Tektronix.

Indeed, the adoption of 3D packaging is in its early stages, and it's important to address several critical challenges before widespread implementation can occur. Let's delve into some of these challenges, particularly focusing on design considerations for manufacturing (DFM), design for test (DFT), and design for reliability (DFR):

• Design for Manufacturing (DFM): As 3D packaging technologies mature, ensuring efficient and cost-effective manufacturing processes becomes paramount. DFM principles need to be integrated into the design phase to minimize manufacturing complexities and costs. Designers must consider factors like material choices, process compatibility, and yield optimization.
• Design for Test (DFT): Thorough testing of 3D-integrated systems poses unique challenges due to their vertically stacked nature. Designing for testability is critical to develop test strategies that can comprehensively evaluate the functionality and reliability of all components within the 3D package. DFT methodologies need to address accessibility and coverage concerns in the stacked configuration.
• Design for Reliability (DFR): 3D packaging introduces new reliability considerations, particularly concerning thermal management, stress, and interconnect reliability. Designers must incorporate DFR principles to ensure that the vertically stacked components can withstand environmental stresses, maintain signal integrity, and have a long operational lifespan.

In conclusion, 3D packaging adoption faces significant challenges as these technologies are deployed in harsh environments, factors like design for manufacturing, design for test, and design for reliability will be critical to ensure the success and longevity of 3D-integrated systems. As the industry addresses these challenges, we can expect to see greater adoption of 3D packaging in various applications.