What Works Best For Chiplets

The semiconductor industry is preparing for the migration from proprietary chiplet-based systems to a more open chiplet ecosystem, in which chiplets fabricated by different companies of various technologies and device nodes can be integrated in a single package with acceptable yield.

To make this work as expected, the chip industry will have to solve a variety of well-documented technical and business issues, and it will have to rein in some of the grander visions of what’s possible — at least initially. The basic challenge is aligning domain-specific performance demands of end systems, which contain a growing number of chiplets, with the assembly and packaging capabilities and methodologies of IDMs, foundries, and OSATs. This includes the creation of assembly development kits (ADKs) that are roughly the equivalent of process development kits (PDKs), which today are codified with manufacturing specifications.

A PDK provides the appropriate level of detail needed to develop planar chips, marrying design tools with fab processes to achieve a predictable outcome. But making this work for an ADK with heterogeneous chiplets is many times more complex. Design and assembly teams need to manage thermal, mechanical, and electrical co-dependencies that cause electrical and mechanical stress, resulting in warpage, reduced yield, and reliability issues under real-world workloads. Layered on top of this the business and legal issues related to packaging of different devices from different manufacturers.

“Chiplets are a growing trend, especially in the HPC and networking segments, with potential to scale to other applications,” said Gabriela Pereira, technology and market analyst for semiconductor packaging at Yole Intelligence. “The industry has understood that high-end advanced packaging technologies are needed to connect them — but that’s much more complex than it seems. Connecting chiplets requires the design of high-bandwidth interconnections at the package level, which can take different forms — e.g., 2D, 2.5D or 3D — while ensuring that the thermal and power requirements are fulfilled.”

Commercial chiplet-based devices generally are domain-specific, and sometimes developed for a specific workload. So despite a big industry push to create a LEGO-like mix-and-match ecosystem for chiplets — which today includes multiple IP and EDA vendors, foundries, memory suppliers, OSATs, substrate suppliers, etc. — making this work as planned will require time and a massive amount of work.

Fig. 1: System assembly requires tighter coupling between chipmakers and OSATs. Source: ASE

In creating heterogeneous integrated designs, it’s essential to have much tighter collaboration between foundries, IDMs, OSATs, and PCB manufacturers. And because each chiplet-based system will be customized, the number of assembly processes will grow substantially. For example, one OSAT noted that among its ~5,000 customers, there are ~1,000 different assembly processes.

That diversity in products and processes makes it difficult to achieve predictable results by choosing chiplets from a large menu of options.

“We’ve already encountered a lot of limitations including not only the silicon, but also integration and the ecosystem,” said Lihong Cao, senior director at ASE Group, at MEPTEC’s Road to Chiplets forum. She stressed that customers continue to push for a low-cost chiplet assembly process, which is creating constructive tension between developing a sophisticated assembly process and the economic realities of different industry sectors. Computing devices for automotive have a higher cost sensitivity than for data centers, for example, but their chips operate in a harsher environment over a longer lifetime.

What’s needed is a defined set of assembly process recipes — basically, a highly limited menu of choices — that are specific to the end application (HPC, automotive, RF telecommunications) in order to lower the cost of chiplet-based systems. OSATs and foundries already are moving in that direction for high-performance computing. For example, at its 2024 Direct Connect event, Intel shared its six different package processes for chiplets. TSMC and Samsung also offer defined sets of chiplet processes. But the success of these assembly processes requires engineering teams to co-optimize the flows, processes, and materials to best match the system requirements.

Fig. 2: Integrated platform development requires tightly coupled architectural analysis that co-optimizes the system design to architecture to assembly process and packaging material selections. Source: Applied Materials

“Previously, when we designed a system we only had to be worried about the system requirements. Once we start segregating into dies and reassembling them, we have to start looking at other things. We have to worry about putting them together while considering signal integrity between dies, reliability, thermals, etc.,” said Itai Leshniak, director of AI systems solutions at Applied Materials, at the MEPTEC forum. “If we take the case of AI-based computer vision, we can break it down layer by layer — on the hardware side, determining which computer vision processors, sensors, filters are needed to break it down into the architecture at layer. Then we begin to go through how to package all these chiplets, and then which materials to use and how to take advantage of those materials.”

Materials and assembly processes
Conceptually, design engineers will use chiplets to design a system. However, the co-design and integration is far more complicated than assembling a set of LEGO blocks, because the chiplets, interposers, and package substrates come from different design houses and manufacturing facilities. The advanced packaging technologies used to connect chiplets vary with an alphabet soup of names — FOWLP, FOPLP, CoWoS, etc., each of which poses additional design and material choices along with certain process limitations.

Fig. 3: There are a multitude of choices in multi-die packaging from the high-level layout to substrates, materials, bonding methods, and cooling materials. Source: Synopsys

Currently engineering teams determine the tradeoffs among the different packaging options to select materials, derive a process recipe, and determine design rules.

Materials are a good starting point. “Materials are very important because they enable new products and packaging technologies,” Tanja Braun, deputy group manager at the Fraunhofer Institute for Reliability and Microintegration IZM. “As you move into more advanced packaging, process is getting much more complex because you are putting more things together. In the end, it’s a combination of equipment, materials, and process development.”

There are three thermal parameters that are critical in package assembly processes — coefficients of thermal expansion (CTE), glass transition temperature (T_g), and thermal conductivity. These factors affect how a material behaves in manufacturing to packaging processes, as well as how it behaves in the field.

“Challenges for our materials include temperature limitations of different die,” said Rama Puligadda, CTO at Brewer Science. “We have to ensure that the temperatures used for bonding materials don’t exceed the thermal limitations of any of the chips that are being integrated into the package. Additionally, there may be some subsequent processes like redistribution layer (RDL) formation or molding. Our materials have to survive those processes. They have to survive the chemicals they come in contact with throughout the packaging process scheme. Mechanical stresses in the package add additional challenges for bonding materials.”

Within a stack of chiplets-on-substrate with an optional interposer, their material attributes affect the thermal-mechanical stresses between neighboring materials, as well. This directly impacts interconnect dimensional control over a large area substrate area.

“If you go work the numbers, you will find that the level of tolerance and control required is frightening,” said Dick Otte, CEO of Promex Industries. “You’re talking about controlling dimensions equivalent to the width of a grass blade over the length of a football field, so that’s roughly 1 in 100,000.”

The goal is uniform heating of the structure in reflow in order to attain the best process results and to avoid cracking. “When you’re taking it through a 250 degrees centigrade temperature change, then you need to heat up slowly so that the top doesn’t get hot before the bottom does,” said Otte.

Multi-physics to comprehend co-optimization
Multi-physics modeling has become the go-to method for co-optimizing packaging design and assembly process development. That affects both permanent and temporary materials, as well the placement of processors, memories, and other components.

“You always looking to what the customer needs electrically, because that’s going to help define the material set. The material set is broadly applicable to a bunch of speed ranges. As long as you don’t step outside of those electrical specifications, theoretically you should be okay,” said Mike Kelly, vice president of advanced package and technology integration at Amkor Technology.

To save many iterations of empirically based development, engineers can use physics-based simulations to understand the impact of a material set’s properties impact on the assembly process, power/thermals, and mechanical vibrations.

Consider that HPC chiplet products can consume ~1,000 watts at peak performance so the power and thermal interactions need to be fully understood.

“We’ve struggled, as everybody has, with this blizzard of complexity in the different techniques. Not only do they vary across different vendors, but they’re also varying over time,” said Marc Swinnen, director of product marketing at Ansys. “Our approach has been to identify the essentials that need to be worked on. We work jointly with customers to develop a simulation flow that actually achieves what is needed now.”

Materials are just one piece of the puzzle. “Then there’s the assembly stresses that need to be modeled to know whether you can correctly assemble this device. The third one is mechanical vibration,” Swinnen said. “Can your device withstand those regular vibrations? Modeling these attributes ties directly into our mechanical analysis tools — acoustic, thermal, vibration, etc. In the end, you’re going to have to do physics simulation. We’re trying to make it accessible to people in many different forms. But the bedrock of our tool offerings is that we have the meshing simulation and analysis. It’s a question of getting the data in the right format in a way that’s practical and usable.”

Evolving assembly design kits
For conventional packages, OSATs provide design rules for each packaging technology. These need to consider electrical, mechanical and thermal design requirements and manufacturing process limitations. In effect this is a multi-dimensional bounding box. Suppliers perform iterations with the customer to create a product specific process recipe.

Rules cover the macro-level attributes. “At a minimum, what you see from design rules is maximum package size, maximum silicon size, and whether silicon can be [mounted] on both sides of the substrate, such that when you follow these constructions the final product will have a lifetime of 1,000 thermal cycles, for example,” said Fraunhofer’s Braun.

In addition, design rules need to describe routing constraints for the interposer and/or redistribution layer, such as RDL line widths and spaces, ball-grid/pillar/pad size and pitches, and the maximum number of interconnections.

Breaking up a monolithic HPC device into multiple dies shifts some of the semiconductor design/process complexity into the packaging space. That makes things much more complicated. Consider that to connect 10 dies requires on order of 100,000 traces within the interposer’s or substrate’s redistribution layer.

To cope with the complexity at the chip level, the IC industry has long relied upon process design kits (PDKs) to capture design rules in an electronic file that can be imported into EDA tools. Their counterparts, assembly design kits (ADKs), are relatively immature.

“We call it Smart Package,” said Amkor’s Kelly. “It’s an ADK that we give to every customer who’s doing their own design. It is a set of macros, and a customization of a database tailored to a customer’s particular design. For chiplets, it is a high-density fan-out package technology. And it’s cognizant of the limitations for metal density and metal spacing, etc. This makes it easier for us to do design rule checks (DRCs).”

But right now, with the level of customization still required, how an ADK is derived and what it entails is in flux. Partnerships between EDA tool vendors, OSATs, and semiconductor device providers are required.

“We come from the IC world where everything is very rigid,” said Kenneth Larsen, director of 3D-IC product management in Synopsys‘ EDA Group. “On the OSAT side, and maybe this is because it’s so custom, design rules seem like a data sheet. Then you build and optimize the products over time or in collaboration with the OSAT. It’s not an electronic exchange. In the IC world, this would be totally unheard of. While it is possible to tweak a few things, you have a qualification process. And it seems like that’s not there yet for packaging.”

Materials and associated assembly recipes ultimately drive what’s possible for a chiplet-substrate stack in terms of pillar pitch, RDL line widths and spaces, bonding processes, and chiplet placement tolerances. But within a handful of ADKs, there are many possible interactions to consider.

The current focus is on co-optimizing the system design with the chiplet assembly process, leading to an assembly process development flow (see figure 4). This flow considers the needs of customization of an assembly process, and it creates the necessary design rules to be used by package designers.

Fig. 4: Chip-package hybrid flow. Source: ASE

“First you need to define your structure using chiplets. Are you using substrate RDL, 2.5D RDL, or a bridge? After that you need to consider your structure’s materials. What kind of material do you choose to fulfill your electrical performance and the mechanical stress requirements,” said Cao. “After that, you do pre-analysis to ensure all the structures and materials you use are workable in terms of electrical, warpage and mechanical stress.”

The design planning flow also includes the evaluation of die-to-die interconnects through the documents for co-design sign-off.

Conclusion
Before chiplet-based designs can be enabled outside the IDM model, the industry needs to complete the ecosystem that bridges the manufacturing and design complexity. This is because the need to co-optimize the system architecture based on materials, process, and integration capabilities is essential. While this would be easier with a set of well-defined products for the chiplet ecosystem to drive forward on, that has not happened yet.

Engineering teams across the design and manufacturing stack will need to collaborate to choose the appropriate materials, architectures, processes, etc., to develop a final chiplet-based product that is designable. As ASE group’s Cao noted, “An integrated design and manufacturing ecosystem is important. It is very critical to have collaboration among IDM, vendors, materials suppliers. Everyone needs to work together to really enable integration for the real applications.”

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