Challenges in Predicting Thermal Integrity of 2.5D and 3D-ICs

Thermal Integrity Challenges in 2.5D and 3D-IC Designs: Impacts on System Behavior and Field Reliability

As silicon interposer technology has advanced, with larger interposers containing multiple dies or chiplets, thermal integrity has become a critical concern. With high power requirements and increasing performance demands, accurately predicting thermal effects in heterogeneous silicon interposer designs has become more difficult. Existing tools for thermal modeling are often disconnected and not fully integrated, making it challenging to ensure reliable sign-off schedules with confidence.

One of the challenges is that traditional approaches used for board-level and package-level designs may not directly apply to 2.5D and 3D-ICs. The landscape has shifted, with chip engineers now taking on the responsibility of thermal design, and boundaries between different domains, such as packaging and IC teams, becoming blurred. This can result in issues of ownership, resolution, and information integration.

Despite the technical aspects of solving thermal challenges, the real hurdle lies in organizational boundaries. As multiple dies from different technologies and foundries are integrated, each die stack is unique, and there is no established process for consolidating all the relevant information into a single tool. Bridging these gaps and addressing the complexity of thermal integrity in 2.5D and 3D-ICs remains a significant challenge for the semiconductor industry.

Effective Communication among Stakeholders for Successful Chip Design

For successful chip design in 2.5D and 3D-ICs, collaboration and communication among all parties involved, including chip designers, EDA tool providers, foundries, and packaging houses, are crucial. Even within a single foundry, coordination is required to set up the necessary infrastructure and processes for chip integration.

Different user bases with varying resolutions of operation, such as chip designers focusing on microns and packaging/systems teams working in millimeters, need to align their approaches and understand each other's requirements.

In addition, the unique material properties of silicon interposers introduce new challenges. Unlike traditional boards or organic substrates, silicon interposers require different practices for maintaining balance and testing system limits. Previous assumptions may not hold true, as physics can change, and what used to be second-order effects can now become first-order effects. Therefore, understanding the nuances of the material and its behavior through mathematical modeling is critical.

In summary, effective communication, coordination, and understanding of material properties are essential for successful chip design in 2.5D and 3D-ICs, involving collaboration among different stakeholders in the ecosystem.

Changing the Starting Points in Heterogeneous Integration Systems

Heterogeneous integration systems are typically built incrementally, starting with the package substrate of a system and then mounting an interposer on top of it. The interposer serves as a bridge that connects various components, including die or chiplets, through micro-bumps or copper-to-copper connections. Depending on the design integration plan, additional die or chiplets can be added on top of the interposer, some of which may be further stacked in a 3D configuration. The interposer also hosts the chiplets and die, delivers supply power to them, and contains the power delivery network for all the components.

However, this integration of different components with varying power consumption and heat generation can result in power and thermal integrity problems. For example, the power and heat generated by chiplets mounted on top of the interposer can cause power integrity issues and thermal integrity problems, including the risk of one IC burning a neighboring IC. Additionally, with 3D stacked die that include high bandwidth memory, power and heat problems can also lead to signal integrity problems. Therefore, designers play a critical role in ensuring successful power delivery, heat dissipation, and signal integrity in heterogeneous integration systems.

Modeling an interposer-based heterogeneous design is challenging due to the many variables involved. Designers need to make assumptions about material deflection and other factors, but determining the right assumptions can be complex. Abstraction level plays a role in the accuracy of models, and gross modeling and averages may be used for solving thermal aspects. However, with newer technologies and the integration of different technologies, such as finFETs, thermal issues can become more challenging due to the use of insulating materials with low thermal conductivity. This can result in the need for additional solutions, such as through-silicon vias (TSVs), to dissipate heat from hot transistors.

Despite the challenges, there are simulation and modeling tools available for understanding heat flow and thermal conduction in heterogeneous integration systems. Fourier's Law of heat conduction can be used to describe how heat dissipates in a system and how the heat sink affects the temperature difference of the system. However, mapping Fourier's Law through a silicon interposer can be complex, as it involves modeling the thermal resistance of multiple components in the system, including the chiplet, interposer, package, and heat sink.

Changing the starting points in heterogeneous integration systems, such as mounting chiplets on top of an interposer, can introduce power and thermal integrity problems. Designers play a crucial role in ensuring successful integration and addressing these issues through modeling and simulation, although challenges remain in determining the right assumptions and accurately modeling complex systems. Understanding heat flow and thermal conduction is critical in mitigating power and thermal integrity issues in heterogeneous integration systems.

The evolution of solutions is yet to be determined.

Aitken highlights the current trend of broad experimentation, but anticipates that the industry will eventually converge on common approaches. In the past, there were a limited number of package options characterized by the package vendors, making it predictable to integrate designs into them. However, nowadays there are numerous package designs, even within the constraints of silicon interposers, with varying ways of assembling them. There is no clear consensus on which design is superior, which means manufacturers may need to support custom solutions for each customer and potentially for each design. This necessitates cautious approaches to redesigns to ensure safety while being mindful of customization.

The introduction of new materials will also bring about additional challenges. Designers may lack historical data on the thermal conductivity and tensile strength of these materials, and there may be variations among different lots. Thermal analysis may be performed without grids, but other downstream processes may require grid-based data, adding complexity to the design process. The industry has traditionally employed guard-banding, allowing for safety margins by assuming a certain percentage of variability. However, this approach may leave potential performance gains untapped.

Another challenge is making critical decisions early in the design process. Architecture considerations, such as package size and heat dissipation, need to be addressed through budgeting and approximate models. However, with advancements in technology and increasing heat generation from newer designs, customers now need to assess if their package and heat dissipation methods are sufficient to maintain reliability, taking into account factors such as electromigration, which is highly dependent on temperature. Engineering teams strive to address these concerns earlier in the design process, but often lack comprehensive information on the technology they plan to use, leading to uncertainties and the need for careful evaluation.

In conclusion, the evolutionary changes in the design of complex heterogenous systems pose significant challenges for design teams. While there are techniques available to address these challenges, such as reducing power, balancing power density, smartly partitioning chips and chiplets, and implementing cooling solutions at both device and system levels, there is no one-size-fits-all solution. The increasing variety of package options and materials, the lack of consensus on optimal designs, and the need to make decisions early in the design process all contribute to the complexity of the problem. As the industry continues to evolve, designers will need to stay informed about the latest technologies and approaches, and carefully consider the trade-offs to ensure reliable and efficient performance of heterogenous systems in the face of thermal challenges.