Die cracking, solder joint fatigue, warpage, and delamination are just a few of the possible mechanical failures.
The benefits of 3D IC architectures are well-documented – smaller footprints, lower power, and increased performance. However, the move to heterogeneous 3D designs also introduces a host of new challenges that must be carefully navigated.
As chip designers integrate multiple dies and technologies into a single 3D package, the interactions between the chip and package become increasingly complex and critical to overall device functionality. Compared to traditional 2D ICs, 3D ICs present new failure mechanisms that can severely impact reliability.
Die cracking, solder joint fatigue, warpage and delamination are just a few of the mechanical failures that can arise in 3D ICs. Figure 1 shows a SEM image of die cracking. These issues are often caused by thermal mismatch, differential expansion and residual stress within the package. There are also more subtle device-level stresses that can alter electrical characteristics through piezoresistive effects.
Fig. 1: An illustration of die cracking.
Stress is a factor in design reliability. While all circuits integrated at the die level could be impacted by chip-package stress, devices that require extra reliability will be most at risk for failures. Specifically, devices that use piezoelectric changes to determine device function will need early modeling and validation to ensure that neither the packaging process nor the operating temperature ranges will put the device out of spec for piezoelectric mobility changes. This level of reliability will also be required for sensors integrated in health monitors, smartphones and even IoT, transportation and infrastructure devices. To ensure reliability, it will be key to verify, both in the early design phase and final sign-off mode, that the circuit level performance will not be negatively impacted by chip/die level stress.
Accurately evaluating and mitigating these stress-related challenges is essential for the successful adoption of 3D IC technology across high-performance computing, automotive, IoT, and other markets. Designers must have the tools and methodologies to simulate mechanical stresses throughout the 3D IC assembly, from early feasibility analysis to final design sign-off.
Evaluating stress in 3D IC packages requires close collaboration between IC designers and package design teams. The first critical step is to accurately describe the 3D IC package, from the millimeter-scale substrates to the nanometer-scale chip features. Leveraging standards like 3Dblox can help establish a common language for this package description.
The complexity of 3D IC packages can vary greatly, but regardless of the architecture, rigorous analysis is needed to identify potential stress-related reliability impacts. This analysis may need to happen before all package details are finalized, requiring a modular, flexible approach.
In the early design stages, initial assumptions about materials and component sizes may be necessary, but these can be updated as the package design matures. Even with an incomplete die-level design, knowledge of the technology can enable accurate initial estimates of stress profiles across the die. As the package components are finalized, the description can be updated for more precise stress modeling.
Another key consideration is understanding the different stakeholders involved in the stress analysis and how their perspectives may influence the process. The solution space should consider a collaborative co-optimization between IC designers, package designers and others.
The physical material properties of the assembly components must also be accounted for, though any one team may only have complete information within their domain. Simplified “black box” assumptions may be necessary for unknown components. Navigating confidentiality concerns around proprietary details is another complicating factor.
Finally, it is critical to fully capture the stresses imparted by the packaging process itself, which can vary significantly across assembly stages and thermal environments (Figure 2). This requires a deep understanding of the assembly flow and advanced finite element analysis techniques.
Fig. 2: Stresses can be imparted throughout the 3D IC package assembly flow.
After the initial 3D IC package definition and stress analysis, designers must interpret the resulting stress values. The first step is to quickly assess whether calculated stresses exceed basic material thresholds for package components. This could be a simple go/no-go determination, or it may require more complex, iterative re-analysis and design rearrangement.
Early in the design process, when there are still degrees of freedom available, a full-package stress analysis can provide valuable, informative results. This allows chip-level designers to consider the package composition and its potential impact on the die-level design.
As the package planning progresses and component placements become more locked in, the focus can shift to evaluating the specific interfaces between the package and integrated chiplets/ICs. Understanding stress profiles at these critical junctions helps identify regions where sensitive device placement should be avoided. “What-if” analyses may also be necessary to determine if macro placements will exceed mechanical or electrical stress thresholds.
Finally, a comprehensive sign-off analysis of chip-package interaction stresses must be performed to validate that circuit performance remains within acceptable tolerances, ensuring the reliability of the completed product. Figure 3 illustrates the visual stress distribution mapping that can be leveraged in this validation step.
Fig. 3: Stress distribution maps.
Beyond the initial go/no-go assessments, designers may need to consider other analysis techniques and visualization methods. Detailed, region-specific evaluations can provide insight into localized areas of concern, while global package-level analyses reveal broader stress patterns and trends. Factors like shear stress, warpage, and piezoresistive impacts may also need to be considered.
This diverse set of stress analysis approaches all offer unique and valuable information about how the calculated stresses will impact the 3D IC design’s performance and reliability. Both the overall package integrity and the implications for individual circuit components are crucial for the successful development of these complex heterogeneous systems. [2]
The shift to heterogeneous design and advanced packaging has introduced a new level of complexity when it comes to managing stress-related challenges. But this is true of all new technologies – physics has been in the driver’s seat, and will continue to be as we navigate this new 3D terrain.
The complexity of physical verification in 3D ICs will require the ingenuity of many to converge on stable, reliable, and accurate results for both inspection and manufacturing. Physical stress will become a major factor in successful die and package-level design.
The simulation tools for 3D IC thermo-mechanical stress evaluation have mostly been custom in-house software. However, commercially available electronic design automation (EDA) tools are emerging that can provide a more comprehensive and integrated approach. These solutions enable tighter integration with the design data and tools that IC and package engineers already use, streamlining the workflow.
Importantly, commercial tools also enable a more collaborative, co-optimization approach between the IC and package design teams. This is crucial given the intricate interplay between the two domains. As 3D IC complexity continues to escalate, access to such automated, integrated stress analysis capabilities will be essential for ensuring reliable, high-performance designs.
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