Reliability On The Road: Multiphysics Design For Automotive 3D-ICs

Anyone who has purchased a car over the past decade knows that there has been a huge increase in the amount of compute processing involved in today’s modern automotive industry. Advanced chips for diagnostics and entertainment as well as logic associated with advanced sensor technology and automated assist features have quickly become key requirements that drivers rely on every day to ensure a safe and comfortable commute. It is noted that the average modern car today has somewhere around 1500 computer chips and the most advanced cars can have as many as 3000! Figure 1 shows a wiring harness diagram that connects all the electronics in a modern car.

Fig. 1: Wiring harness diagram. Some vehicles can have nearly 40 different harnesses to connect all the ICs.

Like so much of the semiconductor industry, the automotive industry is now turning towards 3D-IC design techniques to help decrease price per chip as well as reduce power consumption and to fit the chips into tight locations. But, as the greater semiconductor market is seeing, 3D-IC design is not without its own unique set of challenges. Specifically, the impacts thermal and mechanical stresses can have on the reliability of those chips can be a challenge. Both thermal and mechanical stresses have impacts on the reliability of chips as well as impacts on the electrical behavior of the chips. Even more challenging, these are not independent variables. Thermal impacts mechanical stresses. Mechanical stresses can alter a material’s thermal properties. Both impact the electrical signals, including power, but power is the direct source of heating. Addressing these intertwined effects requires a multiphysics approach to simulation and analysis where all impacts are analyzed together.

What’s special about automotive ICs?

In typical home or office compute needs, these impacts can be largely mitigated or at least constrained due to their environment. Cloud or in-house hardware is typically stored in a climate controlled clean-room environment. Unfortunately, doing the same for an automotive application is impossible. Vehicles need to be built to withstand difficult driving terrains and potentially significant temperature impacts. In fact, there are very strict guidelines put on the automotive industry in the form of ISO 26262 and other compliances to help ensure that all cars can drive safely and reliably for 10 years or longer.

For auto manufacturers, that means that the chips, including 3D-ICs, the batteries that power them, and the cable harnesses that connect it all together need to undergo careful design and testing. This is no small feat considering cars and trucks come in all shapes, sizes and weights, are exposed to hot sun or cold winters, face potholes, and may have to make very fast stops. How can IC designers, who design chips without knowledge of the final automotive system, ensure that with all this variation, the car as a whole continues to function as designed?

Thermal in automotive 3D-ICs

The first key to this problem is through careful design. Every part must be considered in detail. Consider for example thermal impacts. Chips can get hot. Depending on their placement within the car, that heat can propagate to mechanical structures which can then be damaged or malfunction. Likewise, mechanical parts, especially the engine or EV batteries or brakes can get very hot. If located near sensitive 3D-ICs, the electronics can fail. The same can be said for mechanical stresses. Mechanical stress can impact transistor behavior, ultimately potentially impacting the electrical reliability in the course of driving. Further, exposure to heat can change the mechanical properties of the materials in the chip, further impacting the stress impacts. Imagine the differences between a small and light weight compact car versus a large industrial truck hauling heavy materials. Different kinds of vehicles require unique decisions on best placements of 3D-ICs to avoid heat and stress impacts.

Similarly, pulling power from the car battery to pass through the cable harness to the chiplets cannot be ignored. Long cables with high resistivities can further generate heat, which can impact the reliability cables, chiplets or their interfaces. The location of the 3D-IC components within the automotive system must be carefully evaluated.

Safeguarding all of this requires careful simulation from the bottom-up. Typical 3D-IC simulation must consider the thermal impacts created by a combination of the power, in this case from the battery, and the thermal boundary conditions for each chiplet within the context of the 3D-IC package assembly. This requires a very detailed level of thermal analysis that has not traditionally been considered in most compute applications. Given that in an automotive application both the battery operation and the thermal boundary conditions can vary significantly, worst case conditions need to be applied. By capturing the thermal model of the chiplets and the assembly, a more accurate model at the printed circuit board (PCB) level can be achieved (figure 2). Further, this accuracy can also be passed through modern formats up into the full automotive system level. This is sometimes called a co-design and co-optimization approach, in which the silicon, package and system can share data to inform each step along the development of the final product (and select data from the final product can feed back to inform the design and process of the components).

Fig. 2: Colormaps and colormap animation allow IC designers and package engineers to see the thermal distribution across all 3D components simultaneously throughout the simulation.

Stress in automotive 3D-ICs

The same kind of approach can be used with respect to mechanical stresses (figure 3). The impacts of mechanical stresses can be captured at the chiplet, assembly, PCB and ultimately to the full automotive system, including the cable harnesses. Considering the specific automotive application and expected conditions with respect to vibration and distortion and coupling with the captured thermal impacts, it is possible to make the careful decisions about chiplet, package, and board placement within the car to ensure safe, comfortable and reliable driving for many years.

Fig. 3: Mechanical stresses like die warpage substrate cracking can affect the chiplet, assembly and PCB. Mechanical stress can affect the full automotive system and must be considered starting at the IC design and verification stage.

Summary

The automotive industry is increasingly adopting 3D-ICs to enhance performance, reduce power consumption, and fit advanced electronics into compact spaces. However, designing reliable automotive chips presents unique challenges due to the harsh environments vehicles endure, such as extreme temperatures and mechanical stresses. Thermal and mechanical effects are interdependent, requiring a multiphysics approach to simulation to ensure chip reliability. Accurate modeling at every level—from chiplets to the full automotive system—is essential for safeguarding performance and meeting industry standards like ISO 26262.