Electronic Design
Thermal Modeling Takes The Heat Off Of Automotive Silicon Design

Thermal Modeling Takes The Heat Off Of Automotive Silicon Design

 

 

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Why is it important to dissipate heat? For most semiconductor applications, quickly moving the heat away from the die and out toward the larger system prevents highly concentrated areas of heat on the silicon.

Typical operating temperatures for silicon die range from 105°C to 150°C, depending on the application. At higher temperatures, metal diffusion is more prevalent and eventually the device can fail from shorting. 

The die’s reliability depends greatly upon the amount of time that’s spent at the high temperatures. For very short durations, a silicon die can tolerate temperatures well above the published acceptable values. However, the device’s reliability is compromised over time. 

Due to this delicate balance between power needs and thermal limits, thermal modeling has become an essential tool for the automotive industry. In the automotive safety industry, the current drive is for smaller assemblies with lower part counts, which forces semiconductor providers to include more functions with higher power consumption. 

The higher temperatures generated ultimately will affect reliability and, in turn, automotive safety. But by optimizing the die layout and power pulse timing early in the design cycle, designers can provide an optimized design with fewer silicon test builds, leading to a quicker development cycle time.

Semiconductor Thermal Packaging

The automotive electronics industry uses various semiconductor package types, from small, single-function transistors to complex power packages with more than 100 leads and specially designed heatsinking capabilities. 

Semiconductor packaging serves to protect the die, provide electrical connection between the device and external passive components in the system, and manage thermal dissipation. For this discussion, we’ll focus on the semiconductor package’s ability to conduct heat away from the die. 

In leaded packages, the die is mounted to a metal plate called the die pad. This pad supports the die during fabrication, and it provides a good thermal conductor surface. A common semiconductor package type in the auto industry is the exposed pad, or PowerPAD-style, package (Fig. 1).

The bottom side of the die pad is exposed and soldered directly to the printed-circuit board (PCB), providing a direct thermal connection from the die to the PCB. The primary heat path runs down through the exposed pad, which is then soldered to the circuit board. The heat is then dissipated through the PCB into the surroundings. 

Exposed-pad-style packages conduct approximately 80% of the heat though the bottom of the package and into the PCB. The other 20% of the heat dissipates from the device leads and sides of the package. Less than 1% of the heat dissipates from the top of the package.

A similar leaded package is the non-exposed pad package (Fig. 1, again). Here, plastic fully surrounds the die pad, providing no direct thermal connection to the PCB. Approximately 58% of the heat dissipates from the leads and sides of the package, 40% from the bottom of the package, and approximately 2% from the top.

Heat transfer occurs via conduction, convection, or radiation. For automotive semiconductor packaging, the primary means of heat transfer are through conduction to the PCB and by convection to the surrounding air. Radiation, when present, represents a minor portion of the heat transfer.

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Thermal Challenges

Operation, safety, and comfort automotive systems rely heavily on semiconductors. They’re now common in body electronics, airbags, climate control, radio, steering, passive entry, anti-theft systems, tire monitoring, and more.

Despite many new applications for semiconductors in the automotive industry, three traditional areas still maintain individual environmental requirements: inside the vehicle cab, the panel firewall, or under the hood. In conjunction, three factors continue to challenge the automotive environment: high ambient temperatures, high power, and limited material thermal-dissipation properties.

Temperatures for automotive environments versus other environments are typically worlds apart. Generally, consumer-electronics temperature commonly resides at 25°C, with upper limits around 70°C. On the other hand, electronics inside the passenger compartment of the car or panel applications will run at temperatures up to 85°C (see the table). 

In firewall applications, where electronics are located between the engine compartment and the vehicle’s cab, devices can be exposed to ambient temperatures up to 105°C. Under-hood applications require operation in an environment with ambient temperatures up to 125°C.

Thermal considerations are especially important in safety-related systems, such as power steering, airbags, and antilock brakes. In braking and airbag applications, power levels of up to 100 W can be expected for short duration (~1 ms). 

Increased functionality demands plus concentrated multiple sources drive the die’s high power. Die temps for some automotive-application semiconductors can reach up to 175°C to 185°C for short periods of time. Typically, this is the thermal shutdown limit for automotive devices. 

Thermal demands increase with the addition of more safety features. Though airbags have been common in vehicles for longer than a decade, some cars now come with as many as 12 airbags. During deployment, multiple airbags require a sequenced operation and create a much greater thermal design challenge compared to a single traditional airbag.

Regarding thermal challenges in terms of material properties, it’s no secret that there’s a concerted effort to reduce cost in automotive assemblies. Plastic materials are replacing metal modules and PCB enclosures. Plastic enclosures have the benefit of being cheaper to produce. They also weigh less. The tradeoff for lower cost and reduced weight, however, is a reduction in thermal performance.

Plastic materials have very low thermal conductivity, in the range of 0.3 to 1 W/mK, so they function as thermal insulators. No doubt, then, that the changeover to plastic enclosures will limit a system’s heat dissipation, increasing the thermal load on the semiconductor device. 

Why Modeling?

Within the automotive semiconductor industry, modeling activity typically focuses on the thermal performance and design of a single device. Careful simplifications can be assumed to obtain modeling data. 

System-level simplifications, such as eliminating extraneous low-power devices from the model, using simplified rather than detailed PCB copper routing, or assuming chassis is at a fixed temperature for heatsinking, can all streamline the thermal model for fast solver times. In addition, they will still deliver an accurate representation of the thermal impedance network.

Package-level thermal modeling makes it possible to review potential packaging design changes in advance without costly development and testing activity, eliminating material builds. Semiconductor packaging design can be varied to allow for the optimal thermal dissipation depending on the application’s needs.

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With exposed-pad packages like PowerPAD, heat can quickly dissipate from the die to the PCB. Variations such as larger die pads, better connection to the PCB, or improved die pad design offer ways to improve a device’s thermal performance.

Thermal modeling is also used to review the impact of potential material changes in a device. Thermal conductivity of packaging materials can vary widely, from as low as 0.4 W/mK (thermal insulator) to more than 300 W/mK (good thermal conductor). Using thermal modeling techniques helps balance product cost versus performance. 

Verification Of Modeling

For critical systems, careful lab-based analysis can determine thermal performance and operating temperatures. However, lab-based measurement of these systems may be time-consuming and costly. Here, thermal modeling is instrumental in addressing the system’s thermal needs and satisfying operational requirements.

In the semiconductor industry, thermal modeling has become an early part of the concept testing and silicon die design process. The ideal thermal modeling flow begins months before fabricating any die. The IC designer and thermal engineer review die layout and power losses for the device.

Then, the thermal modeling engineer creates a thermal model based on this review. Once thermal-model results are complete, the designer and modeling engineer review the data and tune the model to accurately reflect possible application scenarios.

Verification of all finite element analysis (FEA) modeling is highly recommended. Texas Instruments’ policy is to run correlation studies comparing thermal-modeling results with a system’s physical measurements. 

These correlation studies highlighted several areas of potential error, including material properties, power definition, and geometry simplification. While no model will be a perfect duplication of a true system, careful attention must be paid to the assumptions made during modeling to ensure the most accurate system representation.

For material properties, published values often show the bulk conductivity of a particular material. Yet in semiconductor applications, thin layers of material are commonly used, and the increased surface area of the material can cause a decrease in thermal conductivity compared to the bulk value.

Carefully note the power represented in the model, because applied power on a device during operation can vary with time. Power losses in the board or other areas in the system may also impact the die surface’s true power.

Types of Modeling

To aid in semiconductor package design, there are four main types of thermal models: system level, package level, die level, and die-level transient analysis. In the automotive semiconductor arena, system-level thermal modeling is important because it shows how a particular device will perform in a specific system.

At a basic level, automotive semiconductor thermal modeling takes the PCB into account because it’s the primary heatsink for most semiconductor packages. The composition of the PCB, including copper layers and thermal vias, should be added to the thermal model to accurately determine thermal behavioral analysis. Furthermore, include any unique geometry, such as embedded heatsinks, or metal connections like screws or rivets. 

Forced airflow around the system and PCB can also play a role in the convective heat transfer from the system. Typically, the semiconductor industry targets thermal modeling on a single, high-power device. Other power components on the PCB, though, may play a large role in the system’s overall performance too.

Simplifying the inputs from these packages, yet still maintaining a level of accuracy, often requires compact models. Compact models are less complex networks of thermal resistance that provide a reasonable approximation of the thermal performance of these non-critical devices.

In smaller and lower-pin-count devices, other methods can be used to improve thermal performance (Fig. 2). For instance, fusing several of the package leads to the device’s die pad can significantly improve the overall junction temperature without impacting the device’s operation. 

Modeling Assumptions

Die-level thermal analysis begins with an accurate representation of the silicon die layout, including any powered regions on the die. In simple cases, assume that the power is evenly distributed across the silicon.

However, most die layouts have power in uneven patterns across the die, depending on functionality. This uneven power distribution can be critical to the device’s overall thermal performance. For thermally critical devices, pay close attention to the power structure’s location on the die. 

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In some thermal software programs, the die layout can be entered using comma-separated variable (.csv) inputs (Fig. 3). This allows for an easy transfer of information between die layout and thermal-modeling software. Depending on the device’s complexity and the power level, these powered regions on the die can vary from two to three locations to several hundred. 

The thermal-modeling engineer should work closely with the IC designer to identify the powered regions for inclusion in the thermal model. Often, small, very low-powered regions can be grouped into larger regions, simplifying the thermal model while still accounting for the device’s overall power. Similarly, in a thermal model, background or quiescent power can be used across the die’s surface to account for a large percentage of the non-critical low-power die structures.

  Device functionality frequently requires high power over small areas on the die. These high-powered regions can lead to localized heating regions, which may be significantly hotter than the surrounding silicon. 

Thermal modeling helps highlight thermal problems in which clusters of medium power silicon products located in close proximity may cause residual heating and possible thermal stress to the die under assessment. 

Models can also aid in the placement or calibration of embedded thermal sensors on the die. Ideally, a temperature sensor is placed at the center of the highest-powered region on the die. Yet due to layout constraints, this is often not possible. When located away from the center of the powered region, the temperature sensor is unable to read the device’s full maximum temperature. 

Thermal models can be used to determine the thermal gradient across the die, including at the sensor location. Then the sensor is calibrated to account for the temperature difference between the hottest region and the sensor location.

The aforementioned model types all assume a constant dc power input. In actual operation, though, device power varies with time and configurability. By designing the thermal system to account for only the worst-case power, the thermal load may become prohibitive.

Transient thermal response can be reviewed using one of several different methods. The simplest method is to assume a dc power source on the die, then track the thermal response of the device as a function of time. A second method inputs a varying power source and then uses thermal software to determine the final steady-state temperature.

The third and most useful transient modeling style is to view the “response with time” of varying power in multiple die locations (Fig. 4). With this method, you can catch interactions between devices that may not be apparent under normal conditions. Transient modeling is also helpful in viewing the full duration of certain die operations that occur separate from normal device function, e.g., a device’s power-up or shutdown mode.

In many automotive systems, such as braking actuation or airbag deployment, the device power remains at a low level for the bulk of the device’s lifetime. In the case of an airbag system during deployment, the power pulse can reach very high power for short durations.

Improvements

Design optimization and lower overall temperatures are the goals of thermal modeling for the automotive semiconductor industry. Lowering the operating die junction temperatures improves a device’s reliability. 

Small enhancements to the system, board, package, or die potentially leads to dramatic improvements in final temperatures. Device and system limitations can eliminate some of these suggestions, though. 

Methods for boosting thermal performance include airflow, conductive heat paths, or external heatsinking. Another is to provide more metal area for heat dissipation, such as by adding external heatsinks, metal connection to chassis, more layers or denser copper layers on a PCB, thermally connected copper planes, and thermal vias.

Thermal vias located below the exposed pad of a device help to quickly carry heat away from the device, as well as speed dissipation to the rest of the circuit board. Semiconductor device packages are designed to quickly move the heat away from the die and to the larger system.

A semiconductor package can be improved thermally with higher conductivity materials, direct attachment to a PCB such as PowerPAD, leads fused to the die pad, or mounting locations for external heatsinks. The semiconductor die itself allows for many possible ways to minimize overall temperature. Of course, the best way to lower temperature is to lower power.

For silicon circuit design and layout, good thermal practices include larger heat-dissipation areas, locating powered regions away from the edges of the die, using long and narrow powered regions instead of square regions, and providing adequate space between high-powered regions. Silicon is a good thermal conductor with a conductivity of approximately 117 W/mK. Allowing the maximum amount of silicon around a powered region improves the device’s thermal dissipation.

For transient power on a die, staggering power pulses to decrease instantaneous power will lower overall temperatures. This results in either a long lag time between power pulses so that the heat can dissipate, or high power events are shared over several areas on the die. These transient variations allow the thermal system to recover before more heat is applied. By carefully designing the die, package, PCB, and system, a device’s thermal performance can be dramatically improved.

References

1. To learn more about thermal modeling and other automotive semiconductor devices, visit www.ti.com/automotive-ca.

2. Download an application note for PowerPad at www.ti.com/powerpad_slma002d-ca.

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