Completely Overcoming the Heat Dissipation Problem in Automotive Semiconductor Design (Part 2)

　　对于汽车半导体而言，单一装置的散热效能及设计通常是建立模型的重点，必须审慎简化，才能取得建立模型资料。去除模型中多余的低功率装置、简化PCB铜线佈线、假设基座对在固定温度散热，加速完成散热模型，才能準确呈现热阻抗网路。

　　Package-level thermal modeling can further examine possible package design changes without the need for high-cost development and testing, thereby eliminating material construction. Because semiconductor package design can be changed to achieve the best heat dissipation effect according to application requirements. In automotive semiconductor components, exposed pad packages such as PowerPAD allow the die to quickly dissipate heat to the PCB. Packages such as enlarged die seats, improved PCB connections or base designs are all designed to achieve more effective heat dissipation performance. Thermal modeling is also used to check the potential impact of material changes in the device. The thermal conductivity of packaging materials varies greatly, ranging from 0.4W/mK (thermal insulator) to 300W/mK (good thermal conductor). Establishing a thermal model helps to balance product cost and performance.

　　Building model validation

　　对于重要的系统而言，审慎的实验数据分析可决定散热效能及运作温度，但实验测量这些系统相当费时且耗成本。散热模型能够确实符合系统的散热及运作要求，在半导体产业中，散热模型已经成为概念测试及硅晶设计过程的前置作业，理想的散热模型建立流程会在晶片生产前几个月进行。积体电路设计人员和散热工程人员负责先检查装置的晶片配置及电源耗损，然后散热模型工程人员依据检查结果，建立散热模型，一旦散热模型结果备齐，设计人员及建立模型工程人员将检查数据并调整模型，以準确反映可能的应用情形。

　　汽车半导体产业多年来使用散热模型提升产品设计，由其是有限元素分析（finite element analysis; FEA）验证的模型。TI的塬则是先比较散热建立模型结果与系统的实体测量，以进行相关分析。这些相关分析着重于潜在误差，包括材质、电源定义及尺寸简化。没有任何模型能够完全呈现实际的系统，因此必须注意建立模型期间所做的假设，以确实呈现最準确的系统。

　　对于材质而言，发佈的数据通常呈现特定材料的容积传导率，不过应用材料表面的反应会影响散热效能。必须注意模型中呈现的功率，因为运作期间施加于装置的功率会随着时间而变化。电路板或系统其他区域的电源耗损可能也会影响晶片表面的实际功率。

　　Model type

　　Based on the thermal conditions of a specific project, there are four main thermal models for automotive semiconductors that can be used to understand and verify thermal performance: system level, package level, die level, and die transient analysis.

　　系统层级散热建立模型相当重要，因为模型可呈现特定装置在某些系统中运作的效能。基本上，汽车半导体散热建立模型将PCB纳入考量，因为PCB是大多数半导体封装的主要散热器。PCB的铜层和散热孔结构都必须包含在散热模型中，才能準确判断散热行为。如果系统使用嵌入式散热器之类的元件，及螺丝或铆钉作金属连接，都必须纳入模型中，以判断对于装置的散热效能所产生的影响。

　　强制对流（forced airflow）及PCB周围的空气流通对于系统导热也相当重要。半导体的散热模型通常是针对单一高功率装置，但PCB的其他电源元件对于系统的整体效能也相当重要。若要简化这些封装的输入，并维持準确度，通常可使用精简模型。精简模型是简化的热阻抗网路，合理估算PCB上的较不重要的装置所达到的散热效能。

　　In small devices with low pin counts (see Figure 2), other methods can be used to improve heat dissipation. By attaching multiple package pins to the base of the device, the overall junction temperature can be greatly improved without affecting the operation of the device.

　　Figure 2: For an 8-pin SOIC package, the junction temperature can be as low as 25°C after the multiple pins are connected to the base.

　　Building model assumptions

　　晶粒分析首先需要準确呈现硅晶配置，包括晶粒上任何用电的区域。简单来说，可以假设电源平均分配到晶片的各个区域，不过，对于大多数的晶片配置而言，皆会因为功能而出现供电不平均的情况，这种现象对于装置的整体散热效能至关重要。针对重视散热功能的装置而言，必须特别注意晶片的用电结构。

　　在某些散热软体程式中，可使用逗号分隔变数（.csv）来输入晶片配置，如图3所示。如此即可在晶粒配置与散热模型软体之间轻鬆传输资讯。视装置的复杂度及用电量而定，这些用电区域可能有两到叁处，甚至数百处。散热模型工程人员应该与IC设计人员密切合作，找出哪些用电区域应该纳入散热模型。考量装置的整体用电时，通常可以将用电量较小的小区域合併为大区域，以简化散热模型。在散热模型中，也可以在晶粒表面使用背景功耗或静态功耗，以考量大部份的非重要低功耗晶粒结构。

　　Device functions often require more power than a small area on the die can provide. These high power areas can cause that area to overheat, becoming significantly hotter than the surrounding area. Adjacent moderately powered dies can cause residual heat and thermal stress on the die under test, and thermal modules can also reveal these thermal issues.

　　Models can also be used to help place or adjust the location of embedded temperature sensors. Temperature sensors are best placed in areas of highest power usage, but this is often not possible due to configuration constraints. If not placed in the center of the power usage area, the temperature sensor cannot read the actual maximum temperature of the device. Thermal models can be used to determine the thermal gradients on the die, including at the location of the sensor. The sensor can be adjusted to account for the temperature difference between the hottest area and the sensor area.

　　The model types mentioned above all assume that the input power is a steady DC power supply. In actual operation, the device power supply will vary over time and configuration. If the cooling system is designed to account for the worst-case power usage, the cooling load will become quite severe. There are many ways to observe transient thermal response. The simplest method is to assume a DC power supply on the chip and then track the thermal response of the device over time. The second method is to input different power supplies and use thermal software to determine the final steady-state temperature. The third and most practical method for transient modeling is to observe the change in power supply over time at different locations on the chip, as shown in Figure 4. This method can be used to understand the interactions between devices that are not present in normal operation. Transient models are also useful for observing the entire operation of a chip outside of normal device operation, such as the device power-on or power-off mode.

　　Figure 3: Thermal modeling software uses comma-separated variable input to generate a detailed die configuration and display potential hotspot locations on the die surface.

　　Figure 4: Thermal response over time on the surface of a semiconductor device. In this case, different areas of the die receive power in an interleaved manner. Thermal modeling allows for a closer look at the die temperature over time.

　　在煞车制动或安全气囊配置等许多汽车系统中，装置用电量在装置使用寿命期间都相当低。对于安全气囊系统，电源脉衝会短时间升高。

改善之道

　　For the automotive semiconductor industry, the purpose of thermal modeling is to optimize the design and reduce the overall temperature. As long as the operating chip junction temperature is reduced, the reliability of the device can be improved. Small improvements in the system, board, package or die can significantly improve the final temperature. However, device and system limitations may make some of these options unapplicable. This article still lists several best practices for system cooling.

　　There are many ways to improve the thermal performance of a system or PCB, including airflow, conductive paths, or external heat sinks. Providing more metal area for heat dissipation can improve thermal performance, including external heat sinks, metal connections to the base, more layers or denser copper layers on the printed circuit board, copper layers connected for heat dissipation purposes, and thermal vias.

　　The heat dissipation vias located under the exposed pads of the device dissipate the heat inside the device, allowing the rest of the circuit board to dissipate heat faster. The design of the semiconductor device package allows the die to dissipate heat quickly. Improvements in semiconductor packaging heat dissipation include more conductive materials, direct attachment to the PCB such as PowerPAD, and pins connected to the die seat or bonding locations of external heat sinks. There are many ways to reduce the overall temperature of the semiconductor die itself, and the best way to reduce temperature is to reduce power consumption.

　　对于半导体电路设计及配置，良好的散热做法包括扩大散热区域、找出晶片边缘外的用电区域、使用狭长形用电区域而非方形区域，及使高用电量区域之间有充足的间隔距离。硅本身是热良导体，导热性约为117W/mK。只要用电区域周围有最多的硅，即可改善装置的散热效果。对于晶粒上的暂态电源，只要将电源脉衝交错而降低瞬间功耗，使电源脉衝的间隔时间加长，让热度能够散出，或者将高用电量元件分配在不同区域，即可降低整体温度。

　　These transient changes can cool the cooling system. With careful design of the die, package, PCB and system, the thermal performance of the device can be greatly improved.

　　Conclusion

　　The automotive industry has unique requirements for high reliability. The number of automotive electronic components for safety, comfort and entertainment continues to increase, and as the electronics of automotive semiconductor devices shrink in size and increase in complexity, new devices run hotter than older ones. Thermal modeling ensures that thermal requirements are adequately met. By optimizing die placement and power early in the design phase, and improving thermal performance at the package and system levels, designers can deliver the best design for their customers.

　　Table 1. Upper ambient temperature limits for automotive applications