The guidelines offered are intended to aid a designer in identifying and mitigating multiple areas of potential concern while designing high accuracy (±0.1°C) temperature sensing circuits. This is demonstrated via an example of a recent CBT design where thermal, electrical and mechanical aspects were concurrently addressed with appropriate trade-offs implemented. These design considerations shall assist the designer to:<ul><li>Learn how to recognise the design challenges, trade-offs, and mitigation techniques associated with the development of an accurate CBT sensing device.</li><li>Learn how to design a reliable rigid-flex printed circuit board for remote patient monitoring applications.</li><li>Apply design guidelines for heat flow and mechanical structure considerations.</li><li>Apply design guidelines for rigid-flex PCB manufacturing.</li></ul><h1>Overview of the CBT Device Design</h1>The CBT patch was developed as a flexible wearable thermal sensing device that accurately estimates human CBT (Figure 1a). Figure 1b highlights the main components of the thermal device. The device comprises four temperature sensors (MAX30208) separated by the materials with different thermal conductivities to accurately quantify CBT. The temperature sensors provide 0.1°C accuracy with a supply voltage of 1.8V, supporting low power operation. One temperature sensor device is located at the centre of the PCB, two temperature sensor devices are located at the PCB middle and edge, and the fourth sensor is located on the tip of a flexible finger, which is bent back in the neck area towards the centre of the rigid-flex PCB (Figure 1c).<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMwOTM0MDk3LTYwMDk5MS1maWctMDEuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 1. CBT device design. (a) Concept of a wearable thermal device applied to the forehead to estimate human CBT; (b) 3D exploded view of the CBT patch; (c) Human tissue side of the flexible CBT patch; (d) Side view of the flexible CBT patch.The CBT patch is intended to monitor human patients in pre-op, operative, and post-op surgery settings. Such environments have typical ambient temperature ranges of 20°C to 24°C and maximum air convections rates of 5W/m2 K. Normal forehead core body temperatures are in the range of 36°C to 38°C. Below 36°C is a condition called hypothermia and above 38°C is a condition called hyperthermia. Both conditions are considered serious and subsequently necessitate monitoring during all phases of surgery.<h1>Layout Design Considerations for Heat Flow</h1>The CBT patch device is designed to measure vertical heat flow from the human tissue surface using two&nbsp;<a href="https://www.analog.com/en/products/max30208.html" target="_blank" rel="nofollow">MAX30208</a> temperature sensing devices. This is shown in Figure 2. The MAX30208 temperature sensors are shown with TS labels. The other two temperature sensors (shown in Figure 1) help account for lateral heat losses. Data from the temperature sensors, combined with thermal models for the conductive plug and insulator, are used to accurately estimate CBT on human foreheads.To accomplish this task, the rigid-flex PCB with temperature sensing circuits needs to:<ul><li>Incorporate temperature sensors with high accuracy.</li><li>Have low enough power consumption (temperature sensor) to not adversely impact the thermal system of interest.</li><li>Have signal traces that are adequate in size for electrical signal transmission.</li><li>Have signal traces with dimensions that minimise heat flow from (or to) the MAX30208 temperature sensor such that the thermal system is not adversely impacted.</li><li>Have signal traces with dimensions that minimise heat radiation from the PCB traces (that is, I2R loss) into the conductive plug area.</li></ul><img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMwOTc0MzkxLTYwMDk5MS1maWctMDIuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 2. The primary temperature sensing path (not to scale).By implementing select thermally conductive/insulating materials, the physical structure establishes a design allowing for an accurate estimate of CBT on the forehead. Combining this mechanical design with accurate low power temperature sensors, such as the MAX30208 device, a successful design is produced. However, the electrical connections necessary for electronic devices conduct heat as well - a phenomenon not desired!Figure 3 highlights the main heat flow paths of interest. One would like to design the thermal resistance of the PCB traces to be very large compared to the conductive plug to keep these heat loss (or gain) errors negligible.<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMDA5ODgwLTYwMDk5MS1maWctMDMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 3. A simplified thermal schematic highlighting main heat flow paths.As heat and electricity are both transported via electron motion, they are intimately related to each other. According to the Wiedermann-Franz Law2, the ratio of thermal to electrical conductivity is approximately constant for different metals at the same temperature. In other words, what’s good for thermal resistance is bad for electrical conduction, and vice versa. Fortunately for this use case, with a fairly limited temperature range, the application of alternate metals from those commercially available was not required.While implementing a commercially available metal for signal and power traces, a thermal-electrical design trade-off is still needed for the rigid-flex interconnections. The design formula for electrical and thermal resistance is given in Figure 4. The thinner and longer the rigid-flex PCB trace is, the larger the thermal resistance. This provides a means to set the thermal resistance greater than the primary conductive plug, minimising thermal leakage (that is, error) from the CBT thermal system. Unfortunately, the electrical resistance increases as well. This can lead to several adverse effects such as power line voltage drop, increased temperature of the PCB trace, and an increased RC time constant for the I2C communication lines.<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMDQzNjQ3LTYwMDk5MS1maWctMDQuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 4. Electrical and thermal conduction of PCB traces.Before considering the PCB traces, the conductive plug thermal behaviour should be first evaluated to establish a design baseline. The heat path has a cylindrical form factor as shown in Figure 5.<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMDcwMDAzLTYwMDk5MS1maWctMDUuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 5. Thermal transfer in the conductive plug.The thermal resistance of the CBT patch conductive plug can be calculated knowing the material conductivity and dimensions as follows:<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMDk0OTQxLTYwMDk5MS1lcS0wMS5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" class="fr-fic fr-dii">Turning our attention to the thermal resistance of the PCB trace, we need to consider several issues:<ul><li>The PCB trace thermal resistance should be significantly greater than the CBT patch conductive plug (for example, RTH&nbsp;(PCB trace) ≥100• RTH&nbsp;(conductive plug)).</li><li>Power requirements of the temperature sensing device (for example, the MAX30208) need to be known and the PCB traces dimensions designed to minimise associated heat loss from the traces into the BCT patch conductive plug. This is greatly mitigated by employing a low power temperature sensor device such as the MAX30208.</li><li>The PCB traces that are in contact with the conductive core need to be checked for potential heat radiation. The smaller the traces for thermal consideration, the larger the I2R heat transfer error can occur.</li><li>The total length of the PCB traces for a given cross-sectional area should be adequate to ensure a large thermal resistance (compared to the CBT thermal conductive plug).</li></ul>Figure 6 displays the thermal/electrical characteristics of various utilised PCB metals. As the commonly implemented metals in PCB manufacturing (for example, gold, copper, silver, and aluminum) have thermal and electrical conductivities within the same order of magnitude, the specific material selection does not dominate our choice. Copper was chosen for cost, availability, and mechanical flexibility (to be discussed in the next section).<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMTEyMDM1LTYwMDk5MS1maWctMDYuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 6. Common PCB conductor properties.While copper’s thermal conductivity is over 1000 times larger than that of CBT patch conductive plug, the dimensions of the thin copper traces were chosen to achieve a much greater thermal resistance than 49.8K/W (that is, the CBT patch conductive plug thermal resistance).The PCB traces are composed of ½oz copper (17.3µm thick) core, 1.5µm nickel layer, and a 0.1µm gold outer layer. Given the relative dimensions of these, all forthcoming calculations assume only the copper core PCB traces.<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMTc1NzQ5LTYwMDk5MS1maWctMDcuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 7. The MAX30208 temperature sensor power and signal line PCB traces.Note: While a smaller trace width is desired for increased thermal resistance, PCB fab houses will have constraints on their minimum trace widths. For example, we initially wanted 2.5mil trace widths, but eventually went with the fab house recommendation of 3mil trace width for our prototype build.Further, each MAX30208 temperature device has a set of four (4) traces of equivalent dimensional design. See Figure 7. Thus, the overall thermal resistance is decreased by four times (that is, four thermal paths in parallel), yielding:<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMjE0NjA3LTYwMDk5MS1lcS0wMi5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" class="fr-fic fr-dii">Note: While the thermal resistance is adequate to isolate the heat conduction inside the conductive core, the heat generated in the interface board needs to be considered. If large enough, heat energy can be conducted back into the CBT patch causing error. This was not an issue for our evaluation system.<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMjYyMzU3LTYwMDk5MS1lcS0wMy5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" class="fr-fic fr-dii">Figure 8 highlights the approximate thermal resistance of each set of four temperature sensor I/O lines to the output connector, CN1.<img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMzE1MzMyLTYwMDk5MS1maWctMDguanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 8. PCB trace thermal resistance estimates.<h1><img data-fr-image-pasted="true" src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNzQ4NDMxMzM5OTU4LTYwMDk5MS1maWctMDkuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==" class="fr-fic fr-dii">Figure 9. The CBT patch to interface board connection.</h1>The set of four I/O signal lines with the lowest thermal resistance (for example, TS1-CN1) is about 380× larger than the CBT conductive plug. This meets our design target of being greater than or equal to 100× larger! Additionally, this is further improved by the bus wire extension from the connector CN1 to the&nbsp;<a href="https://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/max30208evsys.html" target="_blank" rel="nofollow">MAX30208EVSYS</a> interface board. Our prototype system used 200mm&nbsp; (7.9inches) long, 28AWG wire that is wrapped around the top of the ear down to the interface board.Note: While the thermal resistance is adequate to isolate the heat conduction inside the conductive core, the heat generated in the interface board needs to be considered. If large enough, heat energy can be conducted back into the CBT patch causing error. This was not an issue for our evaluation system.<h1>Thermal Error Mitigation of the Electrical System</h1> <a href="http://www.wevolver.com" target="_blank" rel="noopener noreferrer"><img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1728563792515-maxresdefault.jpeg" style="width: 100%px;" class="fr-fic fr-dib"></a>

Introduction

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Chapter 1

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Testing the report scheduling

Introduction

1. Chapter 1

2. Chapter 2

Testing the report scheduling

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Chapter 1

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Chapter 2

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