Rigid-flex circuit design considerations for wearable temperature sensor applications
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Rigid-flex circuit design considerations for wearable temperature sensor applications

Posted Date: 2024-02-04

This article aims to help designers identify and respond to multiple potential issues when designing high-precision (±0.1°C) temperature sensing circuits. It uses a recent CBT design as an example to illustrate the thermal, electrical and mechanical aspects, and provides Appropriate trade-offs have been made between these aspects. These considerations will help designers:

・Learn how to identify the design challenges, trade-offs, and countermeasures associated with developing high-precision CBT detection devices
· Learn how to design reliable rigid-flex printed circuit boards for remote patient monitoring applications.
・Apply design guidelines to heat flow and mechanical structures.
・ Rigid-flexible PCB is being manufactured

CBT Device Design Overview

As a flexible wearable thermal detection device, the CBT patch can accurately estimate human CBT (Figure 1a). Figure 1b shows the main components of the thermal detection device, which consists of four temperature sensors (MAX30208). The sensors are separated by materials with different thermal conductivities to accurately quantify CBT. These temperature sensors have an accuracy of 0.1°C and a supply voltage of 1.8V, enabling low-power operation. Among them, one temperature sensor is located in the center of the PCB, two temperature sensors are located in the middle and edge of the PCB, and the fourth sensor is located at the tip of the flexible contact piece, and the edge of the contact piece is folded toward the center of the PCB. (Fig. 1c).


Figure 1. CBT device design. (a) A wearable thermal detection device is placed on 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 used to monitor patient body temperature in pre-operative, intra-operative and post-operative settings. Typical ambient temperatures for this type of environment range from 20°C to 24°C, with a maximum air thermal conductivity of 5 W/m2/K. The normal range for forehead core body temperature is 36°C to 38°C. Anything below 36°C is called hypothermia, and anything above 38°C is called hyperthermia. Both conditions are serious and require monitoring of core body temperature during all stages of surgery.

Layout Design Considerations Regarding Heat Flow

CBT patch products are designed to measure heat flow perpendicular to the surface of human tissue using two MAX30208 temperature sensors. As shown in Figure 2, TS is the MAX30208 temperature sensor. Two other temperature sensors shown in Figure 1 help calculate lateral heat losses. Combining data from temperature sensors with thermal models of conductive plugs and insulating housings allows for accurate estimation of CBT on the human forehead.

To achieve this goal, a rigid-flex PCB with temperature detection circuit needs:

・Integrated high-precision temperature sensor.
・The power consumption of the temperature sensor should be low enough not to adversely affect the associated thermal system.
・Have PCB traces thick enough for signal transmission.
・Signal traces should be sized to sufficiently reduce heat flow from (or to) the MAX30208 temperature sensor to avoid adverse effects on the thermal system.
・Signal traces should be sized to minimize heat radiation (i.e. I2R loss) from the PCB trace to the conductive plug area.


Figure 2. Main temperature detection paths (not drawn to scale).

By using appropriate thermally conductive/insulating materials and designing their physical structure, it is possible to accurately estimate forehead CBT. Successful product designs can be achieved when combined with a high-accuracy, low-power temperature sensor such as the MAX30208. However, electrical connections such as PCB traces on electronic devices can also conduct heat – something we don’t want!

Figure 3 shows the associated heat flow paths. We want to design the PCB traces to have a much greater thermal resistance than the conductive plugs to ensure that the error caused by these additional heat losses (or gains) is negligible.


Figure 3. Simplified thermal schematic showing the main heat flow paths.

Since both heat and electricity are transported through the movement of electrons, they are closely related. According to the Widmann-Franz law, the ratio of thermal conductivity to electrical conductivity of different metals at the same temperature is approximately a constant. In other words, the greater the thermal resistance, the worse the conductivity and vice versa. Fortunately, in this use case, since the temperature range is quite limited, common commercially available metals will suffice.

Although commercially available metals are used for signal and power traces, there are still thermoelectric design trade-offs when combining rigid-flex PCB interconnects. The formulas for electrical resistance and thermal resistance are shown in Figure 4. The thinner and longer the traces of rigid-flex PCB, the greater the thermal resistance. Therefore, the traces can be made thinner and longer so that their thermal resistance is greater than the conductive plugs to fully reduce the heat leakage (i.e., error) of the CBT system. Unfortunately, the resistance of the traces also increases accordingly. This will bring some adverse effects, such as power supply trace voltage drop, PCB trace temperature increase, and RC time constant increase of I2C communication lines.


Figure 4. Electrical and thermal conductivity of PCB traces.

Before considering the thermal resistance of PCB traces, we should first evaluate the thermal behavior of the conductive plug to establish the design baseline. The heat conduction path of the conductive plug is cylindrical, as shown in Figure 5.


Figure 5. Heat conduction of conductive plug

According to the conductivity and size of its material, the thermal resistance of the CBT patch conductive plug can be calculated as follows:

When considering the thermal resistance of PCB traces, we need to consider several issues:

・ PCB trace size needs to be designed according to the power requirements of the temperature sensor (such as MAX30208) to minimize heat loss from the trace to the CBT patch conductive plug. Using low-power temperature sensors such as the MAX30208 can greatly reduce this heat loss.

・ PCB traces in contact with the conductive core also need to be checked for potential heat radiation. The smaller the trace, the greater the I2R loss.

・For a given cross-sectional area, the total length of the PCB trace should be sufficient to ensure a large thermal resistance compared to CBT thermal plugs.

Figure 6 shows the thermal/electrical properties of various commonly used PCB metals. Since the thermal and electrical conductivities of these metals (such as gold, copper, silver, and aluminum) are in the same order of magnitude, the specific material chosen is less important. Copper was chosen here for reasons of low cost, easy availability, and mechanical flexibility (discussed in the next section).


Figure 6: Conductivity of common PCB metals.

Although the thermal conductivity of copper is more than 1000 times greater than that of the CBT patch conductive plug, choosing a thinner copper trace size can achieve a much greater thermal conductivity than 49.8 K/W (that is, the thermal resistance of the CBT patch conductive plug). block.

PCB traces are composed of a 1/2-ounce (17.3-micron-thick) copper core, a 1.5-micron-thick nickel layer, and a 0.1-micron-thick gold plating layer. Considering that the relative sizes of the nickel layer and the gold plating layer are small and negligible, in all subsequent calculations, it is assumed that the PCB trace consists only of copper cores.


Figure 7. MAX30208 temperature sensor PCB power and signal traces.

The width of each PCB trace is 76.2 microns (3 mils), so:

Note: Although we would like to use smaller trace widths to increase thermal resistance, PCB manufacturers have restrictions on minimum trace widths. For example, we originally wanted a trace width of 2.5 mils, but ended up going with the manufacturer's recommended trace width of 3 mils.

In addition, since each MAX30208 temperature device requires four equal-sized PCB traces (Figure 7), that is, four thermal paths in parallel, the thermal resistance of the overall PCB trace is also reduced by a factor of four, that is:


Figure 8 shows the approximate thermal resistance of the PCB traces from the four temperature sensors to connector CN1.


Figure 8. Estimated PCB trace thermal resistance.


Figure 9. Connection of CBT patch and interface board.

As shown in Figure 8, the PCB trace with the lowest thermal resistance (such as TS1-CN1) is about 380 times greater than the thermal resistance of the CBT conductive plug, which meets the design goal of greater than or equal to 100 times. In addition, an extension cable from connector CN1 to the MAX30208EVSYS interface board further improves this performance. Our prototype system uses 200 mm (7.9 inches) of 28 AWG wire, wound from the CBT patch through the top of the auricle to the interface board.

Note: Although this thermal resistance is sufficient to isolate heat conduction inside the conductive core, we still need to consider the heat generated by the interface board. If this heat is large enough, it will be conducted back to the CBT patch causing errors. The temperature sensor used in our evaluation system consumes very little power, so this is not a problem.

Reduce thermal errors in electrical systems

When it comes to the electrical system, we will focus on two main aspects: (1) the heat generated by the MAX30208 device itself (such as self-heating), and (2) the heat generated by the PCB traces (such as thermal radiation). Both heat sources will input (or output) heat to the CBT patch, thereby adversely affecting the thermal performance of the system. Figure 10 shows a schematic diagram of the MAX30208 circuit design.


Figure 10. MAX30208 functional diagram.

The MAX30208 (±0.1°C accuracy, I2C) digital temperature sensor was chosen because of its high accuracy and low power consumption. The CBT chip electrical system is powered by a 1.8 V regulated DC power supply on the MCU interface board. The I2C pull-up resistor is an important source of heat and is located on the MCU board, not on the CBT patch rigid-flex PCB.

Most of the power consumption comes from the I2C signal line and power supply, and the power consumption in continuous operation is about 810 μW. Since the temperature signal does not change very quickly, periodic sampling can be used, which not only helps with data management but also reduces overall power consumption, which in turn helps reduce heat dissipation in the MAX30208 device itself and in the signal and power traces.

When the integration period is 15 ms and the sampling rate is 1 Hz, the average power consumption of the MAX30208 is approximately

Although package thermal resistance is often provided in data sheets, designers must be cautious when using package thermal resistance to estimate heat flow. This is because both θjA (junction-to-ambient thermal resistance) and θjC (junction-to-case thermal resistance) are evaluated based on the JEDEC environment, which may differ significantly from actual applications. They are often used to measure a chip's quality when comparing competing devices.

Therefore, we do not recommend using ambient temperature to infer the junction temperature, especially for this application where the temperature sensor is mounted between insulating and non-insulating materials.

Since the temperature measurement circuit of MAX30208 relies on integrated circuits, the first thing we need to pay attention to is the self-heating of the chip. The chip is used to measure the external temperature at the top (or bottom) of the package, so assuming the case temperature is the same as the chip temperature, we can estimate the temperature error due to self-heating of the chip as follows:

This error is over 100 times lower than the MAX30208's accuracy (eg, ±0.1°C), so we can accept the assumption made above that the case and die temperatures are the same.

Note: This assumption cannot always be made when an accurate measurement of chip temperature is required. One available technique is to use ESD diodes on the IC input/output lines as temperature sensors to measure the temperature rise of the IC chip.

Next, we consider the I2R losses of the PCB traces in the conductive core area. As shown in Figure 8, the distance from TS1 or TS4 to the outer edge of the conductive core is 7.5 mm. Using the resistance formula for a single PCB trace (see Figure 4) and the conductivity of copper, we can calculate the following:

This is negligible for the thermal system of this example. For the case where periodic sampling is implemented, the error will be smaller than this. In short, the thermal error caused by the MAX30208's self-heating and thermal radiation from the conductive core PCB trace has little impact on the system.

At the same time, the line voltage drop is also within the acceptable range. The maximum length of wire is 88mm (TS4 to CN1), plus 200mm of 28 AWG wire (0.32mm diameter) to the MAX3020x interface board. Using the calculation formula for resistance, the following results can be calculated:

This voltage drop is small enough that power supply suppression issues do not occur.

The above are the main thermal dissipation and electrical design considerations for rigid-flex PCB used in CBT patches, but we still strongly recommend that before prototyping the first patch, a thermal finite element analysis (FEA) is performed to analyze the transient status to verify. Thermal and capacitance are not discussed in this article because they have little impact on performance in this application. However, we recommend that thermal and capacitance be analyzed during the design phase as well.

Figure 11 shows the electrical schematic of the CBT device, focusing on how to achieve electrical interconnections and traces to slow down heat flow in a two-layer polyimide rigid-flex PCB board.


Figure 11. CBT chip electrical schematic diagram.

Layout design considerations to ensure mechanical structure reliability

Rigid-flex circuits use a hybrid structure of traditional rigid PCB and flexible PCB. While the circuit is mechanically flexible to conform to the human forehead, it requires mechanical rigidity in several key locations. they are, respectively:

・ Nine connection points for SMT components.
・Circuit contacts extending from the circular circuit area to the temperature sensor (TS4).
・The circuit contacts extend from the circular circuit area to the connector (CN1).
・Boundary of rigid-flexible circuits.

Rigid-Flex Circuit Design Considerations for Wearable Temperature Sensor Applications SMT components are often connected using reflow soldering. Therefore, these components are often mounted on rigid PCB materials to maintain the integrity of the solder joints. Since flexible PCB materials require fewer stress relief pieces, SMT components must be soldered with care. Even if a system is subject to relatively few physical disturbances, it requires careful assembly to ensure long-term reliability.

Typical PCB reinforcements use FR4, polyamide, polyimide and/or metal. Our CBT patches use 4 mil-thick polyimide in the flexible area and 12 mil-thick polyimide in the reinforcement area. To increase stiffness, we reinforced the flexible contact circuit with metal sheets.

The CBT patch prototype will be made into a flat rigid-flexible component and then statically bent twice. As shown in Figure 10, the circuit contacts extending from the circular circuit area to the TS4 temperature sensor require two 90-degree bends during final assembly.


Figure 12. Static bending of TS4 flexible circuit contacts.

The TS4 flexible circuit contact design uses a brick-shaped metal sheet to reduce metal fatigue caused by one-time static bending. Figure 13 shows these staggered brick-pattern reinforcements that relieve mechanical stress at the rigid-flexible boundary. Additionally, the intermittent brick pattern eliminates heat conduction along these metal paths. This design technology is also used for the circuit contacts extending from the circular circuit area to the connector (CN1).


Figure 13. Staggered brick pattern of flexible contact reinforcement.

Other aspects to consider include avoiding 90-degree corners (which create stress concentration points, for example) and the installation of prefabricated components.

Manufacturing Considerations and Guidelines

In order to design stable and reliable products, we recommend that designers work closely with PCB assembly plants. Before fabricating the first device, all electrical, thermal, and mechanical design details should be reviewed. In many cases, manufacturers have alternative materials and/or technologies that can be used to improve designs.

In the process of developing a rigid-flexible PCB assembly process for CBT patches, several major difficulties caused by the use of reflow soldering materials and reflow soldering curves must be overcome. We initially used standard reflow solder, which resulted in delamination of the PCB (see Figure 14). As an insulator, air pockets can affect the flow of heat through rigid-flex PCBs, which is particularly detrimental to thermal design. We eventually mitigated this problem by using an alternative low-temperature eutectic solder. To achieve acceptable yields, the reflow profile must be fine-tuned multiple times.


Figure 14. Rigid-flexible combined PCB layering of CBT patches.

in conclusion

This article discusses design considerations to help address the technical challenge of high-precision heat flow applications, namely how to use a high-precision, low-power device, such as the MAX30208 temperature sensor, to meet the performance requirements of a core body temperature patch. As long as the appropriate components are selected and good design techniques are applied to properly balance thermal, electrical, and mechanical performance, a successful design can be achieved.


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