Feature: Medical


in the conductive core area. Referring to Figure 8, the distance from TS1 to the outer edge of the conductive core is 7.5mm. Using the formula for electrical resistance (Figure 4) of a single PCB trace and the resistivity of copper, we calculate it with:


Using the maximum current of 383µApk


for the SCL and SDA


signal lines, we calculate the heat radiation value for a single PCB trace as follows:


Figure 12: Static bends in the TS4 flex finger Tis is negligible with respect to the thermal system of interest.


Further, this is an absolute worse-case level if periodic sampling is not implemented. In conclusion, the thermal errors originating from both the MAX30208 self-heating and PCB trace heat radiation in the conductive core have insignificant effects. Te line voltage drops are within acceptable levels, too. Te


longest wire length is 88mm (TS4 to CN1) plus 200mm of 28 AWG (0.32mm diameter) wire, which connects to the MAX3020x interface board. Using the same formula for electrical resistance for the longest line, we calculate the following:


Figure 13: Staggered brick-pattern flex finger stiffeners


Using the maximum current of 67µA for VDD line voltage drop as follows:


, we calculate the


Tis is small enough that no issues with respect to power supply


rejection will arise. While we have highlighted major thermal and electrical design


considerations for the rigid-flex PCB used in the CBT patch, finite element analysis (FEA) is highly recommended. An FEA (thermal) was conducted for transient analysis validation before prototyping the first patches. However, thermal and electrical capacitances have not been discussed here. While neither had any significant effect on performance in this application, it is recommended that they be analysed during the design phase. Figure 11 shows a schematic diagram of the CBT device that


Figure 14: CBT patch rigid-flex PCB delamination


highlights the PCB trace branches used to mitigate heat flow, as well as to implement electrical interconnections in the 2-layer polyimide rigid-flex board.


Given that this is over 100 times smaller than the accuracy of


the MAX30208 (for example, ± 0.1°C), the assumption that the case and die are at same temperature is acceptable. However, this may not always be the case, which will require characterisation. One useful technique is to use an ESD diode as a temperature sensor to characterise the temp rise of the IC die, commonly implemented on IC I/O lines. Ten we need to check the I2


24 March 2024 www.electronicsworld.co.uk


Layout design considerations Whilst this circuit is designed with mechanical characteristics for attachment to the human forehead, there are several critical locations that need rigidity: • Te nine SMT component attachment sites; • Te circuit finger extending from the circular circuit area to the temperature sensor (TS4);


R heat radiation of the PCB traces


• Te circuit finger extending from the circular circuit area to the connector (CN1);


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48