Feature: Medical
Whilst copper’s thermal conductivity is over 1000 times larger
than that of the CBT patch conductive plug, the dimensions of the thin copper traces were chosen to achieve a much greater thermal resistance than 49.8K/W (the conductive plug’s thermal resistance). Te PCB traces are composed of a ½ oz copper (17.3µm thick) core, a 1.5µm nickel layer and a 0.1µm gold outer layer. Each PCB trace was fabricated with 76.2µm widths, yielding:
Figure 5. Thermal transfer in the conductive plug Furthermore, each MAX30208 device has a set of four traces of
equal dimensions (Figure 7), thus the overall thermal resistance is decreased by four times (that is, four thermal paths in parallel), yielding:
Figure 8 shows the approximate thermal resistance between
each set of four temperature sensor I/O lines and the output connector, CN1. Te set of four I/O signal lines with the lowest thermal
resistance (in this case TS1-CN1) is about 380× larger than the CBT patch conductive plug, which meets our design target. Tis is further improved by the bus wire extension from the connector CN1 to the MAX30208EVSYS interface board. Our prototype system uses 200mm-long 28 AWG wire that is
Figure 6: Common PCB conductor properties
wrapped around the top of the ear down to the interface board. Note that, whilst the thermal resistance is adequate to isolate the heat conduction inside the conductive core, the heat generated in the interface board must also be considered. If large enough, this heat can be conducted back into the CBT conductive patch, causing error. Tis was not an issue for our evaluation system.
Figure 7: MAX30208 power and signal line PCB traces
to be assessed for potential heat radiation. Te smaller the traces for thermal consideration, the larger the I2
R heat transfer error.
• Te 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). Figure 6 shows the thermal/electrical characteristics of
various PCB metals. Commonly-implemented metals in PCB manufacturing like gold, copper, silver and aluminum have the same order of magnitude thermal and electrical conductivities, so we chose copper because of its low cost, high availability and mechanical flexibility.
22 March 2024
www.electronicsworld.co.uk
Figure 8: PCB trace thermal resistance estimates
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