For time response of PCB material the only data taken is for the EVKit PCB. The time response of this PCB is around 15sec. What happens when the PCB is placed next to the skin is that the outer layers of skin (epidermis and dermis) and PCB are at thermal equilibrium in about 30 seconds. During this exchange of heat from the skin to the PCB the skin drops about 1.5 oC. For the PCB to increase its temperature to the temperature of the skin before contact the system is reliant on heat flowing from the blood into the skin and then into the PCB. This is a much slower heat transfer process with a time constant of around 15 to 30 minutes depending on location on the body and difference in the person.
It is important for the customer to understand this limitation to be successful in making an accurate and timely temperature sensor.
The key design considerations are to maximize the contact area with the skin and minimize the thermal heat capacity of the sensor. (i.e. make the PCB as small as possible or use Flex circuit). The other important consideration is steady state heat loss to the environment which can keep the sensor from ever reaching the initial skin temperature. The thermal resistance to the environment must be much greater than the thermal resistance of the heat flow from the blood through the skin and contact material to the sensor, or the 0.1 oC accuracy will not be realized. Thermal resistance to the environment is best kept high by minimizing copper trace thickness connecting to the MAX30205, and not having ground planes on the PCB. Most heat flow in a PCB is through the copper and all the heat flow in a flex circuit is in the copper.