Reduce impedance for power traces of a decoupling Cap when ground planes are not directly under

Question

I am using an SHTC3 temp and humid sensor to send readings using I2C lines to the MCU.

Based on the datasheet for SHTC3 to make sure the sensor is reading ambient temp without the interference of the heat coming from traces and planes that connect to the MCU and other heat producing ICs, it is recommended to limit the trace lines to the MCU and not to have copper filling under the SHTC3 chip. As well as adding an enclosure shield to prevent heat radiation.

When adding a decoupling cap on the power lines to the chip, there are the traces that need to connect to the planes and that requires a trace spacing as seen below. Best way is to increase the trace width to the cap to lower impedance to the cap since there is no copper filling under or around the cap. Does this seem accurate?

Here is the schematics:

I would appreciate any feedback you all could provide. Thank you so much!!

Answer 1

Your design seems accurate to me.

I still have some suggestions for improvement:

• You could reduce the decoupling loop even more by moving the cap tighter to the SHTC3 sensor.
• The traces from the planes to the cap could be smaller to prevent heat transfer and they should be as close as possible to each other to prevent inductive coupling.
• Please double check the ground pad of the SHTC3 (It could be required to have it connected to GND.)

Answer 2

It's always a good idea to keep impedance of power traces in mind and design accordingly. On the other hand, for sensors like this it's essentially trivial. As long as the decoupling capacitor is reasonably close, the length and style of power traces doesn't matter much. I'm very confident that you can operate this device even on a several meter long power cable without any negative effect. The main reasons for this are that the power consumption is marginal (<1 mA) and the digital chip internals are few and running at a low speed. The most noise on the supply lines will be caused by the I2C interface, but even this is specified to run at less than 1 MHz and has really slow edges on the data lines (>20 ns by definition).

Apart from this, following @stefan-wyss suggestions is a good idea to keep coupling of noise and heat as low as possible.