I have a design with capacitive buttons on a FPC (Flexible Printed Circuit) that connects to a PCB with a CY8CMBR3108 controller. The Application Note at https://www.cypress.com/file/46366/download does not have a good example on how to construct the shield electrode on a FPC.
Section 4.4.4 best describes what I intend to design but the section does not show a shield electrode. In this section, #1 states that the hatch fill for the shield signal should only be in the top layer. However, #2 states to place a ground trace in parallel to the sensor trace in the FPC connector. My question is: do I need a ground fill in the FPC if I am using a shield signal? If so, where do I put the ground fill?
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I am not using a PCB. The buttons are on a FPC (flexible printed circuit) and the FPC connects to a PCB where the CY8CMBR3108 controller is. The shield is on the FPC where the buttons are. Please re-read my initial post and Section 4.4.4 of https://www.cypress.com/file/46366/download.
There are two reasons you may require a shield - one for liquid tolerance and the second for reducing the Cp of the sensors. In both cases, you need to provide the top layer with a hatched shield.
Now, if your design has a possibility of touch from the opposite side or if there are noisy signals behind the FPC, then it is recommended to used a hatched shield at the bottom layer as well to prevent any of these from being detected as a touch. In that case, the FPC also follows the same rules as a normal FR4, having a hatch on the top and the bottom layer.
Having a hatched ground on the opposite layer will increase the Cp of the sensors and of the shield and is not recommended (if your design has a small Cp then this might be alright).
Thank you for providing this information. You stated that it is recommended to use a hatched shield on the bottom layer as well. However, this information contradicts what is provided in Section 4.4.4. Section 4.4.4 states that the hatch fill should only be on the top later. Please see attached.
You need a hatch in the bottom layer only if your design has a possibility of touch from the opposite side or if there are noisy signals behind the FPC. If this is not a concern, then you need the hatch pattern on the top layer only. This is application-specific and dependent on the way the sensors are going to be placed.
If there are noisy signals or the possibility of touch from the other side - use hatch on both layers.
Else, if the FPC is well protected and there is no way for an end-user to touch the FPC on the opposite side, a hatch on the top layer is only required.