CapSense button optimization for space constrained application

[MaRa_4092696]

Please advise on the best button and slider design for this application, using the CY8CMBR3106S

I have two separate sets of sensors: one to detect a "swipe up" or "swiped down" action (function A), and a different set to act as 3 independent buttons (function B). Each set must fit within a dedicated 17.3 x 5.3mm PCB section that is not shared with any other circuitry. The CY8C device resides in a different section of the PCB but very close. The PCB is a 6-layer rigid-flex stackup, the segments holding the cap sensors can be either 2-layer flex or the full 6-layer stackup.

Function A:

Only the swiping action is needed, exact centroid is not necessary. Current plan is a 3-segment slider with segment width=4.8mm, air gap=0.5mm, segment height=4.85mm, no top ground hatch. Average trace length to CY8C is 10mm

Function B:

Independent button presses. Current plan is round buttons with 4.85mm diameter and approximately 1.38mm spacing between the buttons, no top ground hatch. Average trace length to CY8C is 72mm

In both cases space is available on either the bottom flex or an internal rigid-flex layer for ground hatching.

Given that the available space prevents sensor shapes that meet the recommended minimums, what is the best option for optimizing the space I do have?

2-layer flex vs 6-layer rigid-flex?

add top ground hatching at the expense of smaller sensor size?

smaller buttons with increased spacing or larger buttons with minimal spacing?

rounded-square buttons to achieve maximum sensor area or circular buttons?

for the swipe sensor, is the chevron shape preferred even if I don't care about centroid?

Thank you for your advice

[Vison_Zhang · Accepted solution]

1. 2-layer flex vs 6-layer rigid-flex?  2 layer flex is fine.

2. add top ground hatching at the expense of smaller sensor size?  Since no enough space to place ground hatch on top layer, ground hatching can be cancelled i think, but you must consider ESD problem of sensor.  I recommend you not only series a 560 ohm R on sensor line, but also plus a 10pf cap between sensor pin and ground(cap must be placed near to sensor pin). In that way, we will get a RC low-pass filter on each sensor line. 

3. smaller buttons with increased spacing or larger buttons with minimal spacing?  I recommend you increase spacing for independent button, 1.38mm is too less. I recommend you use rounded-square buttons, spacing between the buttons increase to 2mm.

4. rounded-square buttons to achieve maximum sensor area or circular buttons? rounded-square buttons

5. for the swipe sensor, is the chevron shape preferred even if I don't care about centroid?  Chevron shape is prefer.

Note：The bottom layer of the FPC can be laid with a very sparse ground grid (like 10% filling gnd hatch) or just blank. Do not lay solid GND floor or dense ground hatch, it decrease sensitivity of the sensor.

[MaRa_4092696]

Thank you