MSP430FR2633: mutual capacitance button design for different overlay thickness

Hi Lenio,

Great question.  The overlay thickness primarily drives the spacing between the RX and the TX.  From an E-field point of view, the farther away the RX is from the TX, the higher the E-field between the two will penetrate the dielectric material (i.e. the overlay) and the air above it.  It is this field that the user interacts with above the dielectric.  Thus, we want to get more field out and above the overlay where the user can disturb it.  If the RX and TX are very close together and the overlay is very thick, the field will concentrate inside the dielectric and the user won't be able to generate as strong of a change in that field.

So, for a thicker overlay dielectric you would want to add a little bit of separation between the RX and TX.  This will help you with sensitivity going through the thicker overlay.  For a very thin overlay, you can bring the RX and TX a little bit closer together without giving up much in terms of sensitivity, and this might help you with having less interference from anything underneath the electrode on a different layer, ect.  Also, a common request with mutual buttons such as this is placement of an LED backlight in the center of the button.  This also requires that the RX be a little bit closer to the TX to allow space for the LED in the center.

The dimensions provided in the drawing you reference above are general guidelines of what has been found to work well for a variety of implementations; it is not a hard rule.  That geometry would work for a 3mm overlay- it will just be a little less sensitive to touch than a button that has a little bit more spacing between the TX and RX.  We have designs out there that are <6mm in overall height, as opposed to 10 or 12, and these can work well with a 3mm overlay.

Ultimately, if you can optimize the layout and add another 0.1mm to 0.3mm or so of spacing between the RX and TX, you will be rewarded with a bit better sensitivity with that 3mm overlay, which will help you reduce your measurement time.  Reduced measurement time means you can improve response time, power consumption, or noise immunity.

Regards,
Walter

Hi Lenio,

The electrode area is dependent on the size of the object interacting with the sensor, typically a human finger, and the desired sensitivity which will not change based on overlay thickness. The general rule of thumb is that the spacing between the RX and TX electrodes is 1/2 the overlay thickness, therefore a 3 mm thickness results in a 1.5 mm spacing. You can review the CAPTIVATE-PHONE design for a mutual capacitance button example.

Regards,
Ryan