Inquiry about Capacitive Touch Through Metal Using MSP430™ MCUs With CapTIvate™ Technology

Hi, MJ, 

MJ Jeon said:

Do I really need a gap?

Yes. For metal touch, the gap is mandatory because there will be a deformation when finger touch the metal plate. The deformation will cause a capacitance change between the metal plate and the sensor pad since the metal plate is grounded. If the gap is replaced by a rigid material, there will be no deformation when finger applies a force on the metal plate. So for the mechanical design, the gap is needed, and the metal plate thickness should be not too big so that the finger force can cause a deformation of the metal plate. Since the deformation is very small (CapTIvate can sense very small capacitor change), the metal can recover to being flat due to its resilience. After the metal plate recovers, next finger press can be applied. 

MJ Jeon said:

Are there any comparison data between general capacitive touch solutions and metal touch solutions? (data such as recognition rate)

Since the metal touch operating theory is different from the normal capacitive touch, there is no comparison data for them. 

Normal capacitance touch is to sense the capacitance change caused by finger touched the sensor pad via some medium like glass, acrylic. 

The metal touch is to sense the capacitance change caused by the metal plate deformation. There are many factors to affect the user experience: metal plate thickness, metal plate Young's module, gap (spacer) thickness, spacer size, force applied on the button. 

There is a deformation evaluation tool in the metal touch app note: Capacitive Touch Through Metal Using MSP430™ MCUs With CapTIvate™ Technology (Rev. A). You can evaluate the metal touch feasibility based on your choose for metal material, metal plate thickness, gap thickness, etc. You can use this tool to evaluate your solution. 

Thanks, 

Lixin