• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 79 subscribers
  • Views 1076 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

MSP430FR2633: Using CapTIvate on bottom layer of multilayer board

Oleg Kobrin
Expert 1890 points

Part Number: MSP430FR2633
Other Parts Discussed in Thread: TM4C1294KCPDT,

Hello!

We are developing the lab device and now I'm looking for new keyboard design for it. The CapTIvate technology looks very useful here, as the device is small, and intended to be used in enviroments where some chemically active liquids can splash on keyboard, and also operator can have wet fingers, also in gloves.

The design of the device includes mainboard, which is attached directly to front panel, which is 1mm thick plastic. So I decided to print cap sensing buttons on the bottom side of the board, which is faced to front pannel. But, the board is rather small, and contains many components, including TM4C1294KCPDT MCU. 

So the questions:

As the board is 4-layer, with power planes inside, what is the impact to sensors, if it is printed on the bottom side of the board and ground plane below? What is the impact, if some components, including 25-MHz crystall resonator or MCU or MSP430FR2633 itself is located on the top side of the board right above the sensors? Any other guidelines?

Thank you!

  • 0
    TI__Guru 56685 points

    Hello Oleg,

    Placing components behind sensors is not an issue with smart board design. Its even easier with 4-layer boards as the components are further away. Now, the real killer of your sensitivity will be the ground plane right behind the sensors. If I understand your stack-up right, you have a solid ground plane on layer 3, and sensors on Layer 4. For max sensitivity, you would have no ground plane underneath the sensors (or around the sensors on same layer), and a little bit past the sensors as well. Sometimes this is not practical or some ground is needed for noise immunity purposes. In these cases it is suggested to place a low density hatched ground instead of a solid pour. Typically it is hatched in a directions that is 45 degrees off of your sensor traces to minimize coupling.

    Further information about grounding layout and CapTIvate sensors can be found in the Design Guide section of the CapTIvate Technology Guide ( )..

  • 0 in reply to Jace H
    Expert 1890 points
    Thank you! It's clear. I'll try to place poligon pour cutouts below sensors in both GND and 3.3v layers, and use hatched pours where it is not possible.

    BTW, link to guide is broken.
  • 0 in reply to Oleg Kobrin
    TI__Guru 56685 points
    Fixed the link. It was picking up the closing ")" for some reason.

**Attention** This is a public forum