Sensing Technologies

[DaFa_1662356]

Hello All,

I am trying to settle on a 6-layer design for a PCB that has a 5LP and several CapSense objects. I used App Note

AN85951 as my guide. My design must be liquid tolerant as well. In the app note they say:

For two layers, layer 2 should be the driven shield and not a GND mesh, can that be right?

For the four layer they say layers 1 and 2 should both have the driven shield, is that correct?

I build a demo board, 2 layers, and layer 2 was the GND mesh, seems to work well.

For my production board I need 6-layers and I am unsure how to proceed. Here is one idea:

• L1 - Sensors and shield mesh
• L2 - GND mesh, but it must be offset "far" away from L1. If not the sensitivity of the buttons will be compromised. Also, we need the GND above the traces so we can run traces anywhere we want on lower layers. If we put GND on L5 for example. Then use L2 for sensor traces only, how would you ever get the traces from the sensors down to the uP without going under other sensors? There is too much in the way blocking, right? When the GND is in between we can run traces how we like, so I'm told. This is why GND needs to be L2 I believe.
• L3 - Traces
• L4 - Traces
• L5 - Power, voided under sensors
• L6 - Components

Any comments will be appreciated!

[VenkataD_41]

Hi,

For CapSense based applications we recommend you to go with PSOC 4000S/4100S/4100S plus/PSoC 6 based devices. These devices have high SNR for CapSense sensors. You can connect your PSoC 5LP based device to the above devices through I2C/SPI/UART interfaces.

The application note that you are looking is for PSoC 4 and PSoC 6 devices.

>>'For two layers, layer 2 should be the driven shield and not a GND mesh, can that be right?"

--> If you need liquid tolerance for your board you need to use shiled hatch else you can go with ground hatch. Having ground hatch has better noise immunity than shield hatch.

>>"For the four layer they say layers 1 and 2 should both have the driven shield, is that correct?"

--> Same as above. Shield is only required when you need liquid tolerance. Else ground should be used.

So, if you need liquid tolerance keep shield hatch in the sensor layer around the sensors and in the layer below the sensors.

Since you have 6 layers, we recommend you to keep one layer full of GROUND for Electro Magnetic Compatibility. This can be either 3rd or  4th layer.

Thanks and regards

Ganesh

[DaFa_1662356]

Hi Ganesh,

Thank you for the response. Just to be clear:

So for liquid tolerance on a two layer board you are recommending:

L1 - sensors and shield

L2 - Shield??

Same question for the 4-layer version:

L1 AND L2 should both be shield??

If I put GND on layer 3, can I rout traces anywhere on layer 4? By this I mean can traces from a button

run under a different button? I know this should not be done ABOVE the GND layer. I am asking

about routing rules BELOW the GND layer.

[VenkataD_41 · Accepted solution]

Hi,

Yes. If you need liquid tolerance, the Layer 1 and Layer 2 should be shield. (for both 2-layer and 4 layered boards).

>>"If I put GND on layer 3, can I rout traces anywhere on layer 4? By this I mean can traces from a button

run under a different button? I know this should not be done ABOVE the GND layer. I am asking

about routing rules BELOW the GND layer."

--> Sensors in Layer1.

--> Traces routed for different sensors in Layer 2. Traces can go underneath the sensors in layer 2also (in case if routing becomes complex). The trace width should be less than 7 mil.

--> Sensor traces should not go in parallel with any switching traces.

Thanks

Ganesh