4 Replies Latest reply on Apr 6, 2017 12:03 PM by m_shihab_2337616

    PCB layer stack up and routing for lots of capacitive touch buttons and surface mount LEDs

    m_shihab_2337616

      Hi,

         

      I am working on a design which has about 30 capacitive buttons. Each button needs an LED for backlight. 

         

      I know that recommended approach in this scenario is to use back-firing LEDs but there are many other components in the design that need to be placed on the bottom side of PCB. 

         

      So I was wondering, Can I place surface mount LEDs (e.g., 0603 size) at the center of sensor electrodes?

         

       

         

      My other question is: What layer stack up should I choose?

         

      There will be sensor electrodes and LEDs only on the top most layer. The second layer can be used for sensor traces. Plus I need at least two layers for digital signals. From what I have read so far, I need a hatched ground plane between sensor signal layers and digital signal layers.

         

      so should I select a six-layer stack up? and what signal-plane configuration?

         

       

         

      Regards

        • 1. Re: PCB layer stack up and routing for lots of capacitive touch buttons and surface mount LEDs
          rjks

          Hello,

             

           

             

          The capsense buttons traces needs to be complete circle or rectangular shape. By placing LED on the top layer is not possible for this reason.

             

          What i would suggest it to have a 4 Layer board, the stack up is suggested in the getting started with capsense document.(link below- Page 82)

             

          This will give enough room since you will have CapSense traces in  separate layer.

             

          http://www.cypress.com/file/41076/download

             

           

             

          4-Layer PCB:

             
                
          •  Place the sensors on the top layer of the PCB.
          •     
          •  Route the sensor traces in the layer-2.
          •     
          •  Place a hatch fill of 7-mil trace and 70-mil spacing and connect it to ground in layer-3.
          •     
          •  Place components in the bottom layer, The unoccupied areas can be filled with a hatch copper fill of 7-mil trace and 70-mil spacing and should be connected to ground.
          •    
             

           

             

          Thanks,

             

          Ranjith

          • 2. Re: PCB layer stack up and routing for lots of capacitive touch buttons and surface mount LEDs
            m_shihab_2337616

            Ranjith,

            Thank you very much for your response.
            I have read that document before. On page 71 of that doc, under "Recommended Button Shapes", it shows a button layout with LED hole at the center. Now I understand that this hole is supposed to be in the PCB for back-firing LEDs. But I have a lot of components to place at the bottom layer and if I place back-firing LEDs at the bottom, there won't be enough space for other components.
            So I was thinking of making hole only in the copper (not through the PCB) and place surface mount LED at the center. Is that not possible? I also understand that I shouldn't be placing any conductive objects or digital signals (e.g. LED drive signals) within 4mm of CapSense traces, but is it possible that I try to handle this problem in firmware. For example, I disable sensor scan while I am sending PWM signal to LEDs and once LEDs are stable, I re-enable sensor scan?

            PCB stackup:
            The stackup you mentioned is great but it leaves me only one layer (the bottom layer) for components and signal traces. It seems really difficult to manage that many traces using only one signal layer. I think I need at least two signal layers.
            Is there any possible stackup that I can use to get one extra signal layer?

            Thanks again for taking the time to respond. I really need more guidance.

            • 3. Re: PCB layer stack up and routing for lots of capacitive touch buttons and surface mount LEDs
              rjks

              Hello,

                 

               

                 

              We do not recommend the LEDs on the top later in between the button. But you can make a prototype and test the setup at your end.

                 

              But what i would suggest is to have the below stackup so that you have another layer for other traces, this should free up space in the bottom layer for the LEDs.

                 

               

                 

              Top layer-sensors

                 

              Layer2-Capsense traces

                 

              Layer3-GND

                 

              Layer4-PWR

                 

              Layer5-Other traces

                 

              Bottom layer- The device

                 

               

                 

              Thanks,

                 

              Ranjith

              • 4. Re: PCB layer stack up and routing for lots of capacitive touch buttons and surface mount LEDs
                m_shihab_2337616

                Thank you Ranjith for your valuable suggestions.

                   

                Is there some recommendation for minimum separation between sensor pads layer and GND layer?

                   

                Regards