Is a completely copper-free inner layer acceptable in an AISLER 4-layer 1.6mm PCB?

ℹ️ Q & A Help Zone PCB Q&A
1

I am designing a 4-layer capacitive trackpad based on Azoteq’s AZD068 recommendations.

The intended stack-up is:

  • F.Cu: sensing electrodes
  • In1.Cu: intentionally no copper beneath the sensing area
  • In2.Cu: ground plane
  • B.Cu: components and routing

The absence of copper on In1.Cu is intentional, because copper directly behind the sensing electrodes would increase capacitive loading.

Could you please confirm the following?

  1. Is it acceptable for In1.Cu to be completely empty, or at least empty over a large part of the PCB?
  2. Will plated through-vias still have a continuous barrel through the empty In1.Cu layer?
  3. Will AISLER preserve the empty layer exactly as submitted? Is it worth adding at least a route along the layer contour?
  4. Are there any concerns regarding warpage, lamination quality, or manufacturability due to the strongly unbalanced copper distribution?

The board is intentionally designed this way for capacitive-sensing performance, so adding a copper fill on In1.Cu would not be desirable.

2

Hi @Ilia ,

Thank you for reaching out!

  1. In principle, large copper-free areas on inner layers can lead to quality concerns, as these regions may present an increased risk of resin starvation during the lamination process. In unfavourable cases, this can result in local lamination issues such as reduced interlayer adhesion. However, based on our experience, this phenomenon is very rare and typically only occurs in very large PCBs or in particularly challenging stackups. For normally sized PCBs, we would not expect any issues.
  2. Yes. Plated through-holes will still have a continuous plated barrel, even if there is no copper on In1.Cu around the via. Just make sure that there are via pads on both outer copper layers.
  3. Yes, we will manufacture the inner layer as provided. Adding a route along the contour is not necessary. It’s actually not desired, as you should maintain a copper clearance of at least 300 µm to the contour.
  4. See 1.

Kind regards,
Manuel