Detail PCB Stackup File for Manufacturing
After the planning stage, it’s time to define and document layer stack up requirements for your printed circuit board clearly and thoroughly. Take this chance to talk with your PCB manufacture to determine materials for your PCB board and what specific requirements you need to consider in your later design .
As far as materials go, you’ll likely be working with one of these:
FR-4. This is a great material when you’re working with clock speeds of < 5Gbps and is classified as a low speed material. FR-4 has a decent ability to control impedance and is also known for its low cost.
Nelco, SI, or Megtron. In the realm of high speed design, you’ll likely be working with these materials. Each is suited for 5-25 Gbps clock speeds.
Rogers. If your first high speed design is pushing 56Gbps, then you’ll likely end up using a Rogers laminate. This is a high frequency, high-temperature material known for good impedance consistency, but it’s also expensive to produce.
Once you know the material you’ll need to have your board manufactured; now it’s time to keep some other layer stackup strategies in mind.
First, you’ll always need to have a signal layer placed adjacent to a plane layer to provide your signals with an efficient return path.
It’s also worth considering routing all of your high speed signals on the inner layers of your board between planes, which will provide shielding against any external emitted radiation.
Lastly, also consider using multiple ground planes in your layer stack up. This will help to lower your reference impedance and reduce common-mode radiation from affecting your circuit.