What is back-drilling and when is it necessary for RF via performance at high frequencies?

Back-drilling is the process of removing the unused portion (stub) of a plated through-hole (PTH) via that extends beyond the signal layer, eliminating the stub resonance that degrades RF performance at high frequencies: (1) The via stub problem: a standard PTH via extends from the top to the bottom of the PCB (through all layers). If the signal transitions from Layer 1 to Layer 3 on a 10-layer board: the via provides the connection from L1 to L3 (the functional portion). The via continues from L3 to L10 (the unused stub). The stub acts as an open-ended transmission line that resonates at: f_resonance = c / (4 × L_stub × √Dk_eff). Where L_stub is the length of the unused via portion. For a 60 mil (1.5 mm) stub in Dk = 3.5 substrate: f_resonance = (3 × 10^8) / (4 × 0.0015 × √3.5) = (3 × 10^8) / (0.01122) = 26.7 GHz. At the resonant frequency: the via stub creates a notch (deep null) in the signal transmission. The S21 drops by 10-30 dB at the resonant frequency. (2) Back-drilling process: a controlled-depth drill removes the stub from the back side of the board. The drill diameter is larger than the via drill (to ensure complete copper removal): via drill = 10 mil, back-drill = 14-16 mil. The drilling depth is controlled to stop just below the target signal layer (leaving a stub residual of 5-10 mil). The residual stub is short enough to push the resonance well above the operating frequency. (3) When back-drilling is necessary: rule of thumb: back-drill when the stub length exceeds λ/20 at the maximum operating frequency. At 10 GHz: λ ≈ 16 mm in Dk = 3.5. Stub > 0.8 mm (32 mil): back-drilling may be needed. At 28 GHz: λ ≈ 5.7 mm. Stub > 0.28 mm (11 mil): back-drilling needed for most multilayer designs. At 77 GHz: stub > 0.1 mm (4 mil): nearly impossible to avoid without back-drilling or blind vias. (4) Alternative: blind vias: laser-drilled vias that connect only the necessary layers (no stub at all). More expensive than PTH + back-drilling. Required for HDI and some mmWave designs. Not practical for thicker boards (laser drilling limited to ≈ 8 mil depth per layer).