How do I select the right prepreg for bonding layers in a multilayer RF PCB stackup?

The prepreg (pre-impregnated fiberglass cloth with uncured resin) serves as the bonding layer between cores in a multilayer RF PCB. Selecting the right prepreg is critical for impedance control and signal integrity: (1) Prepreg parameters: Dk: must match or be compatible with the core material Dk. If the prepreg Dk differs from the core Dk: the impedance of traces referenced to a prepreg layer will differ from those referenced to a core layer. This creates impedance discontinuities at layer transitions. Df: should be as low as possible (contributes to dielectric loss in stripline structures where the signal is between two prepreg layers). Resin flow: the amount of resin that flows during lamination. High flow: fills gaps around traces (good for embedding), but may cause resin starvation in other areas (variable thickness). Low flow: less filling capability, but more consistent thickness. Thickness after pressing: determines the dielectric spacing (which determines impedance). The finished thickness depends on the starting prepreg thickness, the number of plies, and the resin flow into the circuit pattern. (2) Matching prepreg to core: for Rogers-based stackups: Rogers provides specific prepregs designed for each core material. RO4450B prepreg: designed for bonding RO4350B cores. Dk = 3.54 (matched to RO4350B Dk = 3.48). Df = 0.004 (matched). RO4450F: faster flow version for filling thick copper patterns. DO NOT use standard FR4 prepreg (Dk ≈ 4.2, Df ≈ 0.02) to bond Rogers cores (the Dk mismatch creates impedance discontinuities and the high Df increases stripline loss). For FR4-based RF stackups: use prepregs from the same material family as the core (e.g., Isola 370HR prepreg with 370HR cores). (3) Multi-ply prepreg: for thicker bonding layers, stack multiple prepreg sheets. The finished thickness of N plies is not simply N × single-ply thickness (the resin flows and compresses during lamination). The fab house has empirical data for the pressed thickness of each prepreg combination. Always consult the fab house for the expected pressed thickness when designing the stackup. (4) Mixed-material bonding: when bonding Rogers RF layers to FR4 digital layers: use a prepreg compatible with both materials (Rogers RO4450B works with both, or use a specialty bonding film like Arlon CuClad). The CTE mismatch between materials must be managed (different z-axis expansion can cause warping).