How do I select between a bonded and an unbonded multilayer PCB construction for RF?
Multilayer PCB Construction Methods for RF Circuits
Modern RF systems increasingly require multilayer PCB stackups to integrate digital control, DC bias, and RF signal layers in a compact package. The method used to join these layers significantly impacts RF performance, particularly at millimeter-wave frequencies where bond ply properties become electrically significant.
Frequently Asked Questions
What prepreg material should I use with Rogers RO4350B cores?
Rogers RO4450F is the recommended bonding prepreg for RO4350B stackups. It has compatible CTE, a Dk of 3.52, and Df of 0.004, closely matching the RO4350B core properties. This combination is widely supported by RF PCB fabricators.
How many RF signal layers can a multilayer RF PCB support?
Most practical RF multilayer PCBs use 4-8 layers, with 2-3 dedicated RF signal layers and the remaining layers for ground planes, DC bias, and digital signals. Above 8 layers, registration accuracy and via drill quality become challenging for RF-critical designs, particularly at mmWave frequencies.
Does the bonding process affect substrate dielectric properties?
The lamination cycle's heat and pressure can slightly alter dielectric properties of some substrates, typically by less than 1-2%. More significant is the dimensional change (shrinkage) during lamination, which can affect transmission line impedance. Fabricators typically compensate for known shrinkage factors by adjusting artwork dimensions.