FR-4 Hybrid PCB Stack-up Optimizes Signal Integrity and the PCB Cost

Hybrid PCB Stack-up Optimizes Signal Integrity and the PCB Cost

Today we're going to talk about hybrid PCB. We mainly refer to high frequency material mixed with epoxy glass.

Let’s see the stack-up to get idea about hybrid structure.

Layer 1 to layer 2 is the core of RO4350B high frequency material and layer 3 to layer 4 is the core of FR-4 epoxy glass. The 2 cores are bonded by a sheet of adhesive (prepreg).

Types of hybrid PCB

The hybrid PCB can be a mixture of FR-4 and high frequency material as abovementioned, another type is a mixture of high frequency material with different dielectric constant (DK), for example RT/duroid 5880 and RO4350B etc.

Why do we use hybrid PCB?

There're 3 main reasons that convince us to use this type of high frequency multilayer PCB, i.e. cost, reliability improved and electrical properties enhanced.

The cost of high frequency material is much higher than FR-4, sometimes, when using hybrid of FR-4 and high frequency material can solve the cost problem.

It causes reliability problems when the CTE value of the PCB material used is relatively high, while some high frequency materials have high CTE characteristics. When a low CTE FR-4 material is combined with a high CTE material to make a multilayer board, the composite CTE is acceptable, thus enhancing reliability.

Most of the time, some lines of the mixed PCB board require very high electrical performance, some do not require high, in this case, the parts with low electrical performance requirements will be used FR4, while the parts with high electrical performance requirements will use more expensive high frequency materials. Some using different DK of material to make mixed PCB, it is also to improve electrical properties, such as in some applications of combiners and filters, using different values of DK material.

                                                                                                           Micro-section of a hybrid PCB

Why choose FR-4?

Mixture of FR-4 and high frequency materials is becoming more and more common because FR-4 and the vast majority of high frequency materials have little compatibility problems.

Lamination

To get a reliable lamination, we first choose PP sheet of high frequency material and use the correct pressing cycle. The adhesive layer of the same material is more favorable for the simpler pressing cycle.