Microwave Tech - Analysis of High-Frequency Multilayer PCB Prepreg

In high-frequency multilayer PCB applications, the use of different Prepregs has different effects on the electrical properties of the material, and the material formulation used to bond high-frequency multilayer films may also vary widely. Many Prepregs are glass fiber reinforced, and there are several commonly used Prepregs that are not woven glass fiber reinforced. Unreinforced Prepreg is usually a thermoplastic polymer film, while the woven glass fiber reinforced Prepreg is usually thermoset and often uses special fillers to improve high-frequency performance.

During lamination, thermoplastic Prepreg needs to reach a melting temperature to achieve bonding between multiple circuit layers. These materials can also be re-melted after multiple layers are bonded, however, re-melting can lead to delamination, which is why it is usually desirable to avoid re-melting. Lamination melting temperatures and remelting temperatures to be aware of vary with the type of thermoplastic Prepreg, with remelting temperatures generally of concern after lamination, processes such as soldering that expose circuits to high temperatures.

Rogers has introduced thermoplastic unreinforced Prepregs commonly used in multilayer high-frequency PCBs such as Rogers 3001 (425°F melt, 350°F remelt), CuClad 6700 (425°F melt, 350°F remelt), and DuPont Teflon FEP (565°F melt, 520°F remelt) adhesive film. Since delamination is considered, the remelting temperature is generally lower than the initial melting temperature, where the material is soft enough to delaminate. At the initial melting temperature during lamination, the material is at its lowest viscosity which allows the material to wet and flow between layers during lamination for good adhesion. As can be seen from the temperature of the different materials, Rogers 3001 and CuClad 6700Prepreg are suitable for multi-layers that are not exposed to high temperatures such as soldering. DuPont Teflon FEP material can be used for multiple layers that will be welded, assuming the welding temperature is controlled below the remelting temperature. However, some manufacturers do not have the capability to reach the initial melting temperature.

There is one exception in thermoplastic unreinforced Prepreg, however, and that is Rogers 2929 Bond Sheet, which is unreinforced, but it is not a thermoplastic, but a thermoset. Thermosets do not have melting and remelting temperatures, but they do have solidification temperatures (during lamination) and decomposition temperatures that should be avoided due to delamination considerations. 2929 Bond Sheet has a lamination temperature of 475°F and decomposition temperatures well beyond lead-free soldering temperatures, so it is stable after multilayer bonding for most high-temperature conditions.

The electrical properties of these Prepregs are as follows: Rogers 3001 (Dk=2.3, Df=0.003), CuClad 6700 (Dk=2.3, Df=0.003), DuPont Teflon FEP (Dk=2.1, Df=0.001) and 2929 (Dk=2.9, Df=0.003).

Another type of Prepreg is a glass fiber reinforced Prepreg, which is usually a combination of woven fiberglass cloth, resin, and some filler. Laminated PCB manufacturing parameters can vary greatly depending on the Prepreg composition. In general, highly filled prepregs typically have much less lateral flow during lamination and may be a good choice if the prepreg will be used to build multiple layers with cavities; The prepreg to which the inner layer is to be bonded has thicker copper, and it may be difficult to laminate well with this low flow prepreg.

There are two types of glass fiber reinforced prepregs usually used in high-frequency PCB manufacturing, namely RO4450B and RO4450F prepregs (Dk=3.5, Df=0.004). The processing parameters of these materials are similar to FR-4, however, they have very good electrical properties at high frequencies. These materials are highly loaded, have low lateral flow when laminated, and are high Tg thermosets that are very stable for lead-free soldering or other advanced processes.

All in all, when designing a multilayer PCB Prepreg for high-frequency applications, there are various tradeoffs that must be considered along with the electrical performance in terms of manufacturing.