Bonding Films Provide Low Dk in Flex PCB Assemblies

If you look at flex assemblies, it seems like there is only a single material option that is used in these systems. The material that is most common in these boards is polyimide, which forms the base and the coverlay for each layer in a flex PCB assembly. For anyone that is new to flex, it might appear that the range of available materials for use in flex PCBs is rather limited, particularly in terms of the available dielectric properties.

Although the Dk value of polyimide films for use in flex assemblies, flexible films are available that can provide low Dk values for signal layers in a flex PCB stackup. These films can be implemented in the standard flex lamination process, but they provide even lower Dk values with lower thicknesses than typical commercially available polyimide films. Make sure to examine the mechanical and thermal properties of these materials before implementing them in a flex PCB layer stackup.

Low-Dk Circuit Board Bonding Sheets

The commercially available sets of low-Dk films for use in flex PCB assemblies are known as circuit board bonding sheets, or PCB bonding sheets. These sheets are available from companies like Panasonic, DuPont, and NAMICS, and they are marketed as a general low-Dk solution for use in flex or rigid PCBs. Circuit board bonding sheets tend to have a common set of material properties:

Compared to the typical Dk value quoted for polyimide films (Dk ~3.5), these materials offer a Dk value that is even lower while providing loss tangents comparable to PTFE. Commercially available bonding films have Dk values reaching as low as 2.5. The thickness varies to allow for a particular target in a flex stackup, which gives a simple method for controlling loss around an interconnect in a flex stackup.

Coverlay vs. Bonding Film in High-Speed Flex

In terms of function, these two materials perform the same task: they bond a copper-clad flex substrate to its coverlay or to the next layer in a multilayer flex stackup. In this way, they modify the Dk value around a trace in a flex PCB.

Impedance-controlled traces in a flex PCB are essentially embedded microstrips (in a 2-layer stack) or striplines (with 3 or more layers). The adhesive covers one half of the trace, while the base flex material (polyimide or alternative) covers the other half of the trace. When a low-Dk bonding film is used, it reduces the effective Dk value and the total dielectric loss seen by a signal propagating on the trace.

A low-Dk bonding film can be used in place of an adhesive layer in a flex PCB stackup.

The main benefit here is in terms of manufacturability and loss. The material allows a designer to use thinner layers for a given target trace width. This means a design can have higher layer count or thinner overall stack without requiring more precise fabrication capabilities. Alternatively, for a given layer thickness, a controlled-impedance trace will need to be wider, which will cause it to have lower conductor losses. Both are beneficial for high-speed and high-density designs.

Other Flex Materials

The standard set of conformal coatings and solder mask materials do have their flex analogues and can be used in flex assemblies.

High-Dk Embedded Capacitance Materials

Some designers only focus on the low Dk side of flex materials, especially if they always try to link low Dk materials with low loss in high-speed PCBs. However, low-Dk materials are not the cure-all for every signal integrity problem, and they create challenges with forming high-density controlled-impedance circuits.

There are high-Dk materials that can be used in multilayer flex assemblies as an embedded prepreg between ground and power layers. These are embedded capacitance films, which can provide a very high Dk value that provides stable power. These films act like large capacitors that provide high planar capacitance out to MHz frequencies, and they can allow for removal of some decoupling capacitors without affecting SI and PI.

• Learn more about embedded capacitance films

Smoother Copper

Copper films used in PCBs have some level of roughness due to their deposition and rolling processes. The standard copper foil type used in flex materials is rolled-annealed copper. This type of copper is smoother than the simple electrodeposited copper foils used in low-cost copper-clad laminates or rigid PCBAs. An alternative low-profile copper foil could also be used in a flex assembly to further reduce conductor losses at high frequencies.

Consider an alternative copper film if you need lower loss in your flex PCB.

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