Multilayer printed-circuit boards (PCBs) have gained in popularity as designers seek to shrink their circuits. They offer the opportunity to stack a wide range of functions on one multilayer design, even combine analog, digital, and microwave circuits on a single circuit assembly. Designers have learned how to stack laminates with different relative dielectric constants, such as PTFE-based laminates and FR-4, in compact multilayer circuits, but creating such circuits requires careful planning and a good understanding of the role that bonding films and prepregs play in multilayer circuits.

One of the most popular PCB prepreg materials is also the dielectric material that forms one of the most commonly used circuit laminates—FR-4. One of the simplest ways to think of a prepreg is as a “glue layer” between two or more circuit layers. A prepreg such as FR-4, which is essentially woven glass and epoxy resin, is cured at a temperature below that of the circuit laminates it bonds together, early in the cure process, flowing around the etched copper traces of the laminates and bonding the multilayer-circuit’s dielectric layers together.

It is not unusual to find a multilayer circuit with a low-loss laminate such as PTFE or RO4000® circuit materials from Rogers Corporation forming the microwave circuit layer and with several FR-4 layers for baseband and digital circuitry. This type of mixed-dielectric multilayer construction is often called a hybrid multilayer circuit. The laminates are chosen for the electrical requirements of the different circuit functions, and the prepregs provide the adhesion between circuit layers. The layers are stacked and clamped under pressure at elevated temperature, typically about 100 to 400 psi at the bonding temperature. The temperature in this process is carefully controlled, with materials suppliers each offering recommended guidelines for their prepregs for temperature ramp and dwell times to achieve good flow around the circuit traces and good adhesion with the dielectric layers. 

Multiple layers of RO4003C™ or RO4350B™ circuit laminates are typically bonded by RO4400™ prepregs to form integrated multilayer microwave circuit assemblies. These materials are formulated to have dielectric constants as close in value as possible, since the effective dielectric constant of a multilayer circuit construction is the average value of the different dielectric constants. In a multilayer circuit construction, laminates are typically referred to as “core” laminates to distinguish them from prepreg layers. With FR-4-based laminates and prepregs, it is generally not possible to exactly match the dielectric constants of the materials. To facilitate flow during the bonding process, FR-4-based prepregs are formulated with less glass reinforcement than FR-4 laminates, and dielectric constant is a function of material content.

But for some cases of PTFE-based laminates and prepregs, and with specially developed materials such as the RO4000 laminates and RO4400 prepregs, it can be possible to achieve a close match in dielectric constant between the laminate and the prepreg. In some cases, it is even possible to fabricate multilayer circuits using the same material, such as PTFE, as the laminate and the prepreg, although these cases require somewhat unique processing conditions.

For example, Rogers RO4003C laminate has a process relative dielectric constant of 3.38 in the z direction at 10 GHz, with dissipation factor (dielectric loss) of 0.0027 at 10 GHz. RO4350B laminate has a process dielectric constant of 3.48 in the z direction at 10 GHz, with dissipation factor of 0.0037 at 10 GHz. RO4450B and RO4450F prepregs, matched to these two laminates are available with different sheet thicknesses, with RO4450B prepreg available in 3.6- and 4-mil-thick sheets and RO4450F prepreg available in 4-mil-thick sheets. For 3.6-mil-thick RO4450B prepreg, the dielectric constant is 3.30 in the z direction at 10 GHz. For 4-mil-thick sheets of the same prepreg, the dielectric constant is 3.54. RO4450F prepreg exhibits a dielectric constant of 3.52 in the z direction at 10 GHz for a 4-mil-thick sheet. Both prepregs have a dissipation factor of 0.004 in the z direction at 10 GHz, regardless of thickness. Both are engineered with glass transition temperature (T_g) of greater than +280°C for stable expansion characteristics over the wide range of circuit-board processing temperatures.

In the case of RO4350B laminate with dielectric constant of 3.48, the thinner RO4450B prepreg has a lower value of dielectric constant while the thicker RO4450B prepreg is slightly higher in dielectric constant. As a rough approximation, the composite dielectric constant can be considered as an average of the two values. More accurately, the dielectric layers are akin to parallel-plate stacked capacitances, and the dielectric-constant layers can be viewed much like capacitance in series. The composite dielectric-constant value of the stacked layers can be approximated by applying a summation formula, with the parallel-plate dielectric-constant values normalized to each layer thickness. Even so, variations can result due to the type of circuit, the thickness, and other factors.

At microwave frequencies, these variations can result in changes in expected impedance for transmission lines. For that reason, Rogers provides a second value of relative dielectric constant for design purposes, such as when using a computer software simulation program. The “design Dk” values for RO4350B and RO4003C laminates are both higher than the process values, at 3.66 and 3.55, respectively, in the z direction at 10 GHz, intended to provide more accurate results when calculating impedance values for dimensions of microstrip and stripline transmission lines.

Taking the average of two or more dielectric constants in a multilayer circuit assembly is really an approximation, since variables such as the copper surface roughness can affect the value of dielectric constant particularly when z-direction dielectric spacing between copper features and planes is thin as is often the case for features that are separated by prepreg bonding layers. A prepreg can also bring the effects of its own dielectric constant to a design, depending upon the circuit layout. Circuits with coupled features or differential-pair transmission lines, for example, will be influenced by the presence of the prepreg material (and its dielectric constant) between them. In such cases, a prepreg layer will exhibit influences in the x-y plane of the circuit and not just in the z direction. 

As detailed in this blog from last August, a laminate’s copper surfaces are often treated to improve adhesion with dielectric materials. Such treatment increases the surface roughness of the copper to enhance adhesion with other materials, but it also increases conductor losses in transmission lines formed on that copper and can impact the effective dielectric constant of dielectric layers, such as prepregs in multilayer circuits. A laminate’s copper thickness can also impact the thickness of a prepreg layer following processing, since the prepreg will flow around the circuit’s traces but also conform to the features of the copper. When attempting to model the combination of prepregs and laminates in terms of average dielectric constant when working with a commercial computer simulation program, suppliers of prepregs can usually provide guidance on recommended values for dielectric constant for a given number of prepreg layers, type of laminate, copper roughness, and other factors.

Finally, for those planning on attending IPC APEX EXPO® Conference & Exhibition (www.ipcapexexpo.org), February 28 through March 1, 2012 at the San Diego Convention Center (San Diego, CA), stop by Booth # 2101 to say hello to representatives from  Rogers Advanced Circuit Materials (ACM) Division. And don’t miss an outstanding technical conference, with two separate technical presentations by, Rogers’ Market Development Engineer John Coonrod: “Thermal Characteristics of PCB Laminates Used in High Frequency Applications,” from 9 to 10 AM on March 1, and “New Developments in PCB Laminates,” from 10:15-11:45 AM, also on March 1.

 Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.