Multilayer microwave PCBs can pack much circuitry into a small volume. But creating a multilayer microwave circuit is more than just stacking layers. It takes planning and knowledge of the multilayer fabrication process. In particular, an understanding of how prepreg materials are used in multilayer circuits can be helpful.
High-frequency multilayer circuits can be constructed with more than one type of transmission line. Stripline is often used, but microstrip and coplanar-waveguide technologies also work fine in a multilayer construction. In single-layer stripline formed with metal-clad dielectric substrates, two metal outer layers serve as ground planes while a single inner metal layer forms the conductor, with dielectric material separating the metal layers. In a multilayer PCB, the layers are counted by the number of conductive layers. Stripline, with its two ground planes and single conductor layer, is technically a three-conductor PCB.
The multiple circuit layers in stripline and higher-layer-count PCBs are held together by a bonding material known as a prepreg. Prepregs act as an adhesive, but they also are substrates with dielectric material properties. Prepregs are typical thermoset materials, uncured dielectric materials that must be cured through high-temperature cycles. At higher temperatures, a prepreg will cure and flow, allowing bonding of multiple circuit layers under pressure. To form a stripline circuit, for example, a dielectric substrate clad on both sides with copper is selectively etched on one side to form the circuitry, with the copper on the other serving as one of the ground planes. Prepreg material is then added on top of the etched circuitry, and a copper layer is added on top of that and the construction is cured at the proper temperature and under pressure to form the stripline circuit board. A stripline circuit can also be formed with less prepreg, by instead using a second dielectric circuit layer that has only been copper clad on one side, with a thinner prepreg layer used to bond its bare side on top of the circuit layer.
In a stripline circuit, connections are needed from the enclosed circuit layer to the ground layers and the outside world, and these are made by means of plated through holes (PTHs)—holes that are drilled through the cladded substrates and coated with copper to form conductive paths. They must be properly aligned to make connections where necessary.
Achieving uniform performance even in a simple multilayer circuit like a stripline construction calls for matching a prepreg to a desired laminate, since the properties of the two materials are typically different. If a laminate and prepreg differ in terms of their coefficients of thermal expansion (CTEs), they will expand and contract in different ways over temperature, posing potential reliability problems. In addition, a prepreg tends to suffer higher electrical loss than a laminate, so the loss of the total circuit “package” must be considered when selecting both a laminate and a prepreg for a multilayer circuit design.
A choice of prepreg material will be dictated by the requirements of an application as well as the choice of laminate materials. In a multilayer construction that might include a mix of transmission-line technologies, such as a stripline inner circuit layer surrounded by two microstrip outer circuit layers, higher-performance laminates might be selected for those outer layers compared to the PCB materials used for the stripline circuits. The prepreg bonding films contribute their own mechanical and electrical properties to the circuit assembly. A variety of different prepreg materials are available, for example, with or without woven-glass reinforcement for support, depending on how much rigidity is needed in the final circuitry structure. It is possible to compare these materials in terms of such factors as ease of fabrication, solderability, and reliability.
The type of circuit can also impact how little or how much a prepreg affects the performance of a multilayer circuit. In an edge-coupled stripline circuit, for example, formed with the addition of prepreg and a top copper-layer ground plane rather than a second laminate, if the spacing between stripline conductors is wide and the conductive traces are tall enough, it will be possible for prepreg material to flow between the conductors. In this way, it acts as a form of dielectric “container” around the conductive traces, with impact, for example, on signal phase at higher frequencies.
Examples of commercial prepregs include 2929 bondply material and RO4450™ prepreg, both from Rogers Corp. (www.rogerscorp.com). The 2929 bondply material is ideal for forming multilayer circuits based on PTFE and other circuit materials. It features a proprietary crosslinking resin system and sacrifices little in terms of performance at higher frequencies, with low relative dielectric constant of 2.9 and low loss tangent of 0.003 at 10 GHz. The RO4450B prepregs are based on glass-reinforced hydrocarbon/ceramic materials. They are available in sheets of several thicknesses, with dielectric constant increasing with thickness. For example, 0.91-mm-thick RO4450B prepreg has a dielectric constant of 3.30 at 10 GHz while 0.101-mm-thick RO4450B has a dielectric constant of 3.54 at the same frequency.
Those attending the IEEE International Microwave Symposium (IMS) in Montreal, Quebec, Canada can learn more about prepreg materials and multilayer circuit boards by visiting Rogers’ staff at booth #2902. And for those who really want to explore prepregs, don’t miss John Coonrod’s Microwave Application Seminars (MicroApps) presentation at 11:05 A.M. Tuesday, June 19. John, Rogers’ Market Development Engineer and author of the ROG Blog series, will discuss “Bonding Materials Used in Multilayer Microwave PCB Applications.” His colleague, Rogers’ Associate Research Fellow, Allen Horn III, will also conduct a MicroApps session, for those concerned with thermal management in higher-power circuits. His talk, “Reducing Active Device Temperature Rise and RF Heating Effects with High Thermal Conductivity, Low-Loss Circuit Materials,” is scheduled for 3:05 P.M. Wednesday, June 20.
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.