While PCB design has relied largely on the tried-and-true FR-4 substrate, many designs require more robust temperature or frequency performance to ensure their circuit performs optimally. When selecting the right PCB laminate, AdvancedPCB (APCB) can help. Table 1 shows an exhaustive list of laminates. Though overwhelming at first glance, each substrate offers distinct advantages for their application of choice. Before selecting a substrate, it’s important to understand its impact on your design. This tutorial is meant to help designers choose a laminate and explain why an alternative to FR-4 could be advantageous.
Table 1: List of laminates offered by AdvancedPCB.
FR-4
For more than 50 years, the FR-4 laminate has been the material of choice of PCB fabrication. It emerged as the preferred option over early G10 and FR-5 counterparts because of FR-4’s thermal performance, dimensional stability, and competitive price. However, FR-4 might not be ideal for circuits requiring better thermal or high-frequency performance, or both.
RF
High-frequency circuit boards, such as satellite-, base station-, cellular-, or radar-based transmitters and receivers, exhibit different performances at varying frequency bands — for example, dielectric constant (Dk), dissipation factor (Df), and Dk stability over frequency and temperature. With typical Df values at ~.020 at 10 GHz and Dk values from 3.8 to 4.8, using a standard FR4 board would yield much higher losses as the PCB frequency rises. This would be an unacceptable degradation in performance, calling for custom dielectrics that dissipate less RF energy at higher frequencies. Instead of basic copper traces, high-frequency circuits rely on carefully laid-out transmission lines; the integrity of the cross-sectional dimensions of these transmission lines is critical in maintaining a constant impedance (often 50 Ω). These circuits include a stripline, microstrip, or grounded coplanar waveguide (GCPW); flex PCBs have variations on these with a meshed ground plane to encourage substrate flexibility. Figure 1 shows the layout and impedance equation of a microstrip. Any variation in impedance across transmission lines would result in unwanted signal reflections and loss (or insertion loss/attenuation). Figure 1: Microstrip transmission line and equation.
These dimensions should be as stable as possible to maintain a constant impedance or minimize changes in electrical length over time and temperature. To accomplish that, incorporate materials with a low coefficient of thermal expansion (CTE) or ones that closely match the copper conductors used within the PCB. This way, when the ambient temperature of the application rises, the dielectric expansion would not adversely impact the design.
As Table 2 shows, Isola, Rogers, and Taconic offer PCB laminates tailored to RF and microwave applications.
Table 2: APCB list of laminates for RF applications.
These laminates often require different types of b-stage/prepreg, or bonding sheets, for multilayer designs. A commonly used laminate is the RO4000 series. It includes the RO4350B, a laminate with a ceramic base that can be manufactured using the standard FR4-type multilayer processes, making it simpler to manufacture. Though the substrate is nearly as simple to work with as the FR4, it exhibits much less loss (Df <.00037), as evidenced by the image in Figure 2. (Figure 2 is taken from an article titled “Circuit Materials and High-Frequency Losses of PCBs,” featured in PCB007 magazine by Rogers’ Technical Marketing Manager, John Coonrod.)
As the table shows, polytetrafluoroethylene (PTFE), or Teflon, is a popular choice for RF design. The dielectric offers very low loss characteristics (Df ~0.001 at 10 GHz), making it preferable for high-frequency transmission lines. There are different formulations and laminates, but they are difficult in multilayer configurations (compared with the RO4000 series) because they tend to require high-temperature bonding films or adhesives.
Figure 2: The insertion loss over frequency of a microstrip using different dielectric materials and a common copper conductor. Source: Rogers Corp.
High-Speed Digital
High-speed digital (HSD) materials also require a low Dk and Df laminate and prepreg. However, it is generally not as critical to have an extremely low Df and as stable a Dk over temperature when compared with higher-frequency RF and millimeter-wave circuits that might leverage PTFE. For this reason, many high-speed digital designs also use the RO4000 series with a hydrocarbon/ceramic base. Advanced FR-4 laminates are also a good option, including Panasonic’s Megtron 6, Isola FR408HR, and Nelco N4000-13ep, because these materials are formulated to have a lower Df and more stable Dk than those of standard FR-4 materials.
As Table 3 shows, a modified epoxy is typically used where the glass transition temperature (Tg) is a factor. This is the region where the material transitions from a rigid glass to a soft, rubbery material; a higher Tg has a rigid and stable structure at elevated temperatures. In HSD design, minute differences between traces cause timing delays between differential pairs. Therefore, strong dimensional stability can be critical.
Table 3: APCB list of laminates for HSD applications.
While epoxy is the most used resin system for PCBs, HSD electronics use modified epoxies, specially formulated to maintain a stable Dk and minimal Df and to optimize solder heat resistance and long-term thermal aging. This way, customers can benefit from the inherent manufacturability of the epoxy resin and the material’s high strength, rigidity, and chemical resistance. Conductive anodic filament (CAF) failure can occur with epoxy resins, creating conductive pathways across the insulating PCB substrate due to the influence of an applied electric field, making it electrically conductive. This often calls for the use of CAF-resistant materials.
Polyimide
Polyimide materials are leveraged for designs that require higher thermal performance, regardless of whether the circuit might run hot with onboard power devices or is operating in environments with a high ambient temperature. These materials have an extremely high temperature resistance, with Tg standing around 260°C with a decomposition temperature (Td) over 400°C. The maximum operating temperature (MOT) can range from 140°C to 210°C, much higher than FR-4 with an MOT of ~130°C. The material has been optimized for CTE control, with a low z-axis CTE and, therefore, optimal dimensional stability.
Flex board polyimide: Pyralux AP
Polyimide films are the go-to choice for flex and rigid-flex board topologies, which, as shown in Figure 3, will include:
Covercoat or coverlay
Copper-clad material used as the base material
Bond ply and adhesive (or prepreg)
Rigid laminates
Both the coverlay and bond ply material use a polyimide film that buffers the board while also flexing with the board itself, while the adhesive layer secures itself to the copper traces. The thermal stability of the polyimide substrate encourages both stability and trace protection through the entire PCB stackup, regardless of flexure.
Figure 3: Cross-sectional diagram of a type 3 flex board.
Conclusion
To better customize a PCB laminate for an application, you can adjust several factors. For example, a higher Tg ensures that the material has more CTE control over high temperatures and less deformation under extremely high temperatures. Other materials can be formulated to lower Df and offer a stable Dk to ensure the substrate offers a reliable impedance over frequency. Depending on the application, consult with the third-party manufacturer to understand the limitations of the materials used in cost, time to fabricate, or performance.
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