Rogers material is the material of choice for high-frequency applications due to its excellent dielectric properties, but its high cost limits its widespread use; FR4 material offers outstanding value for money, yet struggles to meet the demands of high-frequency signal transmission. By scientifically combining these two materials, Rogers hybrid laminates achieve the dual objectives of ‘meeting high-frequency performance standards whilst keeping costs reasonable and controllable’, and meticulous attention to detail during the design phase is key to determining the reliability and cost-effectiveness of these hybrid laminates.
Why Combine Rogers and FR4
The core value of Rogers hybrid laminates lies in resolving the conflict between ‘high-frequency performance’ and ‘cost control’, allowing the strengths of the two materials to complement one another and meet the diverse requirements of mid-to-high-end PCBs. This is also the fundamental reason for their widespread application in fields such as 5G base stations and autonomous driving.
Rogers materials (such as the RO4000 series and RO4350B) possess extremely low dielectric loss and a stable dielectric constant, achieving a stability of dk ±0.05 in the 10 GHz band. They effectively reduce attenuation and interference during high-frequency signal transmission, ensuring signal integrity—an advantage that FR4 materials struggle to match. This makes them particularly suitable for critical signal layers in high-frequency applications such as millimetre-wave radar and satellite communications.
FR4 materials, on the other hand, have become the standard choice in the PCB industry due to their mature manufacturing processes, low cost and good mechanical properties. Their cost is only one-fifth to one-tenth that of Rogers materials, making them suitable for power planes, ground planes or auxiliary signal layers that do not carry high-frequency signals. By laminating the two materials together, there is no need to use an all-Rogers substrate.
Instead, Rogers material is used only for the critical high-frequency signal layers, whilst the remaining layers utilise FR4 material. This approach reduces overall costs by 20%–30% whilst maintaining core performance, whilst also balancing the PCB’s mechanical strength and manufacturability, and avoiding the issues associated with all-Rogers substrates, such as high processing difficulty and susceptibility to brittle fracture.
Furthermore, the hybrid lamination of Rogers and FR4 can meet the functional requirements of complex PCBs. Many high-end electronic devices incorporate both high-frequency signal transmission modules and conventional power supply and signal control modules. Hybrid lamination design enables a single PCB to achieve integrated ‘high-frequency transmission + conventional control’ functionality, reducing product size, improving assembly efficiency, and mitigating the risk of signal interference caused by splicing multiple PCBs together.
This approach of ‘precise matching and material usage on demand’ not only compensates for the shortcomings of FR4 material in high-frequency performance but also resolves the issue of excessively high costs associated with all-Rogers substrates, making it one of the preferred solutions for high-end PCB design.
Thermal mismatch is the most critical technical challenge in the design of Rogers hybrid laminates, and it is the primary cause of board warping, delamination and pad offset. There are significant differences between Rogers and FR4 materials in terms of coefficient of thermal expansion (CTE) and glass transition temperature (Tg): conventional FR4 typically has a Z-axis CTE of 40–60 ppm/°C, an X- and Y-axis CTE of approximately 12–18 ppm/°C, and a Tg of 130–150°C; whereas the Z-axis CTE of the Rogers RO4000 series can be as low as 20–30 ppm/°C, with a Tg exceeding 280°C, offering far superior thermal stability to FR4.
Throughout the entire process of laminate heating, temperature holding and cooling, the two materials exhibit different rates of expansion and contraction, resulting in significant shear stress between layers. When this stress exceeds the bonding strength of the materials, defects such as warping and delamination may occur, affecting the reliability of the PCB board and the precision of subsequent assembly.
To prevent thermal mismatch and warping, one must address the issue at the design stage, optimising comprehensively across three dimensions: material selection, laminate structure and process parameters.
Firstly, materials must be selected with a matched CTE gradient to avoid combinations with excessively large CTE differences. For low-to-medium frequency communication boards, modified high-Tg FR4 materials can be selected, as their CTE is closer to that of Rogers materials, effectively reducing interlayer stress.
If Rogers type PTFE materials are laminated with FR4, filled PTFE materials should be prioritised, as they offer better dimensional stability and more controllable CTE. Simultaneously, the selection of the prepreg is crucial; a PP specifically designed for hybrid lamination, which exhibits good adhesion to both materials, must be chosen. The lamination temperature and flow rate must be matched to avoid uneven resin flow caused by excessive PP flow, or the formation of bubbles and delamination due to insufficient flow, which would further exacerbate stress concentration.
Secondly, there is the symmetrical design of the laminate structure, which is a fundamental principle of hybrid laminate design. A symmetrical laminate allows stresses on the top and bottom sides of the board to cancel each other out, thereby reducing the risk of warping at its source. For example, in a 6-layer hybrid laminate, the high-frequency signal layers (Rogers material) can be placed on layers 2 and 5; the thickness and grade of the corresponding FR4 core and PP film on the top and bottom must be exactly the same to avoid excessive stress on a single side.
Asymmetrical designs are difficult to completely eliminate warping even with subsequent process optimisation and should therefore be avoided at the prototyping stage. At the same time, the judicious placement of ground and power planes, utilising them as buffer layers, can further disperse interlayer stress and enhance the board’s thermal stability.
Finally, there is the need for precise control of lamination process parameters. The lamination profile for hybrid boards cannot simply adopt the standard profile used for conventional FR4; customised adjustments are required: during the heating phase, a step-by-step temperature ramp should be employed, with a dwell time of 20–30 minutes in the 100–120°C range to allow the PP to fully melt and release gases, whilst enabling both materials to gradually adapt to the temperature change, thereby reducing instantaneous thermal shock; The temperature and duration of the holding phase must simultaneously meet the curing requirements of both materials.
For example, when laminating Rogers RO4350 with FR4, the peak temperature can be set at 180–190°C, with the holding time extended to 60–90 minutes to ensure both materials cure fully and prevent subsequent stress release; During the cooling phase, employ gradual cooling, reducing the temperature at a rate of 1–2°C/min to below 100°C, followed by natural cooling, whilst maintaining constant pressure to allow internal stresses to be fully released and to suppress warping and deformation.
Recommendations for Cost-Optimised Layer-Up Strategies
The key to cost optimisation for Rogers mixed-pressure laminates lies in ‘precise material usage, simplified processes and improved material utilisation’. By employing a scientific layer-up strategy, a balance between cost and performance can be achieved without compromising high-frequency performance. Much of the cost wastage in PCB design stems from “over-engineering” – such as the indiscriminate selection of high-end Rogers grades, the use of Rogers material across all layers, or complex laminate structures that drive up manufacturing costs. Consequently, targeted laminate optimisation is particularly crucial.
Allocate Rogers material as required, focusing on core signal layers. The cost of mixed-material PCBs is primarily concentrated in the Rogers substrate; therefore, it is unnecessary to use Rogers material on every layer. By using Rogers material only on core layers for high-frequency signal transmission (such as signal transmission and reception layers in 5G base stations or core layers in millimetre-wave radar), and employing FR4 material for power layers, ground layers and low-speed signal layers, material costs can be minimised.
At the same time, the selection of Rogers material grades should be tailored to product requirements; there is no need to blindly pursue high-end grades: for entry-level high-frequency applications, RO4003C can be selected, as it offers good processability and controllable costs; for mid-to-high-end applications, RO4350B is recommended, balancing stability and cost-effectiveness; for microwave-level applications, high-end grades such as RT5880 should be selected to avoid cost wastage caused by ‘over-specification’.
Optimise the laminate structure and simplify the manufacturing process. Complex laminate structures (such as excessive blind and buried vias or asymmetrical stacking) increase manufacturing difficulty and scrap rates, indirectly driving up costs. During design, the number of layers should be minimised wherever possible; provided functional requirements are met, adopt a simplified structure of ‘fewer high frequency layers combined with multiple FR4 layers’.
Furthermore, avoid unnecessary blind and buried via designs, prioritising through-hole connections to reduce drilling difficulty and manufacturing costs. Furthermore, adopting a “localised mixed-material” process—using Rogers material only in core high-frequency areas and FR4 material in all non-core areas—can further reduce the consumption of Rogers material, yielding significant overall cost savings; in some scenarios, cost reductions of over 28% can be achieved.
Improving material utilisation and reducing manufacturing waste. As Rogers material is expensive, the yield during cutting directly impacts overall costs. During design, panelisation schemes should be optimised by adopting symmetrical layouts and stamp-hole panelisation designs to increase board material utilisation to over 90%, thereby reducing waste from edge and corner scraps; simultaneously, PCB layouts should be planned rationally based on the dimensional specifications of the Rogers material to avoid material cutting losses caused by ill-conceived layouts.
Furthermore, by utilising simulation tools (such as Ansys HFSS and Cadence Allegro) to optimise the layer stack up and impedance design in advance, rework and material wastage caused by design errors can be minimised, whilst also reducing testing costs employing a combination of AOI and flying probe testing, with only critical networks sampled, can reduce testing costs by approximately 40%.
Collaborate with manufacturing processes to reduce overall costs.The design phase should work closely with production, selecting proven manufacturing processes and avoiding complex specialised techniques (such as high frequency laser drilling or special surface treatments). Prioritise cost effective surface treatments such as immersion gold or immersion silver to replace the more expensive electroplated gold process.
At the same time, rational planning of lamination batches should be implemented to avoid laminating mixed-layer boards of different models and thicknesses in the same oven, thereby improving production yield and reducing scrap losses a 10% improvement in the yield of hybrid laminates can indirectly reduce overall costs by approximately 15%.
The design of Rogers hybrid laminates essentially involves striking an optimal balance between performance, cost and manufacturing processes. By mastering these three key aspects—material selection, layer symmetry and process coordination—it is possible to achieve a win-win situation in terms of reliability and cost-effectiveness whilst ensuring high-frequency performance.
