Bondply vs Prepreg PCB: What’s the Difference in Multilayer Stackups?

If you’ve spent any time designing multilayer PCBs, you know what prepreg is — the semi-cured fiberglass-epoxy bonding sheet that holds a stack together during lamination. But once you move into RF, microwave, or PTFE-based multilayer boards, you’ll start seeing a different term on fabricator stackup drawings: bondply. The two materials serve the same structural purpose in a PCB stackup, but they’re not interchangeable, and choosing the wrong one for your application can cost you either signal performance or yield.

This article breaks down the bondply vs prepreg PCB question properly — what each material is, how each is used in multilayer construction, where they differ in electrical and mechanical properties, and how to specify the right bonding material for your stackup depending on whether you’re building a standard digital board, a high-speed design, or a multilayer RF/microwave circuit.

What Is Prepreg in PCB Multilayer Construction?

Prepreg — short for pre-impregnated — is a sheet of woven glass fiber cloth saturated with a partially cured (B-stage) thermosetting resin, most commonly epoxy. It’s the bonding and insulating layer between copper-clad cores in a multilayer PCB. When the stack is assembled and put through a lamination press under controlled temperature and pressure, the resin in the prepreg softens, flows into copper surface features, and then fully crosslinks into a rigid dielectric layer permanently bonding everything together.

In a standard 4-layer FR-4 board using foil construction, prepreg sits between the outer copper foil and the inner core on both the top and bottom of the stack. In a 6-layer or 8-layer board, additional prepreg layers fill the gaps between each core pair. Prepreg does two things simultaneously: it bonds the stack mechanically, and it forms the dielectric between adjacent copper layers — meaning its Dk and Df values directly affect impedance control and signal loss on every layer interface in the board.

Prepreg Glass Styles and Resin Content

Not all FR-4 prepregs are identical. They come in different glass weave styles, and each style has a different thickness per ply and a different resin-to-glass ratio. The most common glass styles used in standard PCB fabrication are:

Glass Style	Nominal Thickness (cured)	Resin Content	Typical Use
106	2.0–2.5 mil	High (~65–72%)	Thin dielectric layers, fine pitch HDI
1080	2.8–3.5 mil	Medium (~52–65%)	General multilayer, thin cores
2116	4.5–5.0 mil	Medium (~48–55%)	General multilayer
7628	7.0–8.0 mil	Low (~42–45%)	Thicker dielectric, structural layers

Higher resin content means more flow during lamination, which is useful for filling over dense copper features but can introduce thickness variability. Multiple thin plies (e.g., three 106 sheets) are often preferred over a single thick 7628 sheet where tight thickness tolerance matters for impedance control.

Prepreg is available in standard FR-4 epoxy, high-Tg epoxy, halogen-free formulations, polyimide, and low-loss variants such as Rogers RO4450B and RO4450F — glass-reinforced hydrocarbon/ceramic prepregs engineered to match the RO4000 core laminate series. These specialty prepregs behave much like standard prepregs in a lamination press but are formulated to deliver Dk and Df values appropriate for high-speed and moderate RF applications.

What Is Bondply in PCB Multilayer Construction?

Bondply is the RF/microwave world’s functional equivalent of prepreg. It’s a thin adhesive bonding film used to join multiple layers of high-frequency laminates — most commonly PTFE-based substrates such as Rogers RT/duroid, RO3000 series, or Taconic materials — in a multilayer PCB stack. The PTFE version of a prepreg is known as a bondply, and it plays the same structural role as prepreg in an FR-4 stackup: it sits between core layers and bonds them together under heat and pressure.

The critical distinction is what bondply is made of. Because PTFE substrates are chemically very different from epoxy-glass cores, standard FR-4 prepreg cannot bond to PTFE effectively. Conventional epoxy prepreg also introduces prohibitively high dielectric loss into a stackup that was specifically chosen for its ultra-low loss PTFE layers. Using standard epoxy prepreg to bond PTFE cores would largely defeat the purpose of choosing PTFE in the first place.

Bondply Material Types Used in RF PCB Multilayers

There are several bondply formulations used in microwave PCB construction, each suited to different performance requirements:

Thermoplastic film bondplies include materials based on fluorinated ethylene propylene (FEP), polyethylene (PE), chlorotrifluoroethylene (CTFE), and PTFE film. These materials melt and flow under heat and pressure, bonding PTFE cores mechanically with excellent dielectric performance. Their primary advantage is very low Dk and Df, closely matching PTFE laminate properties. The main limitation is that thermoplastic films are generally unsuitable for sequential lamination — meaning boards using them are typically limited to lower layer counts, and sequential lamination processes used in HDI builds are not compatible with thermoplastic bondply.

Thermoset bondplies are partially cured adhesive systems — the B-stage equivalent in the RF world — that crosslink permanently during lamination. A widely-used commercial example is Rogers 2929 bondply: a non-reinforced hydrocarbon thermoset film available in 1.5 mil, 2 mil, and 3 mil thicknesses, with a Dk of 2.9 and Df below 0.003 at microwave frequencies. Its crosslinking resin system is compatible with sequential lamination processes, enabling higher layer count builds. Rogers RO4450B and RO4450F are glass-reinforced thermoset prepregs designed for the RO4000 hydrocarbon-ceramic core series — technically labeled as prepregs by Rogers, but serving the bondply function in RF stackups.

Third option: fusion bonding uses neither prepreg nor bondply — layers are joined by applying extremely high temperature and precisely controlled pressure directly between PTFE substrates, with no bonding material added. This preserves the pure electrical performance of the stack but requires tightly controlled process equipment and is limited in applicability. It’s used in very high-performance military and aerospace microwave circuits where every 0.001 of Df matters.

Bondply vs Prepreg PCB: Core Differences Compared

The table below summarizes the key differences between standard epoxy prepreg and RF bondply across the properties that matter most in multilayer PCB design.

Property	Standard Epoxy Prepreg (FR-4)	RF Thermoset Bondply (e.g., Rogers 2929)	Thermoplastic Film Bondply (FEP)
Base material	Woven glass + epoxy resin	Non-reinforced hydrocarbon thermoset film	Fluoropolymer (FEP, PTFE, CTFE)
Dk @ 10 GHz	3.8–4.5	~2.9	2.0–2.4
Df @ 10 GHz	0.015–0.025	<0.003	0.0002–0.001
Thickness range	2–8 mil (per glass style)	1.5–3 mil per ply (stackable)	1–5 mil typically
Compatible core materials	FR-4, high-Tg epoxy, polyimide, RO4000	PTFE, RO3000, RT/duroid, RO4000	PTFE, RT/duroid, soft substrates
Sequential lamination	Yes — fully compatible	Yes (thermoset types)	No — limited to simple stacks
Moisture absorption	0.10–0.20%	~0.1%	<0.05%
Lamination temp range	170–200°C	190–230°C	260–290°C (melt temp dependent)
Typical application	Digital, mixed-signal, general PCBs	Microwave, mmWave, PTFE multilayers	High-performance PTFE multilayers
Representative products	Isola 370HR, Panasonic Megtron 6	Rogers 2929, RO4450B/F	Taconic FR-28, various FEP films

Dielectric Performance: Where the Real Difference Lies

For digital and mixed-signal boards, standard FR-4 epoxy prepreg works fine because the signal frequencies stay in a range where a Df of 0.015–0.025 doesn’t cause critical signal attenuation. Push a board into the GHz range on those same dielectric layers and insertion loss climbs sharply. A PTFE multilayer board specifically selected for Df values below 0.002 in its core material would see that advantage erased if bonded with epoxy prepreg at every layer interface. At 10 GHz, the Df of a Rogers 2929 bondply (<0.003) is roughly 5–8x lower than standard epoxy prepreg — a difference that directly translates to insertion loss in stripline circuits where the signal propagates through both the core and the bonding layer.

For a 4-layer PTFE multilayer stripline design, the bonding layer between the two cores sits directly in the signal path. Specifying a low-loss bondply like Rogers 2929 (Dk 2.9, Df 0.003) instead of standard FR-4 prepreg preserves the signal integrity that justified using PTFE in the first place.

When to Use Prepreg vs Bondply: PCB Stackup Decision Guide

Stackup Material Selection Table

Design Type	Frequency Range	Recommended Bonding Material	Notes
Standard digital / consumer	DC to ~1 GHz	Standard FR-4 epoxy prepreg	7628, 2116, 1080, 106 as needed
High-speed digital (SerDes, DDR5)	1–10+ GHz	Low-loss epoxy prepreg	Isola 370HR, Megtron 6 prepreg
Moderate RF / wireless	1–6 GHz	RO4450B or RO4450F (glass-reinforced thermoset)	Pairs with RO4003C or RO4350B cores
Microwave / radar (6–40 GHz)	6–40 GHz	Rogers 2929 bondply or FEP film	Use thermoset for sequential lam builds
mmWave (>40 GHz)	40–100+ GHz	Low-Dk FEP or PTFE film bondply	Dk/Df stability over frequency critical
Hybrid FR-4 + Rogers multilayer	Mixed	FR-4 prepreg for FR-4 sections, RO4450 for RF sections	Discuss CTE management with fabricator
Rigid-flex (PTFE core)	RF frequencies	PTFE-compatible thermoset bondply	Confirm flex zone compatibility

Hybrid Stackups: Using Both Prepreg and Bondply Together

A common approach in mixed-signal and RF designs is to combine standard FR-4 cores for the digital and power sections with Rogers or PTFE cores for the high-frequency RF sections — all in the same multilayer stackup. These hybrid boards require careful bonding layer selection at each interface. The general rule is to use a homogeneous prepreg between layers of similar properties: FR-4 prepreg bonds to FR-4 cores, and RF-compatible bondply or prepreg bonds the RF core layers. At the interface between dissimilar materials — where a Rogers core meets an FR-4 core — the bonding material choice must consider CTE mismatch, lamination temperature compatibility, and the electrical implications of which Dk/Df value appears at that interface.

Sierra Circuits and Rogers Corporation both publish hybrid stackup design guides that address material compatibility and bonding layer selection at dissimilar-material interfaces. These are worth reviewing before finalizing any hybrid stackup with your fabricator.

Bondply Materials from Key Suppliers

Rogers Corporation Bondply and Prepreg Products

Rogers is the most referenced supplier of bondply and high-frequency prepreg materials for microwave PCB construction. Their product line covers the full spectrum from moderate-RF to mmWave applications:

Rogers 2929 Bondply — non-reinforced hydrocarbon thermoset film; Dk 2.9, Df <0.003 at 10 GHz; available in 1.5, 2.0, and 3.0 mil thicknesses; sheets can be stacked for thicker bonding layers; ideal for PTFE-core multilayers including RT/duroid 6000 and RO3000 series; compatible with sequential lamination.

Rogers RO4450B and RO4450F — glass-reinforced ceramic-filled hydrocarbon thermoset prepregs designed as bonding layers for RO4000-series cores (RO4003C, RO4350B). RO4450F is the flame-retardant variant. Dk of 3.2–3.5 depending on thickness and glass fill; Df of 0.004 at 10 GHz; UL 94 V-0 rated; compatible with sequential lamination and lead-free assembly.

Arlon PCB Bonding Films

Arlon PCB produces bonding materials for high-reliability microwave and military PCB applications, including PTFE-based bondply and adhesive film systems for multilayer PTFE constructions. Arlon’s 25N, 25FR, and CLTE-XT laminate families each have corresponding bonding film specifications. For mil-spec and aerospace programs where material lot traceability and qualification documentation are required, Arlon’s bonding materials provide established certification paths alongside their core laminates. Engineers specifying PTFE multilayers for defense radar or satcom programs should request Arlon’s bonding film compatibility matrix alongside their core laminate datasheets.

Taconic and Other Suppliers

Taconic produces PTFE-compatible bonding films for their TLX, RF-35, and FastRise laminate series. Their FR-28 thermoset bonding film is used in conjunction with PTFE composite cores. AGC Multi Material (formerly Nelco/Park) and Isola also publish specialty prepreg options for their respective laminate families.

Practical Fabrication Considerations

Lamination Temperature and Pressure

Standard FR-4 epoxy prepreg lamination cycles run at 170–200°C under press pressures of 200–400 psi. Thermoplastic PTFE bondplies require significantly higher temperatures — typically 260–290°C or above — because the thermoplastic must reach its melt or softening temperature to flow. This has implications for any other materials in the same stack that may be affected by those temperatures. Thermoset bondplies like Rogers 2929 laminate at lower temperatures closer to conventional epoxy prepreg cycles, which is one reason they’re preferred for hybrid builds.

Controlled Resin Flow and Thickness Tolerance

One of the engineering tradeoffs with bonding films versus glass-reinforced prepreg is resin flow behavior. Standard prepreg flows significantly during lamination — filling copper trace height features on inner layers. Bondply films like Rogers 2929 are specifically formulated with controlled flow characteristics: enough to fill trace topography and eliminate voids, but not so much that the bonding layer thickness varies unpredictably. The unreinforced film format also makes 2929 easier to pre-laminate to inner cores before final stack booking — simplifying fabrication of complex via structures.

Layer Count and Sequential Lamination

Thermoplastic bondply films can only be laminated once because they melt and then resolidify but cannot be reliably re-pressed. This limits their use to stackups that can be completed in a single lamination cycle — typically 2- to 4-layer designs for pure PTFE builds. Thermoset bondplies (Rogers 2929, RO4450B) and specialty low-loss prepregs are fully compatible with sequential lamination, making them the correct choice when blind or buried vias require multiple lamination cycles. This is also why Rogers 2929 documentation specifically calls out its compatibility with sequential bonding processes.

Useful Resources for Bondply and Prepreg PCB Stackup Design

Resource	What You’ll Find	Link
Rogers 2929 Bondply Datasheet	Full Dk/Df specs, thickness options, lamination compatibility for PTFE multilayers	rogerscorp.com
Rogers RO4450B/F Datasheet	Glass-reinforced prepreg for RO4000-series hybrid and sequential-lam builds	rogerscorp.com
Sierra Circuits Hybrid Stackup Guide	Design and material selection guidance for mixed FR-4/Rogers multilayer boards	protoexpress.com
IPC-4101 Base Materials Standard	Specification covering prepreg classification, slash sheets, and laminate types	ipc.org
Microwave Journal: Picking Prepregs for Multilayer Microwave PCBs	Technical overview of bonding material selection for microwave multilayer circuits	microwavejournal.com
ASC: Microwave PCB Bonding Methods	Comparison of thermoplastic film, thermoset prepreg, and fusion bonding for RF/microwave PCBs	asc-i.com
Isola 370HR Datasheet	Widely-used high-Tg low-loss epoxy prepreg for high-speed digital multilayers	isola-group.com

FAQs: Bondply vs Prepreg PCB

Q1: Can I use standard FR-4 prepreg to bond PTFE cores in a multilayer RF board?

Technically the lamination will hold, but the result will likely be inadequate for RF performance and potentially unreliable mechanically. PTFE surfaces are notoriously difficult for epoxy resins to adhere to — the chemical inertness that gives PTFE its low surface energy also resists adhesion. More importantly, inserting an FR-4 prepreg layer with Df of 0.015–0.025 between PTFE cores with Df of 0.001–0.002 dramatically increases insertion loss at the layer interfaces. For a stripline structure where the signal passes through the bonding layer, you’ve essentially put a lossy barrier in the middle of your low-loss substrate. The correct approach is a PTFE-compatible bondply or a matched low-loss thermoset film like Rogers 2929.

Q2: What is the difference between Rogers 2929 bondply and RO4450B prepreg?

Both are low-loss bonding materials from Rogers, but they serve different core material systems. Rogers 2929 is a non-reinforced hydrocarbon thermoset film (Dk 2.9, Df <0.003) designed to bond PTFE-based cores including RT/duroid and RO3000 series, where a reinforcing glass fabric would introduce surface irregularity issues. RO4450B is a glass-reinforced ceramic-filled thermoset prepreg (Dk ~3.3, Df ~0.004) engineered specifically as the bonding layer for RO4000-series hydrocarbon-ceramic cores (RO4003C, RO4350B). If you’re building a multilayer board using RO4350B cores, you use RO4450B or RO4450F prepreg. If you’re using RT/duroid 5880 or RO3003 PTFE cores, Rogers 2929 bondply is the appropriate choice.

Q3: Does bondply thickness affect impedance control the same way prepreg does?

Yes — the thickness of any dielectric layer directly affects the characteristic impedance of traces referenced to adjacent copper planes. This applies to bondply just as it does to prepreg. The advantage of bondply films like Rogers 2929 is their availability in precise, well-controlled thicknesses (1.5, 2.0, 3.0 mil), and the fact that multiple sheets can be stacked to achieve an exact total dielectric thickness. Because unreinforced films have more predictable thickness post-lamination than glass-weave-reinforced prepregs (which vary with resin flow into copper features), impedance control using bondply can sometimes be tighter than with standard glass prepreg — particularly in dense stripline layers.

Q4: Can bondply be used in sequential lamination for HDI builds?

It depends on the bondply type. Thermoplastic bonding films (FEP, CTFE) cannot be re-pressed after their initial lamination cycle and are not suitable for sequential lamination. Thermoset bondplies — including Rogers 2929 and the RO4450 family — are compatible with sequential lamination because their crosslinked thermoset structure survives subsequent lamination cycles. Rogers 2929 documentation specifically notes its patented crosslinking resin system enables sequential bonding process compatibility. If your RF multilayer design requires blind or buried vias necessitating multiple lamination passes, a thermoset bondply is the required choice. Confirm sequential lamination compatibility with your fabricator before finalizing the material selection.

Q5: How do I specify bondply vs prepreg to my PCB fabricator?

The most important thing is to specify the exact material by product name and supplier, not just a description. Include the following on your stackup drawing or fabrication notes: laminate manufacturer and grade (e.g., Rogers RO4350B for cores), bonding material manufacturer and grade (e.g., Rogers 2929 or RO4450F for prepreg/bondply), target dielectric thickness per layer after lamination (not just prepreg ply count), and whether sequential lamination is required. If the design is a hybrid FR-4 + Rogers build, identify which prepreg applies at each interface. Experienced RF fabricators like ASC, Taconic, and those supporting Arlon PCB materials will be familiar with bondply-specific lamination cycles and material handling requirements — but they need complete, explicit stackup documentation to quote and build correctly.

Conclusion

The bondply vs prepreg PCB question comes down to what you’re bonding and at what frequency. For standard FR-4 multilayers, epoxy prepreg is the correct and cost-effective choice. For PTFE or hydrocarbon-ceramic RF multilayers operating above a few GHz, standard epoxy prepreg introduces dielectric loss that undermines the fundamental reason for using a high-performance substrate in the first place. Bondply — whether a non-reinforced thermoset film like Rogers 2929 or a glass-reinforced hydrocarbon prepreg like RO4450B — is the correct bonding material for those applications.

The choice between thermoplastic film bondply and thermoset bondply comes down to layer count and process requirements: thermoplastic for simple PTFE builds where maximum electrical performance matters most, thermoset when sequential lamination or hybrid construction with FR-4 sections is required. Always finalize bonding material selection in consultation with your fabricator, and specify materials explicitly on your stackup drawing — the bonding layer is not a detail to leave to interpretation.