What Is PCB Prepreg? Types, Properties & Selection Guide

If you’ve ever worked on multilayer PCB designs, you’ve probably dealt with prepreg—whether you realized it or not. It’s one of those materials that sits quietly in the background of every stackup, holding everything together while you focus on trace routing and impedance matching. But here’s the thing: choosing the wrong prepreg can tank your signal integrity, cause delamination during assembly, or leave you scratching your head over impedance mismatches.

I’ve seen engineers spend hours perfecting their layouts only to get boards back with unexpected Dk variations or lamination voids—problems that often trace back to prepreg selection or handling. This guide breaks down everything you need to know about PCB prepreg, from the fundamentals to practical selection criteria that actually matter in real-world designs.

What Is PCB Prepreg?

PCB prepreg (short for “pre-impregnated”) is a fiberglass cloth that’s been saturated with a partially cured resin system. Think of it as the glue that holds your multilayer PCB together. During lamination, heat and pressure cause the resin to flow, bond with adjacent copper layers and cores, and then fully cure into a solid dielectric structure.

The key characteristic of prepreg is its B-stage curing state. Unlike fully cured core materials, prepreg remains pliable and tacky before lamination. This semi-cured condition allows it to:

• Bond copper foil to cores in foil-build constructions
• Join multiple cores together in cap-build stackups
• Fill gaps around inner-layer copper traces during pressing
• Provide electrical insulation between conductive layers

Here’s a simple analogy: if a multilayer PCB were a sandwich, the cores would be the pre-made patties with copper on both sides, and prepreg would be the cheese slices—they’re solid at room temperature but melt and bond everything together when you apply heat.

How PCB Prepreg Is Manufactured

Understanding prepreg manufacturing helps explain why different styles behave differently in your stackup.

The Impregnation Process

The manufacturing process starts with woven fiberglass fabric. This glass cloth passes through a bath of liquid resin (typically epoxy-based for standard FR-4), where it absorbs the resin completely. The saturated fabric then moves through a treater tower where controlled heat evaporates solvents and partially cures the resin to its B-stage condition.

Several factors during manufacturing determine the final prepreg properties:

Glass weave style defines the fabric’s weight, thickness, and weave pattern. Tighter weaves use more yarn per inch, creating denser, stronger materials.

Resin content is controlled by the amount of resin applied and how much gets squeezed out during processing. Higher resin content means thicker finished prepreg and more material to flow during lamination.

Curing advancement determines how much the resin has polymerized. Too little curing leaves the prepreg too tacky; too much reduces its ability to flow and bond during lamination.

Types of PCB Prepreg by Glass Style

The glass weave style is identified by a number designation that indicates the fabric construction. Each style has different characteristics suited for specific applications.

Common Prepreg Glass Styles

Glass Style	Thickness (Pressed)	Resin Content	Dk (Typical)	Best For
106	0.038-0.050 mm	65-75%	3.0-3.5	HDI, fine-pitch, thin boards
1080	0.060-0.076 mm	60-65%	3.2-3.7	High-speed digital, RF
2113	0.085-0.095 mm	55-60%	3.4-3.8	General purpose
2116	0.095-0.115 mm	48-55%	3.6-3.9	Standard multilayer
7628	0.170-0.210 mm	42-48%	4.1-4.6	Thick boards, cost-sensitive

Glass Style Selection Considerations

Lightweight styles (106, 1080) offer higher resin content relative to glass, resulting in lower Dk values and smoother surfaces. They’re preferred for:

• High-frequency applications where lower Dk improves signal propagation
• HDI boards requiring thin dielectric spacing
• Designs needing better resin flow around dense copper features

Mid-weight styles (2113, 2116) provide a balance between cost, mechanical properties, and electrical performance. They’re workhorses for most standard multilayer PCBs.

Heavyweight styles (7628) cost less and provide excellent dimensional stability, but their rougher surface and higher Dk make them less suitable for controlled impedance or high-speed designs. The thicker glass fibers can also deflect small drill bits, causing hole quality issues in HDI applications.

Types of PCB Prepreg by Resin Content

Within each glass style, prepreg comes in different resin content variants:

Variant	Resin Content	Pressed Thickness	Key Characteristics
SR (Standard Resin)	40-50%	Thinnest	Lower cost, less flow, rigid stackups
MR (Medium Resin)	50-55%	Medium	Balanced flow and thickness
HR (High Resin)	55-65%	Thickest	Maximum flow, void filling, flexible designs

When to use High Resin (HR) prepreg:

• Heavy copper inner layers (2+ oz) that need more resin to fill around thick traces
• Complex designs with significant copper variations across the board
• Applications requiring maximum void-free lamination

When Standard Resin (SR) works fine:

• Uniform copper distributions with 1 oz or less
• Cost-sensitive production volumes
• Designs requiring tighter thickness control

Types of PCB Prepreg by Material System

Beyond standard FR-4 epoxy, prepreg comes in specialized material systems for demanding applications.

FR-4 Standard Prepreg

The most common prepreg material, FR-4 uses a flame-retardant epoxy resin with a glass transition temperature (Tg) of 130-150°C. It’s suitable for:

• Consumer electronics
• Industrial controls
• General-purpose applications
• Operating temperatures below 110°C

High-Tg Prepreg

High-Tg prepreg uses modified epoxy or phenolic resin systems with Tg ratings of 170-180°C or higher. Choose High-Tg when:

• Multiple reflow cycles are required (lead-free assembly typically needs Tg >170°C)
• Operating temperatures exceed 100°C
• Better thermal reliability is needed for automotive or industrial applications

Low-Dk/Low-Df Prepreg

For high-speed digital and RF applications, specialized resin systems reduce dielectric constant and loss. These materials maintain signal integrity at multi-gigabit data rates:

Material Type	Typical Dk	Typical Df	Target Applications
Standard FR-4	4.2-4.7	0.018-0.025	Up to 3 Gbps
Mid-Tier (e.g., Megtron 4)	3.8-4.0	0.008-0.012	3-10 Gbps
High-Speed (e.g., Megtron 6)	3.4-3.7	0.004-0.006	10-28 Gbps
Ultra-Low Loss (PTFE-based)	2.2-3.0	0.001-0.003	mmWave, >28 Gbps

Polyimide Prepreg

Polyimide-based prepreg handles extreme temperatures (up to 250°C continuous) and finds use in aerospace, military, and high-reliability applications. It costs significantly more than epoxy-based systems.

Halogen-Free Prepreg

Environmental regulations increasingly require halogen-free materials. These prepregs use phosphorus-based flame retardants instead of bromine compounds, meeting IEC 61249-2-21 requirements without sacrificing electrical performance.

Key PCB Prepreg Properties Explained

Understanding these properties helps you make informed stackup decisions.

Dielectric Constant (Dk)

The dielectric constant determines signal propagation speed and impedance. Key points:

• Lower Dk = faster signals. A material with Dk of 3.0 allows signals to travel about 25% faster than standard FR-4 (Dk 4.4).
• Dk affects trace width. For the same impedance, lower Dk materials require narrower traces, enabling denser routing.
• Dk varies with frequency. Most materials show 5-10% Dk reduction from 1 MHz to 10 GHz.
• Dk changes during lamination. Prepreg Dk values differ before and after curing—always use post-lamination values for impedance calculations.

Dissipation Factor (Df)

Df measures signal energy lost as heat in the dielectric. This becomes critical at high frequencies:

• At 1 GHz, standard FR-4 (Df ~0.02) loses about 0.3 dB/inch on a typical microstrip
• At 10 GHz, those losses increase to roughly 1.5 dB/inch
• Low-Df materials (Df ~0.004) reduce losses by 60-70%

For designs running at 5 Gbps or above, Df should be a primary selection criterion.

Glass Transition Temperature (Tg)

Tg is the temperature where the resin transitions from rigid to rubbery. Above Tg:

• Mechanical strength drops significantly
• Coefficient of thermal expansion (CTE) increases dramatically
• Material becomes more susceptible to damage

Rule of thumb: Select prepreg with Tg at least 25°C above your peak operating temperature, and 40°C above for high-reliability applications.

Coefficient of Thermal Expansion (CTE)

CTE mismatch between prepreg and copper causes thermal stress. Typical values:

Material	CTE X-Y (ppm/°C)	CTE Z (ppm/°C)
Copper	17	17
Standard FR-4	14-17	50-70 (below Tg)
High-Tg FR-4	12-15	45-55
Low-CTE Materials	8-12	30-40

High Z-axis CTE is the primary cause of plated through-hole fatigue failure during thermal cycling.

Resin Flow

Resin flow indicates how much the resin will spread during lamination. Higher flow prepreg fills gaps better but may be harder to control for tight thickness tolerances. Flow is affected by:

• Resin content percentage
• Lamination temperature profile
• Pressure application timing
• Age and storage conditions of the prepreg

PCB Prepreg vs Core: Key Differences

This comparison trips up many engineers, so let’s clear it up.

Property	Prepreg	Core
Curing State	Partially cured (B-stage)	Fully cured (C-stage)
Copper	None	Copper foil on one or both sides
Flexibility	Pliable, tacky	Rigid
Dk Stability	Changes during lamination	Stable, consistent
Primary Function	Bonding layer, insulation	Circuit layer substrate
Typical Use	Between cores or core-to-foil	Hosts etched copper traces

Stackup Construction Approaches

Foil Build (Most Common) Cores sandwich prepreg layers, with copper foil on outer surfaces. The outermost dielectric is prepreg.

Cap Build / Core Build Prepreg at the center with cores toward the outer surfaces. The outermost dielectric is core material. This approach offers better Dk stability on outer layers—preferred for RF and microwave applications.

PCB Prepreg Selection Guide

For Standard Multilayer PCBs (4-8 Layers)

• Glass style: 2116 or 7628
• Resin content: SR or MR depending on copper weight
• Material: Standard FR-4 (Tg 140°C) or Mid-Tg (Tg 170°C) for lead-free assembly
• Typical stackup: 2x 2116 prepreg sheets per layer transition

For High-Speed Digital Designs (>5 Gbps)

• Glass style: 1080 or 2113 (avoid 7628)
• Material: Low-Dk/Low-Df (Megtron, IS415, Tachyon)
• Key priority: Df < 0.010 at operating frequency
• Watch for: Fiber weave effects causing skew in differential pairs

For RF and Microwave Applications

• Glass style: 1080 or thinner
• Material: PTFE-based (Rogers 4350, RO3003) or specialized laminates
• Key priority: Stable Dk across frequency and temperature
• Construction: Consider cap build for consistent outer-layer Dk

For HDI and Fine-Pitch Designs

• Glass style: 106 or 1080
• Resin content: HR for better via filling
• Key priority: Thin dielectric for tight layer spacing
• Watch for: Laser drilling compatibility with resin type

For High-Temperature Applications

• Material: High-Tg (180°C+) or polyimide
• Key priority: Low Z-axis CTE to protect PTH reliability
• Verify: Material compatibility with your assembly process temperatures

PCB Prepreg Storage and Handling Best Practices

Prepreg is sensitive to environmental conditions. Poor storage is a common root cause of lamination defects.

Storage Requirements

Parameter	Recommended	Maximum
Temperature	0-10°C (refrigerated)	23°C (room temp, reduced life)
Humidity	<50% RH	60% RH
Light exposure	Minimal (avoid UV)	Yellow lighting in work areas
Shelf life (refrigerated)	6 months	Per manufacturer specification
Shelf life (room temp)	3 months	Varies by resin system

Handling Guidelines

Before use:

1. Remove prepreg from refrigeration 6-12 hours before opening
2. Keep sealed until fully acclimated to room temperature
3. Opening cold prepreg causes condensation, leading to moisture absorption

During layup:

1. Handle with clean gloves to prevent contamination
2. Avoid excessive bending that damages glass fibers
3. Use within 24-48 hours once removed from packaging
4. Follow FIFO (First In, First Out) inventory rotation

Signs of degraded prepreg:

• Excessive tackiness or dried-out feel
• Color changes or discoloration
• Resin flaking or powdering
• Failed flow test results

Common PCB Prepreg Issues and Solutions

Delamination

Causes: Moisture in prepreg, insufficient resin flow, contaminated copper surfaces Solutions:

• Pre-bake prepreg if moisture suspected (2-4 hours at 105°C)
• Verify storage conditions and shelf life
• Ensure proper oxide treatment on inner layers

Voids and Air Pockets

Causes: Trapped air, insufficient lamination pressure, low resin content Solutions:

• Use vacuum lamination presses
• Increase lamination pressure by 10-20 psi
• Switch to HR prepreg variant

Resin Starvation

Causes: Heavy copper weight, insufficient resin content, improper pressing Solutions:

• Use HR prepreg for 2+ oz copper
• Calculate required resin volume based on copper distribution
• Adjust lamination profile for longer flow time

Impedance Variation

Causes: Dk variation, thickness inconsistency, fiber weave effects Solutions:

• Request tighter Dk tolerance prepreg
• Use consistent glass styles throughout stackup
• Rotate prepreg orientation to minimize skew

Frequently Asked Questions About PCB Prepreg

What is the difference between prepreg and laminate in PCB?

Prepreg is partially cured (B-stage) and has no copper—it’s the bonding material. Laminate (or core) is fully cured with copper foil already bonded to one or both sides. During PCB manufacturing, prepreg bonds cores together to create the complete multilayer structure.

How does prepreg thickness affect impedance?

Prepreg thickness directly determines the dielectric spacing between signal layers and reference planes. Thinner prepreg = lower impedance for the same trace width. For controlled impedance designs, even 0.5 mil thickness variation can shift impedance by 3-5 ohms.

Can I mix different prepreg types in one PCB stackup?

Yes, mixing prepreg types is common. For example, you might use 1080 prepreg for critical high-speed signal layers (lower Dk, smoother) and 7628 for non-critical power plane separation (lower cost). Just ensure CTE compatibility and verify Dk values for each layer in your impedance calculations.

What happens if I use expired prepreg?

Expired prepreg has advanced curing, reducing its ability to flow and bond properly. This leads to poor adhesion, voids, delamination, and inconsistent thickness. Always verify shelf life and conduct flow tests on questionable material before production use.

How do I choose between FR-4 and high-frequency prepreg materials?

Consider your signal frequency and acceptable loss:

• Below 1 GHz: Standard FR-4 usually works fine
• 1-5 GHz: Consider mid-tier materials if trace lengths exceed 2-3 inches
• Above 5 GHz: High-frequency materials typically required for reliable signal integrity

Also factor in your loss budget—longer traces at lower frequencies may need low-loss materials, while short traces at higher frequencies might tolerate standard FR-4.

Useful Resources for PCB Prepreg Information

Material Manufacturer Datasheets

• Isola Group: isola-group.com/pcb-laminates-prepreg – FR408HR, 370HR, Tachyon specifications
• Rogers Corporation: rogerscorp.com – High-frequency laminate data
• Panasonic Megtron: Available through distributors – Megtron 4, 6, 7 datasheets
• Shengyi Technology: syst.com.cn – Cost-effective alternatives

Industry Standards

• IPC-4101: Specification for base materials
• IPC-1601A: Handling and storage guidelines
• IPC-2141: Controlled impedance design guideline

Stackup Calculators

• Saturn PCB Toolkit: Free impedance and stackup calculator
• Isola IsoStack: isola-group.com/design-tools – Online stackup planning tool
• Polar Instruments Si9000: Industry-standard impedance modeling software

Technical References

• Signal Integrity Journal: signalintegrityjournal.com – Material selection articles
• Altium Resources: resources.altium.com – PCB design tutorials including stackup guidance

Final Thoughts

PCB prepreg might not be the most glamorous topic in electronics design, but getting it right makes everything else work better. Match your prepreg selection to your application requirements—don’t over-specify expensive materials where standard FR-4 works fine, but don’t cheap out on high-speed designs where signal integrity matters.

If you’re unsure about material selection for a specific design, your PCB fabricator’s engineering team can be a valuable resource. They see hundreds of stackups and know what works in production. Providing them with your impedance targets, operating frequencies, and reliability requirements helps them recommend appropriate prepreg combinations that balance performance with manufacturability and cost.