PCB Prepreg: A Comprehensive Guide

October 12, 2023

Introduction

PCB Prepreg, a dielectric substance, is placed between two cores or a core and copper foil in a printed circuit board (PCB) to provide the necessary insulation. You could also refer to it as a binding substance. Either a core and two cores or a core and copper foil are joined. Prepreg is crucial for fabricating multilayer printed circuit boards (PCBs). But what exactly is prepreg? Let’s start with a quick definition.

What is PCB Prepreg?

PCB prepreg refers to fibrous material like fiberglass pre-impregnated with a resin system before the laminating process.

Prepreg typically consists of fiberglass fabric or mats soaked in an epoxy or polyimide resin matrix.
The resin is partially cured to form a tacky, flexible B-stage material ready for lamination.
Prepregs are supplied with protective packaging to avoid damage during handling and shipping.

Prepreg provides a few vital functions when making multilayer PCBs:

It bonds the conductive copper foil layers by applying heat and pressure during lamination.
The composite material is a dielectric to electrically isolate the copper layers while allowing conduction through plated holes.
The fiber weave imparts mechanical strength and dimensional stability to the laminate stack-up.
Prepreg controls the thickness and spacing of the conductive layers within the PCB.

So, prepreg is the fundamental dielectric building block of multilayer PCBs, making high-density, reliable boards possible through their unique properties.

Why is Prepreg Important for Multilayer PCBs?

Prepreg provides several advantages that make multilayer PCB fabrication possible:

The resin soaks and coats the fiber weave, bonding layers under heat and pressure.
It serves as a dimensionally stable electrical insulator between copper sheets.
It imparts structural rigidity for reliable plated through holes and vias.
Low-flow prepregs prevent shorting between high-density traces.
High resin content prepregs control spacing between layers.
Filler particles like silica provide a coefficient of thermal expansion that matches copper.

Without prepreg material as a bonding dielectric layer, a multilayer board could not be held together as a stable multilayer PCB!

So, in multilayer stacks, prepreg forms the fundamental insulating layer for enabling high-density assembly and routing. Understanding its properties provides the basis for fabricating complex, reliable PCBs.

Properties of PCB Prepreg

Internal composition of prepreg — The internal composition of prepreg

The previous section covered a basic introduction to prepreg and its role in PCB fabrication. Now, let’s explore the various components that makeup prepreg and how they contribute to its overall properties and performance. Understanding the prepreg composition will show how to select the optimal material for your application.

Fiber Reinforcement

The fiberglass fabric reinforcement in prepreg provides mechanical strength and dimensional stability to the dielectric layers. The fiber weave style and aerial weight impacts properties like resin absorption, thickness, and dielectric performance.

Fiber Materials

Several different fiber materials are used to create the fabric plies in prepreg:

Glass is the most common fiber material. Provides good mechanical properties and dielectric performance at a low cost. Types include E-glass, S-glass, D-glass.
Aramid – Solid synthetic fibers like Kevlar or Nomex. Used for high-strength flex circuits.
Ceramic – For high-frequency applications. Stable dielectric constant. Costly.
Carbon – High strength and rigidity. Not commonly used due to conductivity.

Glass fabrics are the dominant fiber material used in cost-sensitive commercial PCBs. The glass fiber diameter ranges from 5-15μm, with fine fibers providing a smoother surface finish.

Weave Styles

The arrangement of the glass fiber strands into woven or non-woven configurations determines properties like resin absorption, dimensional stability, and anisotropy.

Woven Fabric

The fibers are interlaced at right angles into an orderly weave:

Plain weave – over and under pattern. Most common.
Twill weave – diagonal pattern.
Satin weave – floats over multiple strands.

Woven fabrics have higher strength but lower resin absorption. Directionality can cause skew issues.

Non-woven Mats

Fiber strands are randomly arranged in a mat with no weaving:

Chopped fiber mats – Short, randomly dispersed fibers.
Continuous fiber mats – Long aligned fibers.

Non-wovens have more uniform resin absorption but lower strength. No skew issues.

Aerial Weight

The aerial weight of the fiberglass is measured in g/m2. Higher weights provide:

More resin retention capacity.
Lower resin flow.
Increased thickness for a given resin content.
Improved dimensional stability.

But come at the cost of decreased flexibility and lower dielectric properties. Weights range from 20g/m2 to over 100g/m2.

Resin System

After curing, the resin coats the fibers and strongly bonds the laminate stack. Fundamental properties depend on the resin chemistry.

Epoxy

The most universally used resin system. Provides:

Good adhesion, processing, and electrical properties.
Temperature rating of 130-180°C based on composition.
Cost-effective choice for FR-4 grade materials.

Modified epoxies like BT (bismaleimide triazine) increase heat resistance further.

Polyimides

Extremely high-temperature resistance above 300°C.
Low Z-axis CTE. Excellent dimensional stability.
Relatively high cost. Used for flex circuits and hot PCBs.

Cyanate Ester

Tightly controlled, low dielectric constant and loss tangent.
High-frequency applications like antennas and radar.
Low moisture absorption. Good chemical resistance.

PTFE Composites

PTFE reinforced with ceramic or glass microparticles.
Ultra-low dielectric constant down to 2.2.
For high-speed digital and RF. E.g., Rogers RO3003.

Others

Polyphenylene oxide (PPO) – low-loss material.
Polyester or phenolic resin – lower performance and cost.

So, resin system selection depends on the electrical, thermal, and cost requirements.

Filler Particles

Inorganic filler particles are added to the resin in prepreg to tailor properties like:

CTE for dimensional stability.
Thermal conductivity.
Dielectric performance.
Rheology and flow.
Adhesion.
Cost.

Typical fillers used include:

Silica – Most common. Quartz or fused silica. Reduces CTE.
Alumina – Enhances thermal conductivity.
Calcium carbonate – Lowers cost.
Aluminium hydroxide – Non-conductive. Chemically resistant.
Boron nitride – High thermal conductivity.

Filler particle shape and size distribution impact packing density and resin flow. Depending on requirements, loadings range from 15% to over 50% by weight.

Flame Retardants

Most PCB prepregs incorporate flame retardant additives to achieve UL 94 V-0 or a similar rating. Common options:

Brominated compounds – e.g., tetrabromobisphenol A.
Phosphorus-containing – e.g., DOPO, phosphates.
Metal hydroxides – e.g., aluminum hydroxide.
Nanoparticles – clay, carbon nanotubes.

Flame retardants function by both physical and chemical mechanisms in the resin.

Types of PCB Prepreg

In the previous sections, we explored what prepreg is and its composition’s various components. Now, let’s examine the different prepreg types commonly used in PCB fabrication.

With a wide range of prepreg materials available, selecting the one optimized for your application requirements and board fabrication process is essential. We’ll cover the fundamental properties and tradeoffs of common prepreg types.

FR-4 Prepreg

FR-4 glass-epoxy prepreg is the universally used standard material for cost-driven commercial PCBs. Key characteristics:

Epoxy resin system with glass fabric reinforcement.
Flame-resistant grade with UL 94 V-0 rating.
Comparatively low cost.
Tg of 130-170°C based on composition.
Excellent processing characteristics.

FR-4 is suitable for up to several GHz low-frequency digital and analog PCB applications. The baseline electrical and thermal performance makes it ideal for controllers, consumer electronics, instruments, and communications gear.

High Tg PCB Prepreg

High Tg prepreg is used for applications requiring high-temperature operation or enhanced thermal reliability.

Tg above 170°C, even over 200°C.
Uses modified epoxy resin systems like BT (bismaleimide triazine) or polyimides.
Improves soldering heat resistance and long-term thermal aging.
Often halogen-free for eco-friendly requirements.

Examples include Arlon 85N, Nelco 4000-13, and Panasonic Megtron 6. High Tg prepregs enable lead-free soldering, component mounting, and quality inspection without warpage or delamination issues.

PTFE Composite Prepreg

PTFE-reinforced prepregs offer an extremely low dielectric constant while maintaining good physical and electrical integrity.

PTFE is impregnated with microfiber glass. E.g. Rogers RO3003 with εr=3.
Significantly lower loss than standard FR-4.
Excellent high-frequency signal performance for RF and microwave.

The ultra-low Dk allows closer trace spacing and thinner dielectrics for improved impedance control, propagation delay, and signal integrity.

Polyimide Prepreg

Polyimide prepregs provide the highest service temperature capability along with outstanding dimensional stability.

Very high Tg > 250°C. Withstands soldering, cleaning, and high-temperature assembly.
Low Z-axis CTE around 20-30 ppm/°C. Minimizes via failures.
Excellent flex life durability. Handles dynamic bending applications.
Relatively high material cost. Justified for demanding applications.

Offerings include DuPont Kapton VN and Hitachi PI-2525. The high thermal rating makes polyimide a top choice for hot aerospace, automotive, and industrial electronics.

Non-Woven Aramid Prepreg

Non-woven aramid prepregs using Kevlar or Nomex offer exceptional layer-to-layer bond strength.

Premium synthetic fibers layered in a random fiber mat.
Highest bond strength to resist delamination. > 3 N/mm.
Well-controlled low Z-axis CTE. Reduces via failures.
Suitable for the large form factor and high-reliability boards.

Isola’s Thermount and Arlon’s Adhesiveless Metal Clad (AMC) are leading non-woven aramid prepregs that deliver robust performance.

Prepreg Handling

Now that we’ve covered the composition and types of prepreg materials, proper handling is critical to maintain consistent properties and avoid defects. Let’s examine key prepreg storage, shelf life, and pretreatment factors.

Storage Considerations

Prepregs are supplied sealed in moisture barrier bags to prevent absorption of ambient humidity. Careful storage is required to maximize shelf life:

Temperature – Refrigerated storage at -18°C is ideal. Room temperature of 18-25°C is acceptable. Avoid excess heat exposure.
Humidity – Storage in sealed bags with desiccant packs is recommended. Avoid condensation inside the packaging.
Light – Protect prepregs from UV and sunlight exposure, which can initiate polymerization. Use yellow light.
FIFO Usage – First-in-first-out stock rotation helps utilize prepreg before expiry. Older material may need additional baking before use.

As advised by the supplier, proper prepreg storage will maintain consistent resin properties and minimize variability.

Shelf Life

The shelf life of prepreg at room temperature storage is typically 1 year from the production date.

Refrigeration can extend this to 18-24 months.
After expiration, additional baking may be needed to reverse partial curing.
Excessive storage can lead to handling issues as the tackiness increases.

Prepreg should be tracked by production lot number and used FIFO to avoid shelf life-related problems.

Pretreatment

Special care is required when taking prepreg out of refrigerated storage:

Allow the sealed bag to warm to room temperature before opening gradually.
This prevents moisture condensation on the prepreg, which can affect resin properties.
Typically, 6-12 hours of warming is recommended.
Avoid directly transferring cold prepreg into the lamination press.

So, proper temperature acclimatization of refrigerated prepreg is vital to prevent defects like delamination or blistering before PCB usage.

Prepreg Testing

Verifying the quality and consistency of prepreg materials is critical to ensure the reliability of multilayer PCBs. Let’s examine some key test methods conducted.

Resin Testing

Resin Content – Measure weight change after burning off resin to determine percentage content. Affects laminate thickness.
Resin Flow – Test viscosity at temperature and pressure. Indicates resin polymerization state and flow during lamination.
Gel Time – Monitor resin cure over time at a set temperature. Provides lamination process window.

Material Analysis

Dielectric Properties – Dielectric constant and loss measured over frequency range. Stable value desired.
DSC – Differential scanning calorimetry detects transitions like Tg. Qualifies degree of cure.
Flammability – UL 94 vertical burning test rating for material flammability.

Lot Acceptance

Received Prepreg – Test samples from each batch to confirm material properties versus certification data.
Control Laminates – Test laminates fabricated from prepreg lot to verify acceptable performance.

Routine testing validates prepreg quality before PCB usage to minimize defects and ensure consistent results.

Prepreg in PCB Fabrication

Now that we understand prepreg materials in depth, let’s examine how they functionally contribute during the multilayer PCB fabrication process:

Bonding – Prepreg reliably bonds together the inner layer circuit foils during lamination under heat and pressure.
Isolation provides electrical isolation between copper sheets while allowing conductivity through drilled holes.
Dielectric Constant – The resin system governs the Dk of the insulating composite layers. This controls impedance.
Dimensional Stability – Low expansion prepregs in the Z-axis minimize registration issues and failures.
Mechanical Strength – The cured prepreg composite lends rigidity and withstands flexing stresses.
Layer Spacing – Prepreg sheets control the distances between adjacent conductive layers.

So prepreg forms a fundamental building block enabling the fabrication of high-density, reliable multilayer boards.

Conclusion

Prepreg forms the fundamental insulating material that makes high-density PCB constructions possible. Its unique properties allow adjacent copper layers to be reliably bonded while providing dimensional stability and dielectric isolation.

With the wide range of prepreg materials available, PCB designers can select the optimal properties like resin type, expansion, thermal rating, and dielectric constant needed for their application requirements and board fabrication process.

Understanding prepreg materials provides the basis for engineering cutting-edge circuits to meet the ever-increasing demands for miniaturization, speed, and reliability.

Frequently Asked Questions

How does prepreg selection influence the PCB fabrication process?

Factors like resin flow, Tg, expansion, and moisture sensitivity impact lamination, hole drilling, and handling. The properties must align with manufacturing capabilities.

What causes prepreg-related defects in PCBs?

Insufficient resin flow leads to poor adhesion. High moisture causes blistering. Large Z-axis CTE leads to cracking. Incompatible prepreg and cores create issues.