What is Prepreg ?

Prepreg (short for pre-impregnated) is a key material used in the multilayer printed circuit board (PCB) fabrication process. It consists of a woven or non-woven fiberglass cloth that has been pre-impregnated with a thermoset resin, typically epoxy. The prepreg provides the adhesive layer that bonds together the copper layers in a multilayer PCB. Understanding what prepreg is and how it works is essential for designing and manufacturing high quality PCBs.

What is Prepreg Made Of?

As mentioned above, prepreg consists of two main components:

Fiberglass

The fiberglass serves as the structural framework that provides mechanical support. It is typically woven fiberglass cloth, although non-woven fiberglass mat is also sometimes used. The weave style (e.g., plain weave, satin weave) and weight (ounces per square yard) can be varied to achieve different mechanical properties. Heavier weights and tighter weaves provide more strength and rigidity. Lighter weights allow more resin impregnation for better layer-to-layer adhesion.

Thermoset Resin

The resin coats and impregnates the fibers, acting as the adhesive glue that holds the PCB layers together. The most common resins used are epoxy, polyimide, or bismaleimide (BMI). Epoxy is the most popular due to its suitable dielectric properties, dimensional stability, and ease of processing. The resin may also contain fillers like silica to modify thermal and electrical characteristics.

Prepreg consists of fiberglass cloth pre-impregnated with a thermoset resin

Prepreg Manufacturing Process

Prepreg is made by impregnating the fiberglass cloth with a carefully controlled amount of partially cured thermoset resin. The process involves:

• Resin preparation - The resin is mixed with curing agents, catalysts, and fillers to achieve the desired processing and properties. The resin is usually in a partially cured “B-stage” to make the prepreg material stable and tacky.
• Impregnation - The fiberglass cloth is passed through a dip tank of resin and then through rollers to squeeze in just the right amount of resin. The dip tank viscosity and roller pressure are controlled to achieve the target resin content or weight per area.
• Drying - The coated cloth passes through a drying oven to evaporate solvents and partially advance the curing process. The B-stage cure makes the resin tacky but not fully cured.
• Roll slitting - The prepreg is sliced into narrower rolls for easier handling. The rolls are packaged to prevent moisture absorption.

Basic steps in prepreg manufacturing

The properties of the prepreg, such as resin tack, drape, and flow can be adjusted during manufacturing. The prepreg maker’s product datasheet provides key characteristics and parameters.

Prepreg Properties

Some key properties of prepreg that influence PCB design and performance:

• Resin Content - The percentage of resin relative to fiberglass by weight, typically around 50%. More resin improves adhesion but reduces thermal conductivity.
• Flow - How much the resin flows under heat and pressure. Higher flow fills spaces but risks excess squeeze-out. Low flow reduces movement but may leave voids.
• Drape - Qualitative measure of limpness and conformability. Improves ply conformance over uneven surfaces.
• Tack - Stickiness of resin. Balances ease of handling against moving during layer alignment.
• Gel time - Time to cure at a given temperature. Longer gel times aid multilayer alignment.
• Shelflife - How long the prepreg can last before the resin advances too far. Typically 1-2 years at room temperature.
• DSC - Differential scanning calorimetry traces measure the cure exotherm. Used to set lamination cycles.
• Tg - Glass transition temperature after cure. Indicates heat resistance.

Role of Prepreg in PCBs

Prepreg serves several key functions in the manufacture of multilayer PCBs:

• Layer Adhesive - Bonds the copper foil layers together into a monolithic structure.
• Imparts Rigidity - Reinforces the PCB for dimensional stability and physical strength.
• Electrical Isolation - Isolates adjacent copper layers, preventing electrical shorts.
• Forms Dielectric - Insulating resin provides the dielectric for layer-to-layer capacitance.
• Heat Dissipation - Fiberglass conducts heat vertically between layers.
• Chemical Protection - Seals and isolates copper traces from the environment.
• Solder Mask Support - Gives an even, non-porous surface for applying solder mask.

Multilayer PCB Fabrication

Prepreg is used during multilayer PCB fabrication in the following manner:

Layer Stack-up

Multiple sheets of prepreg are stacked between copper foil bond-ply layers to build up the multilayer lay-up. The stacking sequence ensures conductive layers are isolated by prepreg insulation.

Prepreg bonding sheets in a typical 4-layer PCB stack-up

Lamination

The layer pack is vacuum bagged and run through a lamination press. Heat and pressure cause the prepreg resin to reflow, bond, and cure into a solid laminate with the copper foils.

Prepreg resin cures during multilayer lamination

Drill and Plating

Through-holes are drilled through the cured laminate. Copper is electroplated to coat the holes and interconnect between layers.

Prepreg provides insulation between plated through-hole barrels

PCB Processing

The panel is further processed by steps like patterning, hole masking, and application of soldermask and silkscreen. The prepreg resin provides mechanical support throughout these processes.

Prepreg resin supports PCBs through fabrication processes

Prepreg Types

Many different prepreg materials are available for PCB fabrication. Common types include:

• FR-4 - Standard glass fabric prepreg with a fire-retardant epoxy resin. Low cost.
• High Tg FR-4 - Modified epoxy for higher heat resistance. Typ Tg 140-170°C.
• Halogen-free (HF) FR-4 - Improved environmental profile with Tg 140-180°C.
• Polyimide (PI) - High performance polymer with very high Tg >240°C.
• Bismaleimide Triazine (BT) – High Tg resin with 170-190°C. Low moisture absorption.
• Cyanate Ester - Low loss polymer used for radar, automotive, and RF boards.
• PTFE - Uses polytetrafluoroethylene (Teflon) resin for low Dk and dissipation.
• Quartz/Woven PTFE - High frequency material with Dk as low as 3.0.

Specialty - Other resins for flex, metal-backed, fire retardant, halogen-free, and low-loss boards.

The choice of prepreg depends on PCB performance requirements, fabrication considerations, and costs.

Prepreg Defects and Problems

Some potential issues with prepreg can cause defects in multilayer boards:

• Dry spots or starved areas with insufficient resin.
• Resin contamination or color variation.
• Wrinkling or rippling due to mishandling.
• Voids or porosity due to moisture absorption.
• Curing and Tg mismatches between prepreg lots.
• Prepreg beyond shelf-life becoming unworkable.
• Cold spots or unbonded areas from uneven curing.
• Fracturing or crazing from thermal stress.

Careful handling, storage, lay-up, and lamination help avoid these prepreg-related defects in multilayer PCBs.

Advantages of Prepreg

Key benefits of using prepreg in PCBs:

• Excellent layer-to-layer bond strength and adhesion
• Consistent and uniform dielectric thickness
• High structural rigidity for panel stability
• Good dimensional stability under temperature
• Tailorable thermal and electrical properties
• Allows high layer count and miniaturization
• Wide offering and availability of materials

Summary

• Prepreg is fiberglass reinforcement impregnated with adhesive resin
• Provides dielectric bonding layers in multilayer PCBs
• Manufactured by dip-coating cloth with B-stage thermoset resin
• Properties can be adjusted by formulation and process
• Stacked between copper foil for laminating multilayers
• Cures during lamination to form integrated laminate
• Numerous types available from FR-4 to advanced low-loss materials
• Careful handling and lamination needed to prevent defects
• Enables high performance, reliable multilayer PCBs

Frequently Asked Questions

Q: What is the typical glass transition temperature of standard FR-4 prepreg?

A: Standard FR-4 prepreg formulations typically have glass transition temperatures in the range of 130°C to 140°C once fully cured. Some "high Tg" versions may reach 170°C Tg.

Q: What weave styles and weights of fiberglass fabric are commonly used in prepregs?

A: The most common weaves used are 106 and 1080 style woven fiberglass, ranging from lightweight 7628 to heavier 2116 or 3313 styles. Weights between 1-5 ounces per square yard are typical for most applications.

Q: How does resin content affect prepreg properties and performance?

A: Higher resin content increases layer-to-layer adhesion but reduces thermal conductivity through the dielectric. It also increases cost. Typical resin content is around 50%, with 45-55% being common.

Q: What is the difference between thermoset and thermoplastic prepregs?

A: Thermoset prepregs use resins that permanently cure when heated, while thermoplastic resins soften on reheating allowing some reworkability. Thermoplastic prepregs have limited use currently due to processing challenges.

Q: How does prepreg storage life compare for freezer versus room temperature conditions?

A: Properly frozen prepreg at -18°C can last 6-12 months before the resin advance affects properties. Room temperature storage typically allows 1-3 months depending on the formulation.