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What is Prepreg in PCBs?

Introduction

Prepreg, also referred to as pre-impregnated composite fibers, is a key material used in the manufacture of multilayer printed circuit boards (PCBs). It is a fibrous reinforcement that has been pre-impregnated with a resin system prior to laminating PCB layers together.

Prepregs provide electrical isolation between the conductive copper layers in a PCB while also offering mechanical stability. They impart crucial properties like dimensional stability, bond strength, heat resistance and dielectric performance. Selecting the right prepreg material is critical for the reliability of multilayer PCBs used in electronics.

This article provides an overview of PCB prepregs covering their composition, properties, types, role in the PCB fabrication process and methods for quality control testing.

Composition of PCB Prepregs

Prepregs used for PCB fabrication are generally composed of:

  • Fiberglass fabric reinforcement
  • Epoxy or other polymer resin systems
  • Filler particles like silica
  • Catalysts and accelerators
  • Flame retardants

Fiberglass Fabric

The fiberglass fabric provides mechanical strength and dimensional stability. Some types used are:

  • 108, 211 fiberglass with 5-15 μm diameter fibers
  • Woven fabric with different weave styles
  • Non-woven random fiber mats

Resin System

The resin coats and bonds together the fibers. Different resin types used are:

  • Epoxy – Most common for FR-4 grade prepregs
  • Polyimide – For high temperature PCBs
  • BT (Bismaleimide Triazine) – Low moisture absorption
  • Cyanate ester – High frequency, low loss PCBs

Fillers

Fillers like fused silica reduce the coefficient of thermal expansion (CTE) of the composite to match copper. They also control flow, adhesion, thermal conductivity and cost.

Other Components

Small amounts of curing agents, accelerators, flame retardants and other additives tailor the prepreg properties.

Properties of PCB Prepregs

Desirable properties in prepregs for quality PCBs:

Dielectric Constant – Stable dielectric constant and low dissipation factor for high frequency performance. Values of 3.5-5 typical for glass fabric-epoxy prepregs.

Dimensional Stability – Low Z-axis shrinkage and expansion under temperature cycling to avoid via failures. Typical X-Y shrinkage < 0.5%.

Bond Strength – Strong adhesion to copper foils and between prepreg layers with good resistance to delamination. Bond strength > 2.5 N/mm.

Decomposition Temperature – High Td above 300°C indicates stable thermal performance and prevents charring issues.

Moisture Absorption – Low affinity for moisture prevents electrical leakage and reliability issues due to vapor pressures. <0.5% uptake standard.

Flow and Filler – Adequate resin flow to fill irregularities but not excessive to avoid shorts between conductors. Filler loading around 15-40%.

Flame Retardancy – High degree of flame resistance with resistance to ignition, burning and smoke generation. UL 94 V-0 rated material.

Types of PCB Prepregs

Many varieties of prepregs cater to different PCB performance requirements:

TypeKey Characteristics
FR-4 EpoxyMost widely used. Good processability. Low cost. Temperature rating 130°C (Tg 135-155°C)
High Tg EpoxyTg > 170°C for higher heat resistance. Ex: BT epoxy, Allied Signal Arlon 85N
PPO (Polyphenylene oxide)Low loss and stable Dk for RF/microwave. Improved Z-axis CTE. Ex: Rogers TMM 10i
Cyanate EsterLowest loss, tightly controlled Dk for high freq. Ex: Taconic TacPreg CE
PTFE CompositeExtremely low Dk ~2.2. Low loss. Ex: Rogers RO3003
PolyimideVery high Tg >240°C, low Z-axis expansion. Ex: DuPont Kapton VN, Hitachi PI-2525
Non-woven AramidHigh bond strength. Low CTE for large boards. Ex: Isola Preperm P92

The choice depends on thermal, electrical and high frequency requirements.

Role of Prepreg in PCB Fabrication

Prepreg performs several crucial functions in the PCB fabrication process:

1. Bonding Layers

Prepreg is used to bond the inner copper layers into a multilayer board through lamination under heat and pressure.

2. Electrical Isolation

It provides electrical isolation between two conductive copper sheets or internal planes while still allowing conduction through drilled holes.

3. Inner Layer Core

Prepreg-copper sheets also serve as the starting cores during the layup of multilayer PCBs.

4. Serving Solder Mask

Special prepreg with photo-sensitive resin can act as the solder mask layer on outer surfaces.

5. Imparting Rigidity

The composite material lends mechanical strength and rigidity to the PCB structure.

6. Controlling Z-axis CTE

Low expansion prepregs control growth along thickness to avoid reliability issues.

7. Providing Heat Resistance

High Tg prepregs allow PCB operation at elevated temperatures.

8. Establishing Dielectric Constant

The resin-fiber matrix governs the dielectric constant of the insulating PCB layers.

PCB Prepreg Handling Considerations

To maintain quality, prepregs require careful handling:

  • Prepregs are supplied sealed in polythene bags to prevent moisture absorption. Unused prepreg should be resealed immediately.
  • Storage temperature should be 18-25°C. Freezing conditions can ruin the material while excessive heat can cause partial curing.
  • Shelf life is typically 1 year at 23°C. Prepregs should be used on a FIFO basis before expiry.
  • Allow refrigerated prepreg to reach room temperature before opening to prevent moisture condensation.
  • Avoid excessive exposure to UV light which can initiate polymerization. Use under yellow light.
  • Use clean lint-free gloves when handling to avoid contamination of the tacky surfaces.

Proper prepreg handling as per manufacturer guidelines ensures consistent performance.

PCB Prepreg Testing

Some key tests carried out on prepreg materials for quality control are:

Resin Flow Testing

  • Resin viscosity measured at standard temperature and pressure
  • Tested after partial curing for specified duration
  • Ensure adequate but controlled flow during lamination

Gel Time Test

  • Monitoring resin polymerization at set temperature
  • Inflection point taken as gel time
  • Checks adequate processing window before curing

Differential Scanning Calorimetry

  • Monitors heat flow versus temperature to detect transitions
  • Gives resin glass transition temperature, cure peaks
  • Indicates degree of curing with heat ramps

Dielectric Constant

  • Measured over frequency range like 10 MHz – 10 GHz
  • Indicates electrical insulation capability
  • Stable value desired through frequency spectrum

Flammability Testing

  • UL 94 vertical burning test rating for flammability
  • High priority for safety critical PCB applications

Prepreg received from supplier is also tested before PCB usage.

Conclusion

Prepreg forms the fundamental dielectric building block of multilayer PCBs that electrically isolates the conductive layers while also imparting mechanical strength. The composition of the resin, fabric reinforcement and fillers that make up prepreg governs the thermal, electrical and reliability performance of the PCB. With the diversity of prepreg types available, PCB manufacturers can select the optimal material suited for their performance requirements and process compatibility. Strict handling and quality control measures for prepreg also minimize variability and ensure consistent results during PCB fabrication. As PCBs become thinner and operate at higher frequencies and temperatures, engineering the prepreg materials is critical for enabling their miniaturization and reliability.

Frequently Asked Questions

Q1. How does prepreg material selection influence the fabrication process of multilayer PCBs?

Some ways prepreg properties affect multilayer PCB fabrication are:

  • Flow viscosity during lamination cycle time and temperature
  • Tg impacts the curing required and potential rework
  • Low thermal expansion suits large area boards
  • High filler suits complex topographies
  • Adhesion levels affect press parameters
  • Moisture sensitivity influences handling
  • Dielectric properties affect impedance control

Q2. What are some troubleshooting tips for issues with PCB prepreg?

Troubleshooting tips for prepreg related PCB issues:

  • Delamination or blistering – Poor adhesion, high moisture absorption
  • Cracking or crazing – Excessive resin flow, filler settlement
  • Poor dimensional stability – High Z-axis CTE material
  • Wicking or shorts – Insufficient resin viscosity
  • High dielectric constant – Incompatible resin system
  • Poor heat resistance – Low Tg material used

So proper prepreg selection, handling and process controls are vital.

Difference between PCB Core and Prepreg Materials

Prepreg Core is the isolating material of the PCB, often referred to as before laminating. Manufacturers mostly use Prepreg as a binding substance. Not just that, experts also employ it as a multifunctional PCB inner conduction material. Thus, once the user laminates the Prepreg & extrudes the half-healed epoxy, it solidifies and runs and binds the multilayer panels together to create a dependable insulator.

When the distinction between both Core and Prepreg is evident, what specific property should you use? When plate, etching, and drying, how do crucial electrical characteristics change? As more manufacturers are aware of the work at GHz frequency range, these principles are vital to size traces of these materials appropriately and prevent difficult signal integrity difficulties.

WHAT IS PREPREG MATERIAL?

Prereg PCB is a conductive substance that planners pack to provide the appropriate insulate between the copper and a core or dual Core of a PCB. Prereg is a level of isolation. Since a copper sheet and a core may be bonded, it may be called a binding material safely. Users may also customize Prereg as special insulators to their demands. A chemical technique may also be used to turn a given section of a Prereg into a conducting zone by combining appropriate catalytic substances and additives.

WHAT IS PCB CORE MATERIAL?

PCB CORE VS PREPREG MATERIALS

The PCB core is a hard foundation material coated on one or two aspects with copper. The CORE is employed in the fabrication of single-sided and two-sided boards and in the manufacture of PCBs of sharing arrangements.

The PCB center consists of the FR4 elements of titanium epoxy laminates and copper traces. The Prereg connected the layers and the PCB core when users heated there.

Experts responded to the Core as the core panel and also to the Core of PCB production. It has a certain copper, width, and firmness of double bread. Its multilayer board is a mixture of Prereg and Core.

HOW IS PREPREG MADE?

Prepreg is a fiberglass fabric/towel coated with a resin preservative, as the name implies. The glass strands are knitted into a glass tissue. This glass fiber fabric is half-dried into material from the B-stage.

Most prepregs are substances of the B-stage. It is vital to track the volume fraction of the material used throughout the production process Prepreg, as it enables the material to be adjusted according to the demand. The number of repetitions of warp and fill affects the epoxy the glass weave can hold.

HOW WE CAN CHOOSE PREPREG MATERIAL:

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Depending on the size and other needs, many kinds of prepregs are integrated into PCBs. According to its resin composition, Prepregs are compatible with standard resin (SR), mid resin (MR), or high resin (HR). The more and more resin it holds, the more costly it becomes.

CORE AND PREPREG MATERIALS:

With clear structural differences in the core prepreg materials, it is critical from a circuit design perspective to get an exact value for the electrical conductivity and the return loss. If your signals have minimal growth time, you can usually get away using a user information sheet value. Once your knee and analog signals have reached the GHz range, attention must be taken with the datasheet values, particularly when modeling interconnect behavior and employing resistance-controlled routing.

The difficulty with data source numbers is that the true electrical conductivity determined is based on the test technique, geometric routing, particular frequencies, convention on the law, and the thickness of the material. John Coonrod spoke extensively about this subject in a recent interview. The fabric pattern of PCB core/prepreg components renders them inhomogeneous and anisotropic, meaning that the main material qualities change in space and various directions. This is why we get fiber weaving phenomena such as excitation of the skew and fiber cavity.

You could wonder, why should a laminate density define the material characteristics? The reason is that the important parameter characterizing the signal behavior is a useful electrical conductivity, which relies on your material’s trace size and layer thickness. 

Finally, copper texture on a particular laminate is the second key characteristic to be considered. The above two studies Will present efficient dielectric constant calculations for the geometries of microstrip and strip lines without the assumption of copper ruggedness. There is, however, a straightforward linear approach to compensate for copper rawness:

Suppose you are working at really high speeds and high frequencies and want very precise connection characterization. In that case, the best choice is to produce a test coupon and utilize standard measurements to calculate the functional dielectric constant. A topology that roughly resembles your anticipated interconnection geometry should be used in your test procedure. This requires some effort at the front, but precise testing and measuring might save you needless prototype runs in the rear.

Suppose you pick from a variety of PCB core versus prepreg material, the Altium Designer.   You will have access, or you may define special material characteristics for exotic substrates, to a component library that offers crucial information on various standardized materials. These capabilities enhance your efficiency and enable you to adjust your design to very particular applications.

THE DIFFERENCE BETWEEN PCB CORE VS PREPREG:

PCB cores and coatings are comparable yet extremely distinct in certain aspects. Your Core is one or more prepreg panels crushed, hardened, and heat-cured, and the Core is covered on each side with a copper foil. The resin is injected with the prepreg material, wherein the resin is solidified but left untreated. Many companies define Prepreg as the duct tape that ties the core components together; the exposure to heat allows the resin to start binding to the next layers when 2 cores are placed at either edge of a prepreg lamellate. The solidified resin cures slowly bypass, and its resultant material characteristics approximate the core layers.

The resin substance contains a glass fabric. The production procedure for this glass fabric is quite similar to that for the fabrication of yarns. The glass fabric may be pretty narrow (e.g., 7638 prepreg) or flexible (e.g., 1081 prepreg), manufactured by the weaver. Any flaws and general yarn uniformity will decide the magnetic characteristics responsible for the scatter, loss, and fiber tissue impact of the signals in the circuit.

According to the resin amount, resin variety, and glass fabric, core PCB vs. prepreg materials may have a slightly differing refractive index. This may be a concern if boards need to be designed that match extremely exact impedances since the absorption coefficient observed on a track relies on the dielectric constants of the material resulting. Not even all prepreg and core materials are mutually compatible, and core/prepreg stacks with widely varying dialectal constants make predicting accurate dielectric constants and inefficiencies in an interconnection problematic.

Every PCB core or prepreg material has a problem with high voltage creeping and leakage current. Copper electromigration and further expansion of conductive filaments are a cause for the FR4 material creeping criteria. This difficulty drove a move to – anti (Non-DICY) in FR4 Prepreg and coatings, albeit with a want to improve the transition of the glass and breakdown temperatures. Phenolic resins offer greater breakdown and transition temperatures than DICY resins and increase isolation resistance upon complete curing.

We can interpret it this way, and The Prepreg is a PCB isolating material. Prepreg shall not be Prepreg before encapsulation, also known as Prepreg. It shall be utilized mainly as a connecting material and an isolating material for a multilayer integrated circuit board’s inner guiding pattern. After lamination, the semi-curated epoxy is ejected, starts to flow, reinforces the multilayer sheets, and produces a durable insulator.

The Core is the essential material for the production of PCB. The Core is also referred to as the core panel, which has some toughness, depth, and double copper bread. Therefore, Core and Prepreg are a mixture of the multilayer board. The distinction between the two:

  • PCB material prepreg and the older material is semi-solid, cardboard-like, the later hard, copper-like,
  • Prepreg is a sticky + dielectric, and the Core is the fundamental PCB material; two functions are entirely distinct;
  • The Prepreg may be curved, not bent to Core;
  • Prepreg is not capacitive; the copper layer between both sides of the Core is a permeable print board.

Final Thoughts

Prereg is a must and not simply a key aspect of the PCB production process when multilayer is included. Without Prereg, manufacturers will have no substance to hold the multiple layers together. The Core and Prereg are the other two elements of the PCB. This Core contains traces of copper as a substance FR4 prepreg. The Core, meanwhile, holds the PCB via Prereg.

 

 

 

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