Printed circuit boards (PCBs) are essential components in almost all modern electronic devices. The core materials that make up PCBs include the base laminate (core), copper foil, and prepreg layers. Selecting the right combination of core, foil, and prepreg is critical to achieving the desired performance, reliability, and cost targets in PCB fabrication. This article provides an in-depth look at the key properties, manufacturing processes, and selection criteria for PCB core, foil, and prepreg materials.

Contents

PCB Core Materials

The core material forms the basic foundation of the PCB providing mechanical support. The core defines important parameters like dielectric constant (Dk), loss tangent (Df), thermal conductivity, CTE, Tg, etc. Common types of core materials include:

FR-4

FR-4 glass epoxy is the most common and cost-effective core material used in majority of PCBs. The key properties of FR-4 include:

• Dk value around 4.5 at 1 GHz
• Df value around 0.02 at 1 GHz
• Tg of 130-140°C
• Good CTE match with copper
• Flame retardant (FR) rating to meet UL94 V-0
• Good mechanical properties

The glass transition temperature (Tg) of 130-140°C allows FR-4 to withstand soldering temperatures. The flame retardant rating and excellent copper adhesion makes it suitable for most general purpose PCB applications.

FR-4 is constructed by impregnating woven fiberglass cloth with an epoxy resin formulated with brominated fire retardants. The resin is cured at high temperatures to form a rigid laminate with the fiberglass cloth embedded. FR-4 offers a good balance of electrical, mechanical, thermal, and fire resistance properties at a relatively low cost.

High Tg FR-4

For PCBs requiring higher temperature resistance, high Tg FR-4 cores are available with glass transition temperatures of 170°C or above. These high Tg cores use modified epoxy formulations to increase the Tg while retaining UL94 V-0 flammability rating.

The key benefits of high Tg FR-4 include:

• Increased thermal capability for lead-free soldering
• Improved thermal cycling reliability
• Higher maximum operating temperature

High Tg cores are more expensive than standard FR-4 but required for many modern PCBs subjected to lead-free soldering temperatures around 260°C. They prevent warpage or delamination during soldering and provide better thermal cycling reliability.

Halogen-free FR-4

Halogen-free or halogen-reduced FR-4 laminates replace brominated flame retardants with phosphorus-based additives to achieve UL94 V-0 rating. Key benefits include:

• Environmentally friendly and safer than brominated FR-4
• Produces less toxic fumes in case of fire
• Complies with environmental legislation prohibiting brominated flame retardants

The drawbacks of halogen-free FR-4 are somewhat higher cost and lower Tg compared to standard FR-4. The Tg is typically in the 130-140°C range. With process refinements, next-generation halogen-free FR-4 offers Tg values approaching 170°C.

High Frequency/High Speed

For high frequency analog and digital PCBs, specialized low-loss laminates are required. These include PTFE-based cores like Rogers RO4000 series and hydrocarbon/ceramic-filled cores like Megtron 6, Nelco N4000-13SI, and Arlon CLTE-XT.

Key properties include:

• Low Dk of 3.0 or less to minimize dielectric losses
• Low Df of 0.004 or less for optimal signal integrity at high frequencies
• Low moisture absorption for stable electrical performance
• Matched Dk and low Df over a wide frequency range

The caveat is that these materials are significantly more expensive than FR-4. They also require specialized fabrication processes due to high melting points and lower CTE.

Metal Core

Metal core PCBs contain a base metal layer like aluminum or copper instead of FR4 laminate. The key benefits of metal core PCBs include:

• Excellent thermal conductivity to dissipate heat from components
• Low CTE for reliability in high temperature conditions
• Good EMI/RFI shielding
• High structural strength

The base metal layer is insulated by bonding thin dielectric layers on both sides. This allows creating a PCB with copper layers on one or both sides of the metal core. The dielectric layers can be standard or high-performance materials.

Metal core PCBs are heavier than FR4-based PCBs. They are used in applications like LED lights, power electronics, and avionics requiring thermal management. The fabrication process requires specialized steps like metal surface treatments.

Copper Foil for PCBs

Copper foil provides the conductive layers for traces, pads, vias, and planes in a PCB. Two main types of copper foils are used:

Rolled Copper Foil

Rolled copper foil is produced by mechanically rolling and annealing electrolytically refined copper. It provides excellent ductility for PCB fabrication. Key properties include:

• Thicknesses from 35 μm to 70 μm
• Excellent ductility and adhesion to laminates
• Good etching characteristics
• Low cost
• Used for standard multilayer PCB construction

The rolling process results in some crystalline elongation which reduces conductivity compared to pure copper. Grain structure also leads to slight surface roughness.

Electrodeposited Copper Foil

Electrodeposited copper foil is plated onto a drum surface to produce a very pure, smooth foil with uniform thickness. Key properties are:

• Thicknesses from 3 μm to 35 μm available
• Exceptional smoothness – RMS roughness below 0.3 μm
• Very high purity – conductivity up to 105% IACS
• Tight thickness tolerances
• Good for fine line PCBs

The smooth surface improves bonding to laminates reducing the risk of lifted traces and pads. The high purity provides higher conductivity resulting in lower losses for high frequency applications.

Electrodeposited foil is more expensive than rolled foil due to more complex manufacturing. It is widely used for multilayer boards with fine features and lead-free processing.

Prepreg for PCBs

Prepreg sheets are B-stage epoxy resin reinforced with glass fabric. Prepreg is used to bond the copper foil to core laminates when making multilayer PCBs. Key properties include:

FR-4 Prepreg

• Compatible with FR-4 cores
• Good flow and fill characteristics for bonding layers
• Standard Tg around 140°C
• UL94 V-0 flame retardant rating
• Low cost
• Suitable for most multilayer boards

FR-4 prepreg contains brominated flame retardants to achieve V-0 rating. It provides the same Dk,Df values as FR-4 cores for good impedance match.

High Tg Prepreg

High Tg prepreg contains modified resin systems to achieve Tg values of 170°C and above to withstand lead-free soldering processes. Benefits include:

• Improved reliability under lead-free soldering thermal excursions
• Allows high Tg cores to realize their full thermal capability
• Enables high layer count boards

High Tg prepregs are more expensive than standard FR-4 prepreg but critical for lead-free compatible PCBs.

Halogen-free Prepreg

Halogen-free prepregs use phosphorus-based chemistry to achieve flame retardance instead of brominated compounds. This makes them environmentally preferable for certain applications.

Many halogen-free prepregs have lower Tg values around 130°C and higher costs compared to brominated systems. However, new formulations are improving thermal performance.

High Frequency Prepreg

Low loss prepreg materials are designed to complement high frequency core materials. Key attributes include:

• Ultra-low loss: Df values below 0.002
• Stable electrical properties over frequency
• Low moisture absorption
• Matched Dk to high frequency cores

These prepregs enable excellent signal integrity in high speed designs over 10+ GHz. The cost is 3-10X higher than FR-4 prepreg.

Selecting Cores, Foils, and Prepregs for PCBs

Here are some key considerations when selecting PCB material sets:

Frequency Range

• Up to 5 GHz: Standard FR-4 cores and prepreg with rolled foil
• 5-10 GHz: Low loss materials for core, prepreg, foil
• Above 10 GHz: Ultra-low loss cores, prepregs, and electrodeposited foils

Board Thickness

• Thin boards below 0.062″: Minimum core thickness with thinner copper foils
• Thick boards above 0.125″: Thicker cores and foils to minimize warpage

Reliability Requirements

• Standard environment: FR-4 material system
• Harsh environment: Use high Tg cores/prepregs
• Thermal cycling: High Tg and low CTE materials

High Speed Signals

• Match Dk of cores, prepregs, and laminates for optimal impedance control
• Minimize glass weave skew with spread glass fabrics

Fabrication Factors

• Higher layer counts: Thinner cores/foils and high Tg prepregs
• Fine features: Electrodeposited foils and compatible prepregs
• Lead-free: 170°C+ Tg materials. Low CTE.

Cost Targets

• Minimize material costs: Standard FR-4 system
• Performance critical: Use high performance materials where needed, FR-4 elsewhere

Proper selection of PCB materials across core, foil, and prepreg enables achieving the right design tradeoffs across electrical performance, manufacturability, reliability, and cost.

FQA

What are the key functions of the core laminate in a PCB?

The PCB core laminate provides the basic mechanical structure and foundation for building the board. It defines key electrical parameters like dielectric constant, loss tangent, and impedance control. The core also determines thermal and fire resistance properties.

What is the difference between FR-4 and halogen-free FR-4 laminates?

FR-4 uses brominated flame retardants to achieve UL94 V-0 fire safety rating. Halogen-free FR-4 replaces these brominated compounds with phosphorus additives to be environmentally friendlier. However, halogen-free cores tend to have lower Tg and higher cost currently.

Why is high frequency PCB fabrication more complex?

High frequency materials like PTFE composites have higher melting points, lower CTE, and poorer adhesion compared to FR-4. This requires specialized processes for lamination, drilling, and imaging to ensure reliability. The materials are also significantly more expensive.

What are the key benefits of using electrodeposited copper foil versus rolled foil?

Electrodeposited copper foil offers improved thickness consistency, smoothness, and purity compared to rolled foil. This improves fine line imaging and etching. The higher purity also gives slightly improved conductivity. However, electrodeposited foil costs more than rolled.

How does prepreg selection influence the performance of multilayer PCBs?

Prepregs must be properly matched to core materials in terms of Dk, Df, and Tg. If prepreg Tg is too low, the benefits of high Tg cores cannot be realized. Low loss prepreg is essential to complement high frequency core materials.