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What is the Core In a PCB?

A printed circuit board (PCB) relies on an electrically insulating core between layers of conductive copper to prevent short circuits. The core forms the foundation upon which all other PCB elements get built upon.

The core material choice significantly impacts costs, performance, manufacturability and reliability in electronics. Engineers balance tradeoffs between mechanical, electrical and thermal properties during selection.

This article provides a comprehensive overview of the various types of cores used in PCBs and their relative advantages.

Functions of the PCB Core Layer

The core layer fulfills several critical roles:

Physical Support

  • Creates a firm substrate for component mounting and copper traces
  • Maintains shape and dimensional stability of PCB throughout product lifecycle

Electrical Isolation

  • Prevents shorting between high speed, high power traces
  • Allows routing of traces on both sides of core without interference

Heat Dissipation

  • Conducts heat away from hot components to keep junction temperatures in check
  • Prevents damage to surrounding fragile parts from excessive temperatures

Environmental Protection

  • Forms barrier against humidity, contaminants reaching electronic components
  • Reinforces PCB rigidity to withstand shock, vibration, flexing damage

Hermetic Sealing (for some material types)

  • Encases electronic modules in airtight enclosure
  • Used heavily in military, aerospace, and medical applications

With power density and performance demands rising each product generation, the core plays an increasingly vital role.

Classification of Core Materials in PCBs


PCB cores can be grouped into three broad material categories:

Organic Cores

  • Base is organic polymer resin system
  • Most common core type
  • Examples – FR-4, CEM-1, FR-5, G-10

Inorganic Cores

Specialty Cores

  • PTFE (Teflon), Quartz glass, Woven fiberglass
  • Feature exceptional thermal or dielectric characteristics
  • Useful for niche applications

Next, we explore the composition, manufacturing process, and relative pros and cons of each core type.

Organic PCB Core Materials

fr4 Epoxy Resin

Organic cores constitute the majority of PCB substrates thanks to their low cost and ease of processing. FR-4 stands as the most popular grade today.


Organic cores contain:

  • Thermoset polymer resin (epoxy, cyanate ester, polyimide)
  • Reinforcement material (woven glass fabric)
  • Fillers (silica)

Flame retardant additives give the FR designation.

Manufacturing Process

Fabrication involves:

Impregnation – Glass fabric gets soaked in liquid state resin

B-stage – Resin viscosity increases to solid yet pliable state

Lamination – Multiple prepreg layers are stacked, compressed under heat/pressure to fully cure resin

Cooling – Crosslinking continues growing polymer chains as temperature gradually lowers

Machining – Sheets get cut to standard PCB panel sizes

Properties and Characteristics

Dielectric Constant4.2-4.5 (FR-4)
Loss Tangent0.02 max
Dielectric Strength400-600 V/mil
Moisture Absorption0.2% max
Thermal Conductivity0.3 W/m-K


  • Low material cost
  • Easy production and drilling
  • Good mechanical strength
  • Flame resistance


  • Anisotropic expansion coefficient
  • Degrades at high temperatures
  • Higher loss tangent above 1 GHz

Organic cores tradeoff electrical and thermal performance for affordability. They serve lightweight, cost driven commercial and consumer electronics well up to 6-8 layer designs.

Ceramic PCB Core Materials

Inorganic ceramic substrates boast exceptional high frequency characteristics thanks to higher rigidity, thermal conductivity and near hermetic sealing.

Alumina substrates reign most popular due to lower cost than Beryillia or Aluminum Nitride. Dimensional stability proves extremely precise.


  • Alumina (Al2O3) – 96%
  • Beryllia (BeO) – 97%+
  • Aluminum Nitride (AlN) – 85%+

They may also contain small percentages of silica, oxides and proprietary additives.

Manufacturing Process

Ceramic production utilizes thin film technologies:

Tape Casting – Slurry poured onto smooth surface, rolled to precise thickness

Blanking – Individual sheets punched into shapes

Registration – Sheets collated, layered & laminated

Co-firing – Baked at high heat to form solid ceramic

Metalization – Vacuum deposition applies patterned copper

Properties and Characteristics

Dielectric Constant9.
Thermal Conductivity301702600.3
Frequency (GHz)15+20+18<5
Loss Tangent0.00030.00050.00040.02


  • Extremely low signal loss
  • High thermal conduction
  • Near zero moisture absorption
  • Fine geometry patterning


  • Difficult machining/drilling
  • Sensitive to thermal shock
  • High substrate cost (10-30x organic cores)

Alumina, AlN, and Beryillia cores enable electronics operating well into microwave frequencies for aerospace systems and wireless infrastructure. The ceramic production process also lends itself towards embedding components or channels within the core.

Specialty Core Materials for PCBs

Multilayer Metal Core PCB

Certain applications require exceptional thermal performance, low dielectric constant or tight impedance control that standard cores cannot satisfy. This spurred development of high cost specialty materials.

Polytetrafluoroethylene (PTFE)

  • Brand names – Teflon, RT/duroid 5870/5880
  • Consists of fluoropolymer resin matrix
  • Features extremely low dielectric constant and loss tangent
  • Poor thermal conduction
  • Mainly used in mm-wave multi-layer boards above 30 GHz


  • Extremely rigid glass-ceramic made of silicon dioxide
  • Nearly zero water absorption
  • Outstanding dimensional stability
  • Low CTE provides unmatched thermal shock resistance
  • Difficult processing restricts use to niche applications

Woven Fiberglass

  • Ultra-thin glass strands woven into mesh cloths
  • Impregnated with epoxy resin
  • Tailorable dielectric constant and thermal expansion traits via weave patterns
  • Primarily found in flex circuits, rigid-flex boards

While cost prohibits mainstream adoption, these specialty cores impart capabilities beyond traditional materials for extreme performance envelopes.

Key Selection Criteria for PCB Core Materials

Ideally, engineers select cores to fulfill performance needs at the lowest feasible cost. This requires balancing tradeoffs between:

Dielectric Constant

  • Controls signal speed, impedance, crosstalk
  • Lower values desired for high speed designs

Loss Tangent

  • Signal loss drops with lower loss tangents
  • Important for RF microwaves, millimeter waves

Thermal Conductivity

  • Cores with higher k values dissipate heat faster from hot components
  • Reduces junction temperatures

Coefficient of Thermal Expansion (CTE)

  • Matching CTE between core, copper, components prevents warping, delamination issues

Moisture Resistance

  • Superior moisture resistance maintains stable electrical performance

Frequency Range

  • Dielectric and loss traits vary over operating frequency spectrum

Fabrication Factors

  • availability, manufacturability, drillability affect cost

Weighing the above parameters against application requirements and cost targets helps drive proper core material selection decisions tailored for every PCB design scenario.

PCB Copper Core Types Comparison

Copper plays the vital role of forming the conductive traces in circuit boards. Enabling higher trace densities proves essential to keep pace with electronics innovation. Semiconductor advancements expand copper usage given their superior signal carrying capacity.

Copper Foil Composition

Electrodeposited copper foil contains:

  • Copper (99%+) – carries signal currents




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