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
- Porcelain ceramic substrate
- Preferred for high frequency applications
- Examples – Alumina, Aluminum nitride, Beryillia
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
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.
Composition
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
| Parameter | Rating |
|---|---|
| Dielectric Constant | 4.2-4.5 (FR-4) |
| Loss Tangent | 0.02 max |
| Dielectric Strength | 400-600 V/mil |
| Moisture Absorption | 0.2% max |
| Thermal Conductivity | 0.3 W/m-K |
Advantages
- Low material cost
- Easy production and drilling
- Good mechanical strength
- Flame resistance
Disadvantages
- 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.
Composition
- 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
| Parameter | Alumina | AlN | Beryillia | FR-4 |
|---|---|---|---|---|
| Dielectric Constant | 9.9 | 8.8 | 6.7 | 4.3 |
| Thermal Conductivity | 30 | 170 | 260 | 0.3 |
| CTE | 7 | 4.3 | 7.4 | 17 |
| Frequency (GHz) | 15+ | 20+ | 18 | <5 |
| Loss Tangent | 0.0003 | 0.0005 | 0.0004 | 0.02 |
Advantages
- Extremely low signal loss
- High thermal conduction
- Near zero moisture absorption
- Fine geometry patterning
Disadvantages
- 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
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
Quartz
- 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
