Introduction
Managing heat dissipation is critical for reliable and efficient PCB design. The choice of printed circuit board substrate material has a major impact on thermal performance. Materials like FR4, ceramic, metal core PCBs, and Rogers laminates each provide different thermal conductivity properties.
This article compares the thermal characteristics of common PCB substrate types. We’ll examine the composition, thermal conductivity, dielectric properties, design considerations, and typical applications for each material class. With an understanding of material thermal capabilities, PCB designers can select the optimal approach for their thermal management needs.
Thermal Conductivity Basics
Thermal conductivity indicates how effectively a material transports heat. It is defined as:
K = Q x L / A x ΔT
Where:
K = Thermal conductivity in W/m-K
Q = Heat flow rate in Watts
L = Material thickness in meters
A = Material cross sectional area in m^2
ΔT = Temperature difference across material in Kelvin
Materials with higher thermal conductivity (K) values conduct heat more efficiently across a temperature gradient.
For PCBs, thermal conductivity directly impacts how much heat from components and traces flows into the board substrate, as well as spreading across the substrate internally. This heat must ultimately be dissipated to avoid temperature buildup.
FR4 Thermal Performance
FR4 glass reinforced epoxy is the most common and cost effective rigid PCB substrate. However, it suffers from poor thermal conductivity:
Composition
FR4 consists of layers of fiberglass fabric reinforcements impregnated with an epoxy resin binder. The glass fibers provide mechanical rigidity while epoxy gives adhesion and environmental resistance.
Typical multilayer FR4 PCB construction
The glass content typically ranges from 30% to 70% by weight depending on thickness. Higher fiber content increases thermal conductivity but reduces dielectric strength.
Thermal Conductivity
The thermal conductivity of FR4 is highly anisotropic due to the woven fiberglass layers:
- In-plane (X-Y direction) = 0.25 – 0.30 W/m-K
- Through-plane (Z direction) = 0.10 – 0.20 W/m-K
The in-plane conductivity is 2-3X higher than through thickness. But even in-plane, FR4 is a very poor conductor compared to metals or ceramics.
Design and Performance Challenges
The limited through-plane conduction hinders heat spreading from hot components into the board:
FR4 exhibits poor thermal spreading from localized heat sources
High power components can experience considerable temperature rise with FR4, risking reduced reliability and performance. Adding thermal vias helps somewhat but is limited by low through-thickness conductivity.
Typical FR4 Applications
Despite thermal constraints, FR4 excels in many applications:
- Consumer electronics
- Computers and peripherals
- Automotive electronics
- Cost-driven designs
- Moderate temperature environments
FR4 provides the most affordable PCB substrate for many commercial and industrial products not needing advanced thermal performance.
Ceramic PCB Thermal Performance
Ceramic printed circuit boards provide an order of magnitude better thermal conductivity than FR4, though at higher cost:
Composition
Ceramic PCB substrates consist of aluminum oxide (alumina) or aluminum nitride ceramic filler particles combined with a glassy binding matrix. This produces a crystalline ceramic material.
Common material grades include:
- Alumina (Al2O3) – 92% to 99% purity
- Aluminum nitride (AlN) – Over 90% purity
A metal layer (copper, aluminum) typically clads the ceramic base to enable circuit patterning.
Aluminum nitride ceramic PCB
Thermal Conductivity
The thermal conductivity of ceramic PCB materials far surpasses traditional FR4:
- Alumina: 24 – 30 W/m-K
- Aluminum nitride: 170 – 250 W/m-K
This enables efficient lateral and vertical heat spreading. The high purity minimizes defects that would impede heat transfer.
Design and Performance
The excellent thermal conduction allows heat to spread across a larger board area:
Aluminum nitride ceramic exhibits superior thermal spreading
Ceramic PCBs can dissipate very high component heat fluxes while maintaining lower temperatures. The rigid structure also resists warping.
Typical Ceramic PCB Applications
The cost of ceramic material relegates usage to high performance and reliability applications:
- High power electric vehicle inverters and charging
- Power supply converters
- Industrial power and control
- RF and microwave boards
- Hybrid microelectronics
- Aerospace and defense electronics
Ceramic PCBs support the demands of power electronics and microwave circuits where thermal and dielectric performance are critical.
Metal Core PCB Thermal Performance
Metal core PCBs utilize a thick metal base layer to maximize heat spreading and thermal mass:
Composition
Metal core PCBs contain a 0.8mm to several mm thick aluminum or copper layer at the core. Dielectric material layers bond circuit layers to the metal foundation.
Typical metal core PCB construction
Common dielectric layers include polyimide films, epoxy composites, and ceramic substrates. Direct Bonded Copper (DBC) integrates a ceramic layer.
Thermal Conductivity
The metal base provides exceptional thermal conduction:
- Aluminum: 150 – 230 W/m-K
- Copper: 400 W/m-K
This enables large heat quantities to be absorbed and spread over the entire board. The thermal mass also promotes lateral conduction.
Design and Performance
Metal core PCBs manage the most extreme thermal loads:
Metal core heat spreading and sinking
The thick metal base acts as both a heat spreader and heatsink, ideal for high power densities. Heat can also be removed via the backside.
Typical Metal Core PCB Applications
The cost and specialized fabrication restricts metal core PCBs to very high power products:
- Power converters and inverters
- Motor drives
- Power amplifiers
- High brightness LED lighting
- Wind and solar power electronics
- Electric vehicle drives
If thermal performance is the paramount priority, metal core PCBs conduct heat extremely efficiently.
Rogers Laminates Thermal Performance
Rogers Corporation (Chandler, AZ) produces premium PCB substrate materials focused on high frequency applications. Many of Rogers’ circuit materials also offer improved thermal conductivity over FR4 while being cost-effective for high volume commercial products:
Composition
Rogers’ materials utilize fluoropolymer resin systems reinforced with glass fabric. Some products incorporate ceramic filler particles to enhance thermal performance.
Typical Rogers laminate PCB construction
Popular materials include RO4350, RO4003, RT/duroid 6202, and LoPro series laminates. They exhibit a good balance of electrical and thermal properties not found in FR4.
Thermal Conductivity
Rogers’ circuit materials span a wide thermal conductivity range:
- RO4350 LoPro: 1.5 W/m-K
- RO4003C LoPro: 0.69 W/m-K
- RT/duroid 6202PR: 1.57 W/m-K
- RT/duroid 6035HTC: 1.7 W/m-K
These values approach ceramic substrate performance levels while using conventional PCB processes.
Design and Performance
Rogers laminates manage heat dissipation effectively while supporting high speed signal designs:
Rogers materials allow strong in-plane heat spreading
Lower thermal resistance improves power handling for a given temperature rise. The coefficient of thermal expansion is also compatible with common FR4 and copper.
Typical Rogers Laminate Applications
The electrical and thermal properties suit Rogers materials to:
- High frequency analog and RF PCBs
- High speed digital designs
- Power amplifiers
- LED lighting
- Automotive circuits
- Motor drives
- Semiconductor testing boards
Rogers PCB substrates balance dielectric, thermal, and economic factors for commercial electronics needs.
Thermal Performance Comparison
Here is a summary comparison of the thermal conductivity properties of common PCB substrate types:
| Material | Composition | Thermal Conductivity |
|---|---|---|
| FR4 | Epoxy + fiberglass | 0.25 W/m-K (in-plane) |
| Ceramic PCB | Alumina or AlN | 25 – 250 W/m-K |
| Metal core PCB | Aluminum or Copper core | 150 – 400 W/m-K |
| Rogers Laminates | Fluoropolymers | 0.7 – 1.7 W/m-K |
This demonstrates the wide range of material options depending on required conductivity levels.
Conclusion
Managing PCB thermal performance requires selecting substrate materials suited to heat dissipation needs:
- FR4 provides a low cost PCB material but suffers thermally
- Ceramic PCBs offer 10-100X better thermal conductivity
- Metal core PCBs maximize spreading and heat sinking
- Rogers laminates balance thermal, electrical, and cost factors
Understanding the composition, thermal characteristics, and typical applications guides appropriate material selection. Thermal modeling informs necessary conductivity levels and guides design tradeoffs.
With the right PCB substrate materials in their toolkit, engineers can develop designs meeting their exact thermal management and performance requirements.
FQA
How does thermal conductivity relate to temperature rise for a given heat dissipation level?
Thermal conductivity determines how efficiently heat transfers through a material. Higher conductivity reduces thermal resistance, lowering component junction temperatures for the same power level.
What are the main reasons higher performance PCB materials like ceramics cost more than FR4?
Reasons include more expensive base materials, thinner substrates requiring tighter process controls, lower fabrication yields, and more specialized fabrication equipment and methods.
When does it make sense to use a more expensive ceramic or metal core PCB instead of standard FR4?
Applications warranting premium materials are RF/microwave boards, power electronics, high brightness LEDs, and other products requiring removal of high heat fluxes that exceed FR4 capabilities.
What are some ways to enhance FR4 thermal conductivity?
Adding more copper layers or planar copper increases conductivity marginally. Thermal vias help conduct heat between layers. But large heat sinks are often still required with FR4.
How does dielectric constant correlate with thermal conductivity in PCB substrates?
Lower dielectric constant materials like fluoropolymers or ceramics generally have higher thermal conductivity than epoxy/glass composites. But other factors like filler contents also affect conductivity.
