The printed circuit board (PCB) substrate, also known as the base material, serves as the foundation upon which the entire board is built. The properties of the substrate have a major influence on the performance, manufacturability and reliability of the PCB. With a wide variety of substrate materials available, from FR-4 to high-frequency laminates to ceramic and metal core boards, selecting the optimal material is critical.
This article provides a comprehensive guide on how to select the best PCB substrate material for a given application. It covers:
- Overview of key PCB substrate materials
- Important substrate properties and characteristics
- Matching substrate properties to PCB requirements
- Substrate recommendations for common PCB types
- High frequency vs. low frequency substrate considerations
- Factors driving substrate selection like cost, manufacturability etc.
- Substrate properties comparison charts
- Real-world examples of substrate selection
With the right knowledge, electrical engineers can choose a PCB substrate that maximizes performance, cost-effectiveness, manufacturability and reliability.
Overview of Major PCB Substrate Materials
There are three broad classes of materials used as printed circuit board substrates:
Organic substrates, also known as laminates, use insulating polymer resins reinforced with fiberglass for mechanical strength. Some common types include:
- FR-4 – Flame resistant glass-reinforced epoxy laminate. Most common low-cost substrate.
- CEM-1 – Cotton paper and epoxy composite. Low cost option for low layer counts.
- FR-5 – Woven glass reinforced epoxy with improved thermal performance over FR-4.
- High-frequency laminates – PTFE or hydrocarbon based composites engineered for optimal RF/microwave performance. Rogers 4000® and 6000® series are common.
- Flexible substrates – Polyimide films and laminates enabling flexible PCBs. DuPont Kapton® HN and Sheldahl® flex materials widely used.
Inorganic substrate materials include ceramics and metal base boards:
- Alumina – Aluminum oxide ceramic board. Provides excellent insulation and thermal properties.
- Aluminum nitride – Higher thermal conductivity than alumina with similar electrical insulation.
- Metal core boards – Base metals like aluminum or copper clad with dielectric for insulation. Highly thermally conductive.
Combine properties of organic and inorganic materials:
- Metal-clad laminates – Organic FR-4 or polyimide laminate bonded to thin metal foil. Combines flexibility and shielding.
- Ceramic-filled PTFE composites – Mix ceramic and PTFE to tailor dielectric properties and CTE. Rogers RO3000® series is a common type.
Now that we have outlined the major substrate material options, let’s look at how specific properties affect PCB performance.
Important Properties of PCB Substrate Materials
There are a number of key material properties that influence the choice of PCB substrate for a given application. Some of the most important ones include:
Dielectric Constant (Dk)
- Defines how readily a material transmits electromagnetic waves.
- Lower Dk allows faster electric signal propagation which improves high frequency performance.
- Typical substrate Dk range: 2.5 to 10.8.
Dissipation Factor (Df)
- Indicates how much signal loss occurs in a material.
- Lower Df reduces losses and increases quality factor Q.
- Typical substrate Df range: 0.002 to 0.025.
Coefficient of Thermal Expansion (CTE)
- Indicates the degree to which a material expands with temperature.
- Matching CTE between layers minimizes stresses and risk of delamination.
- Typical substrate CTE range: 17 to 7 ppm/°C.
- Defines how readily heat transfers through a material.
- Higher values enable better thermal management of heat-generating components.
- Typical range: 0.25 to 170 W/m/K.
Glass Transition Temperature (Tg)
- Indicates the temperature where a polymer transitions from rigid glassy state to soft rubbery state.
- Higher Tg improves material stability at elevated temperatures.
- Typical range: 130°C to 280°C.
- Moisture absorption lowers insulation resistance and can reduce reliability.
- Lower values preferred for stable electrical performance.
- Typical range: 0.1 to 5.0%.
- Indicates expansivity perpendicular to the board layers.
- Lower Z-axis CTE minimizes interconnect stresses and via failures.
- Typical range: 30 to 70 ppm/°C.
Let’s look at how we match these substrate properties to the requirements of different types of PCBs.
Matching PCB Substrate Properties to Board Requirements
To select the optimal PCB substrate material, you need to consider the electrical, thermal, mechanical and physical requirements of the specific application and match candidate materials accordingly.
Here is an overview of how substrate properties align with the needs of some common PCB types:
High Frequency RF/Microwave PCBs
- Critical needs – Low Dk, low Df, stable electrical properties, controlled impedance
- Candidate materials – PTFE or ceramic-filled PTFE laminates like Rogers RO3003; Hydrocarbon laminates like Rogers RO4350
High Layer Count Multilayer Boards
- Critical needs – Dimensional stability, low Z-axis CTE, low moisture absorption, high Tg
- Candidate materials – FR-4, high Tg epoxy laminates, PTFE composites
High Power PCBs
- Critical needs – Thermal conductivity, dielectric strength, Tg, CTE match
- Candidate materials – Insulated metal substrate, ceramic-filled laminates, ceramic boards
High Reliability Aerospace & Defense
- Critical needs – Stable electricals, low moisture absorption, high flexibility
- Candidate materials – Polyimide films, quartz-reinforced laminates
- Critical needs – Lead-free compatibility, thermal stability, flame resistance, robustness
- Candidate materials – FR-4, high Tg laminates with UL 94 V-0 rating
- Critical needs – Flexibility, bend radius, chemical resistance, layer bond strength
- Candidate materials – Polyimide films, reinforced flexible laminates
Let’s look at some specific recommendations for common PCB applications.
PCB Substrate Selection Guidelines
Here are some general guidelines on selecting the optimal PCB substrate material for different classes of boards:
Low Frequency/Digital PCBs
- FR-4 – The baseline cost-effective choice. Serves most digital/low freq needs.
- CEM-1 – Very low cost option for simple double sided boards without plated through holes.
- FR-5 – Improves thermal conductivity over FR-4 while retaining UL rating.
RF and Microwave PCBs
- PTFE laminates – Prime choice for >10 GHz. Low Dk and Df. Examples: Rogers RO3003.
- Hydrocarbon laminates – Cost-effective option for moderate RF requirements. Rogers RO4350.
- Ceramic filled PTFE -Tunable electrical properties. Rogers RO3006.
- Quartz glass laminates – Extremely low loss for high freq. Rogers RT/duroid 6006.
High-Speed Digital PCBs
- FR-4 – Primary choice. Serves most digital applications up to GHz range.
- Halogen-free laminates – For environmental compliance requirements. Tachyon 100G.
- Low-loss laminates – Help maximize signal integrity at multi-GHz frequencies. Rogers RO4350.
High Layer Count and HDI Boards
- FR-4 – Widely used for high layer count boards up to 20+ layers.
- High Tg epoxies – Enables lead-free assembly temperatures. Panasonic Megtron 6.
- Low Dk laminates – Minimizes layer-to-layer crosstalk.
High Power and Thermal Management
- Insulated Metal Substrate (IMS) – Metal core PCBs with dielectric coating. Highly thermally conductive.
- Ceramics – High thermal conductivity and temperature resistance. Aluminum nitride or alumina boards.
- Thermally conductive laminates – Enhanced dielectric thermal conductivity. T-lam and metal-clad laminates.
- Polyimide films – Prime choice. Kapton® or Upilex® films enable high flexibility and bend radius.
- Flexible laminates – Reinforced films for better dimensional stability. Dupont Pyralux® AP.
- FR-4 – Widely used thanks to proven reliability, UL rating and lead-free process compatibility.
- High Tg laminates – Withstand higher temperatures. Panasonic Megtron DZ-2.
- Halogen-free laminates – Meet environmental regulations. Isola 370HR.
This provides a good starting point for selecting substrates based on the PCB application. But there are many other factors driving material selection as we will see.
High Frequency vs. Low Frequency Substrate Considerations
Let’s compare some of the key considerations when choosing substrates for high frequency versus low frequency PCBs:
High Frequency RF/Microwave Substrates
- Minimize dielectric constant and loss
- Stable electrical properties over frequency and temperature
- Controlled dielectric thickness and tolerance
- Smooth copper surface
- Thermal expansion matching between layers
- Moderate glass transition temperatures
Low Frequency/Digital Substrates
- Moderate dielectric constants acceptable
- Loss and impedance control less critical
- Focus on cost, manufacturability and reliability
- Lead-free assembly compatibility
- UL flammability ratings
- Higher glass transition temperatures
While FR-4 is suitable for many digital boards, critical RF boards may require advanced low-loss laminates to achieve peak performance. But cost and manufacturability remain important considerations for any substrate type.
Key Factors Driving PCB Substrate Selection
Some of the main considerations when choosing a PCB substrate material include:
- Dielectric constant, loss, breakdown voltage, leakage current
- Impedance control, signal speed, signal integrity
- Frequency range, power handling, Q factor
- Thermal conductivity, maximum operating temperature
- Thermal management of components and traces
- Temperature cycling resistance, thermal shock performance
- Flexural and tensile strength, impact resistance
- Hardness, chemical resistance, abrasion resistance
- Flammability and fire resistance rating
- CTE and mechanical stability
- Moisture absorption, outgassing properties -Thickness and dimensional tolerance
- Board fabrication process compatibility
- Soldering and assembly process compatibility
- Availability, lead times and cost
- Ease of prototyping and fabricating high volumes
- Shock, vibration and acceleration tolerance
- Withstand operating environment conditions
- Lifetime performance under thermal cycling
- Resistance to common failure modes
Understanding these multiple factors helps guide the substrate selection to match design requirements.
PCB Substrate Materials Properties Comparison
To help compare options, here is a table showing key property ranges for some common PCB substrate material types:
|Material||Dk||Df||CTE (ppm/°C)||Thermal Cond. (W/m/K)||Tg (°C)|
This highlights how properties can vary widely between substrate families. The optimal balance depends on the priorities like cost, performance and manufacturability.
Real-World PCB Substrate Material Selection Examples
To help illustrate substrate selection in practice, here are some examples:
Case 1: Cell Phone RF Front End PCB
Case 2: Space Satellite Communications PCB
Requirements: Extreme stability over temp. cycling, non-outgassing, withstand vibration/shock, moderate freq. range
Selected Substrate: Polyimide laminate with quartz reinforcement
Requirements: High thermal conductivity to manage power dissipation, electrical insulation, metal trace patterns
Selected Substrate: Thermally conductive aluminum metal core substrate
Case 4: Automotive ABS Control Unit
Requirements: Lead-free soldering compatibility, flame resistance rating, withstand under-hood temperatures
Selected Substrate: FR-4 with high Tg resin system (>170°C)
This illustrates how substrate selection depends on balancing the key electrical, mechanical, thermal, physical and manufacturing requirements of each unique PCB application.
Frequently Asked Questions about PCB Substrates
Here are some common FAQs related to selecting PCB substrate materials:
Q: What are some of the newest PCB substrate materials available?
Q: How are substrate requirements different for double sided vs multilayer PCBs?
Multilayer PCBs demand tighter control of Dk and Df for impedance control, lower Z-axis CTE to minimize interlayer stress, and dimensional stability for thinner dielectrics.
Q: What materials enable embedded passives and components?
Q: What substrate materials are suitable for rigid-flex PCBs?
Flexible polyimide films bonded to glass-reinforced laminates like FR-4 provide a solid rigid-flex solution. Adhesives and bondplys are used.
Q: How do material costs compare between standard and advanced substrates?
Standard FR-4 is lowest cost. Prices rise moving to high freq laminates, thermally conductive boards, and advanced ceramics.
Selecting the optimal PCB substrate material requires striking the right balance between electrical, thermal, mechanical and manufacturing properties to meet the needs of the particular application. While standard FR-4 will suffice for many digital and low frequency boards, engineers must take a broader perspective and consider advanced materials for more demanding applications.
The PCB substrate directly impacts many aspects of board performance, manufacturability, reliability and cost. Investing time upfront to make the best material choice based on design requirements will maximize the probability of success, avoid unnecessary risk, and prevent costly redesigns.