The substrate, or base insulating material, is a foundational choice when designing a printed circuit board (PCB). The substrate determines key attributes like thermal and mechanical properties, dielectric performance, manufacturability, and cost. Selecting the optimal substrate requires balancing electrical, physical, and budgetary requirements.

This article provides guidance on PCB substrate selection covering:

• Common substrate materials used in PCBs
• Key factors to consider when choosing a substrate
• How substrate properties impact the PCB design
• Recommended substrates for specialized applications
• New substrate developments on the horizon

Understanding PCB substrate options allows designers to make informed selections best suited for their particular design needs.

Overview of Common PCB Substrates

The most prevalent PCB substrate materials include:

FR-4 Glass Epoxy

The most common and cost-effective substrate. FR-4 is a glass fiber reinforced epoxy resin laminate. Provides reasonable performance across thermal, mechanical, and electrical characteristics.

High Tg FR-4

A variant of FR-4 rated for higher glass transition temperature (Tg) making it more thermally stable for lead-free soldering. More expensive than standard FR-4.

CEM-1

Woven cotton paper base impregnated with epoxy resin. Less expensive than FR-4 but has lower Tg rating. Used for cost-sensitive consumer electronics.

Polyimide

A high-performance polymer substrate exhibiting excellent thermal, chemical, and mechanical stability. Used for flex PCBs and high-reliability rigid boards.

PTFE (Teflon)

Known for thermal stability and chemical inertness. Low dielectric losses make it ideal for radio frequency (RF) circuit boards.

Rogers Laminates

A family of high-frequency circuit board materials with precisely controlled dielectric properties ideal for RF and microwave.

Aluminum

An uncovered metal substrate used for thermal management PCBs to conduct heat away from high-power components.

Isola FR408

A leading-edge halogen-free flame retardant epoxy substrate with improved electrical and thermal performance versus FR-4.

There are many other specialty substrate materials, but FR-4, polyimide, ceramic, and Rogers substrates dominate most commercial PCB production.

Factors in Selecting PCB Substrate

Key considerations when specifying a PCB substrate material include:

Dielectric Constant

A lower dielectric constant (Dk) results in faster signal speeds and lower signal loss/crosstalk. Important for high-speed digital or RF.

Dissipation Factor

Tan delta or dissipation factor indicates dielectric losses converting signal energy to heat. Should be minimized.

Thermal Conductivity

Ability to transfer heat away from components needing cooling. Metals excel here.

Coefficient of Thermal Expansion (CTE)

How much the material expands/contracts with temperature fluctuations. Want to match CTE of mounted components.

Glass Transition Temperature

The Tg indicates the temperature where the substrate transitions from rigid to rubbery state. Should exceed soldering/operation temps.

Moisture Absorption

Tendency to absorb atmospheric moisture affecting dielectric performance. Lower is better.

Mechanical Strength

Rigidity and impact resistance. Glass fiber reinforcement improves strength of epoxy.

Chemical Resistance

Ability to withstand corrosive or solvent agents like acids, alkalis, and cleaners without degrading.

Flame Retardance

Resistance to catching fire. Key for safety in consumer electronics.

Cost

Production cost per square foot is a pragmatic consideration for high-volume commercial PCBs.

Ideally, the substrate meets all electrical, thermal, mechanical, and chemical needs, but tradeoffs exist between performance and cost.

Impact of Substrate on PCB Design

Properties of the PCB substrate affect the circuit board layout and performance:

High-Speed Routing

Substrates with lower Dk allow tighter trace spacing for high-speed signals. Dielectrics control crosstalk.

Controlled Impedance Traces

Track width and spacing are calculated based on substrate Dk to match target loaded impedance values.

Thermal Dissipation

Substrates like aluminum or ceramic aid heat transfer from hot chips or power devices.

Stacking and Spacing

High Tg rating allows thinner substrates reducing interconnect lengths between layers.

Component Selection

CTE of substrate should match mounted components to avoid temperature stress.

Board Flexure

Thin flexible boards bend more easily requiring flexible substrates like polyimide.

Layer Count

More layers allow partitioning high-speed routing across layers. Lower loss materials like polyimide facilitate this.

Substrate properties guide many aspects of PCB design – material selection is rarely an afterthought.

Recommended Materials by Application

Certain substrates are preferable for specific PCB applications:

Digital Circuits

FR-4 is suitable for most consumer digital boards not pushing extreme speeds. High Tg FR-4 provides better lead-free soldering margin.

High-Speed Computing

Advanced dielectric materials from Rogers, Taconic, Isola with low Dk and Df manage tightly spaced traces.

RF/Wireless

PTFE, Rogers 4350, and ceramic substrates maintain signal integrity at GHz frequencies.

Automotive

Aluminum or ceramic substrates withstand high temps while providing electrical isolation in underhood environments.

High Power

Thick copper plates on ceramic substrates paired with thermal vias dissipate waste heat.

Flexible Circuits

Polyimide films like Kapton meet the bending and flexing needs of wearable and foldable devices.

Aerospace and Defense

PTFE composites and high-reliability ceramics withstand vibration, shock, moisture, chemicals.

Matching substrate to application maximizes performance, life span, and safety.

Emerging Substrate Technologies

Ongoing research and development aims to improve PCB substrate materials:

• Nano-engineered dielectrics – Control material structure at nanoscale to achieve lower Dk and Df along with enhanced thermal and mechanical characteristics.
• Photoimageable dielectrics – Substrates that become photosensitive when exposed to light enable direct imaging of vias and trenches.
• Direct metallization – Techniques to selectively metallize a dielectric surface without copper etching. Reduces waste.
• Buried capacitance laminates – Create capacitance between power and ground layers embedded inside the PCB substrate.
• Thermal composites – Dispersed ceramic, metal, carbon filler particles enhance thermal conductivity of base substrates.
• Biodegradable epoxies – Develop bio-based resin systems from plant-derived sources to enhance environmental sustainability.

Many avenues exist for substrates to improve with incremental evolutionary gains or more disruptive technologies.

Conclusion

Choosing the optimal PCB substrate is an important design step impacting electrical, thermal, and mechanical performance. FR-4 remains ubiquitous for cost-driven consumer electronics, while RF, automotive, aerospace and other demanding applications use specialized dielectric materials to achieve superior high-frequency, thermal, and reliability characteristics. Emerging substrate technologies will enable continuing advances in PCB miniaturization, power handling, and efficiency.

PCB Substrate Selection FAQ

What are the most common substrate materials used in PCBs?

The most prevalent base materials are FR-4 fiberglass-epoxy composite, polyimide films, ceramic, PTFE composites, and metal cores. Over 80% of PCBs use FR-4 substrates.

What should designers consider when selecting a PCB substrate?

Key parameters are dielectric constant and loss, thermal conductivity, CTE, Tg rating, moisture absorption, mechanical strength, chemical resistance, flammability, and cost.

How does substrate impact high-speed digital or RF PCB performance?

Low Dk substrates with controlled impedances allow tighter trace routing and spacing while minimizing signal loss, reflections, and crosstalk at high frequencies.

When would metal-based substrates be used?

Metals like aluminum or copper are used when thermal dissipation is critical for power electronics, LEDs, power amplifiers and other heat-generating components.

What new substrate materials are emerging?

Nano-engineered dielectrics, direct metallization techniques, embedded capacitance laminates, thermal composites, photoimageable films and bio-based epoxies provide incremental improvements.