High Tg PCB stands for high glass transition temperature PCB. The glass transition temperature (Tg) is an important property of the laminate material that makes up the PCB substrate.
A PCB’s Tg influences factors like heat resistance, mechanical strength, and electrical performance. High Tg PCBs with specially engineered laminates offer improved properties to meet demanding application requirements.
In this article, we’ll cover:
- What PCB Tg means
- Why high Tg is beneficial
- Typical Tg values for standard FR4
- High Tg laminate materials
- BT Epoxy
- Applications using high Tg PCBs
- Considerations when using high Tg boards
- How to specify high Tg materials
- Testing PCB Tg
After reading this guide, you’ll understand the meaning of PCB Tg, why high Tg boards are used, and how to select the right laminate material when high Tg properties are needed.
What Does PCB Tg Mean?
Tg stands for the glass transition temperature of the laminate material used in a PCB. The laminate is the insulating substrate that forms the core layers of the board on which the copper traces are patterned.
The Tg determines the temperature where the laminate transitions from a rigid, glassy state to a soft, rubbery state.
Below Tg, the material is stiff and dimensionally stable. Above Tg, it becomes progressively softer, weaker, and more thermally expansive.
A higher Tg means the material retains its stiff, rigid properties at higher temperatures. This is desirable for many applications, driving the development of high Tg PCB laminates.
Why is High Tg Beneficial for PCBs?
A higher Tg laminate provides several advantages:
- Improved thermal performance – Can withstand higher temperatures without softening
- Enhanced mechanical strength – Stays rigid and withstands stresses at elevated temps
- Reduced expansion – Less thermal growth deformation above Tg
- Higher heat resistance – Better resilience to intense heat exposure
- Superior electricals – More stable dielectric constant and loss tangent at high frequencies/temps
- Higher reliability – Less prone to problems from temperature fluctuations
For applications like aerospace, automotive, and industrial electronics where high ambient temperatures or intense heat loads are encountered, a high Tg PCB minimizes risks of failure.
Typical Tg Values for Standard FR-4
The most common FR-4 laminate material has a Tg around 130-170°C. Some key points:
- Tg can vary based on resin chemistry and fiberglass type
- Lower Tg at 130-150°C range for baseline FR-4 materials
- Enhanced FR-4 formulations offer higher Tg of 170°C range
- Tg declines after multiple reflows due to physical aging
So while standard FR-4 has moderately high Tg, specialized high Tg laminates offer significantly improved thermal and mechanical characteristics.
High Tg PCB Laminate Materials
Many types of High Tg laminates are available. Some common ones include:
- Extremely high Tg of ~260°C
- Excellent thermal and chemical resistance
- Low Z-axis CTE for reduced expansion
- Flexible substrates available
- Cost is moderately high
BT (Bismaleimide Triazine) Epoxy
- High Tg of ~180°C
- Low xy CTE for dimensional stability
- Low moisture absorption
- Good high frequency dielectrics
- Somewhat more affordable than PI
PPO (Polyphenylene Oxide)
- High Tg of ~210°C
- Low loss and stable dielectrics
- Low smoke and flammability
- Copper adhesion can be a challenge
- Costs less than BT Epoxy
LCP (Liquid Crystal Polymer)
- Very high Tg of 280-320°C
- Extremely low loss, stable dielectrics
- Naturally flame retardant
- Excellent copper adhesion
- Primarily used in RF flexible circuits
- Exceptionally high Tg of ~330°C
- Lowest dielectric constant of any laminate
- Chemically inert and non-flammable
- Requires special processing
- Often avoided except when essential
Ceramic Filled PTFE
- Contains ceramic filler to improve thermal conductivity
- Reduces high frequency signal loss versus pure PTFE
- Expensive material requiring special handling
- High Tg along with ceramic filler for thermal conductivity
- Balances electrical, thermal, and cost tradeoffs
With the wide range of high Tg laminates available, designers can select the right fit depending on application requirements, operating environment, performance needs, and budget.
Applications Using High Tg PCBs
Some examples of products that benefit from high Tg PCB materials include:
- Avionics systems in hot areas like engines
- Reduced thermal expansion improves reliability in flight
- Engine control units and powertrain components
- Better stability and lifetime under-hood at high ambient temps
- Process control systems in hot environments
- Test equipment used under extreme temperatures
Military and Defense
- Mission critical electronics in harsh conditions
- Withstands intense heat from explosions or lasers
- Motor drives, converters, and distribution boards
- Handle heat from high power devices
- Signaling and control systems in hot train cars
- Withstands vibrations and repeated thermal cycles
Oil and Gas
- Downhole drilling electronics
- Survives high temperatures in subsurface wells
Wherever electronics must perform reliably despite intense heat, high Tg PCB materials are the ideal solution.
Considerations When Using High Tg Boards
While enabling improved thermal and mechanical performance, high Tg PCBs also have some design and manufacturing considerations:
- Components must also withstand higher temperatures
- May need active cooling in some cases
- Can be more brittle than standard laminates
- Care needed in handling thinner/larger boards
- Often require higher lamination temps and pressures
- Component attach methods must match higher Tg
- Lead times may be longer for some materials
- Verify supply chain availability
- High Tg laminates are more expensive than standard FR-4
- But enables less cooling or insulation needed
With careful design and process adaption, high Tg boards bring major benefits despite small tradeoffs.
How to Specify High Tg Materials
When selecting PCB laminates, designers should:
- Research material options and properties to determine the right fit
- Specify laminate type, thickness, and Tg on fabrication drawings
- Define construction techniques compatible with the Tg
- List acceptable substitutes if a certain laminate is ever unavailable
- Only use high Tg where truly needed based on environment
- Review material coupons and certs from the PCB supplier
Getting the right laminate material requires upfront planning and tight coordination with your board fabrication partner.
Testing PCB Tg
There are a few methods to verify the Tg of a laminate material or finished PCB:
- DMA – Dynamic mechanical analysis tracks stiffness decline versus temperature
- DMTA – Measures dimensional changes with temperature
- DSC – Differential scanning calorimetry detects change in material heat capacity
- TMA – Measures thermal expansion and contraction response
- Dielectric Testing – Identifies Tg based on changes in dielectric properties
Ideally, the PCB supplier performs release testing to certify the correct laminate Tg. But engineers may also utilize these techniques to validate board Tg if needed.
Frequently Asked Questions
Here are some common FAQs regarding high Tg PCBs:
Q: Does high Tg affect PCB fabrication and assembly?
A: It can – higher lamination temperatures/pressures may be required. The assembly process must also accommodate the higher Tg.
Q: What are the downsides or tradeoffs when using high Tg laminates?
A: Potentially higher cost, more brittle boards requiring careful handling, constrained component selection, and sometimes more complex processing.
Q: How much does using a high Tg laminate increase PCB cost?
A: Increases vary based on Tg level and material volumes, but typically 50-100% over standard FR-4 laminates in smaller quantities.
Q: Can components designed for standard FR-4 use be used on high Tg PCBs?
A: Verify components are adequately rated for higher temperatures. Passives generally tolerate high Tg better than many semiconductors and ICs.
Q: Is it preferable to use very high Tg laminates like PTFE for optimal performance?
A: Not necessarily – balance Tg against cost, manufacturability, and other parameters. Use the lowest suitable Tg for the expected environment.
In summary, high Tg PCB laminates enable improved thermal, mechanical, and electrical performance crucial for demanding applications. By selecting the right high Tg material, engineers can create boards that survive intense heat and stress.
However, high Tg PCBs also introduce manufacturing and component compatibility considerations. This requires planning to ensure smooth fabrication and assembly.
With their many advantages and wide material selection, high Tg boards provide an enabling technology for electronics in harsh hot environments. The improved resilience unlocks innovative applications and superior reliability.