What is Thermal Decomposition (Td) in PCB?A Comprehensive Guide for PCB Engineers and Designers

If you’ve spent any time working with PCB materials, you’ve probably encountered the term “Thermal Decomposition” or “Td” on a datasheet. But what does it actually mean for your design and manufacturing process? As someone who has dealt with countless delamination issues and material failures over the years, I can tell you that understanding Td is absolutely crucial for building reliable circuit boards, especially in today’s lead-free soldering environment.

Thermal Decomposition (Td) represents one of the most critical thermal parameters in PCB laminate selection, yet it’s often overlooked in favor of the more commonly discussed Glass Transition Temperature (Tg). This comprehensive guide will walk you through everything you need to know about Td, from its fundamental definition to practical applications in your design workflow.

What is Thermal Decomposition (Td)?

Thermal Decomposition Temperature, commonly abbreviated as Td, is the temperature at which a PCB laminate material begins to chemically decompose. According to IPC-TM-650 Method 2.4.24.6, Td is defined as the temperature at which the material loses at least 5% of its original mass weight. Unlike the Glass Transition Temperature (Tg), which represents a reversible physical change, thermal decomposition is an irreversible chemical process that permanently damages the material.

When a PCB substrate reaches its Td, the polymer chains in the resin system begin to break down. This breakdown releases volatile compounds and causes the material to lose structural integrity. The result? Your board starts falling apart at a molecular level, and there’s no going back.

How Thermal Decomposition (Td) is Measured

The industry-standard method for measuring Td is Thermogravimetric Analysis (TGA), as specified in IPC-TM-650 Method 2.4.24.6. Here’s how the process works:

1. A sample of the laminate material (typically 10-30 mg) is prepared by removing all copper cladding
2. The sample is placed in a TGA instrument and its initial mass is recorded at room temperature (not exceeding 50°C)
3. The sample is heated at a controlled rate of 10°C per minute from ambient temperature to 550°C under nitrogen atmosphere
4. The instrument continuously monitors the sample’s mass throughout the heating process
5. Two key temperatures are recorded: Td (2%) when 2% mass loss occurs, and Td (5%) when 5% mass loss occurs

The Td (5%) value is the commonly reported specification on material datasheets. Some manufacturers also report Td (2%), which provides an earlier warning threshold for the onset of decomposition.

Thermal Decomposition (Td) vs Glass Transition Temperature (Tg)

One of the most common points of confusion in PCB material selection is understanding the difference between Td and Tg. While both are critical thermal parameters, they represent fundamentally different phenomena:

Parameter	Tg (Glass Transition)	Td (Decomposition)
Type of Change	Physical (phase transition)	Chemical (decomposition)
Reversibility	Reversible	Irreversible
Typical Range	130°C – 280°C	300°C – 400°C+
What Happens	Material softens, CTE increases dramatically	Polymer chains break, material loses mass and integrity
Test Method	DSC or TMA	TGA (IPC-TM-650 2.4.24.6)
Primary Concern	Dimensional stability, CTE mismatch	Material degradation, delamination

The key takeaway here is that Tg represents a transition point where material properties change temporarily, while Td represents a point of no return. Your design should operate well below Tg for normal use, and your manufacturing processes should stay well below Td to prevent permanent damage.

Why Thermal Decomposition (Td) Matters for Your PCB Design

Understanding Td is critical for several reasons that directly impact your product’s reliability and manufacturing success:

Lead-Free Soldering Requirements

The shift to lead-free soldering has made Td more important than ever. Traditional tin-lead solder melts at around 183°C, with typical reflow peak temperatures of 210-245°C. Lead-free solders (like SAC305) require peak temperatures of 240-270°C, putting significantly more thermal stress on your PCB materials.

Here’s the problem: conventional FR-4 materials often have Td values around 300-310°C. While this sounds like adequate margin, studies have shown that materials begin losing resin mass well before reaching the 5% threshold. At lead-free soldering temperatures, standard FR-4 can lose 1.5-3% of its resin content, which can cause delamination, void formation, and long-term reliability issues.

Multiple Reflow and Rework Cycles

Modern PCB assembly rarely involves just a single pass through the reflow oven. Consider a typical double-sided SMT assembly with through-hole components:

• First reflow cycle for top-side SMT components
• Second reflow cycle for bottom-side SMT components
• Wave soldering for through-hole components
• Potential rework operations for defective joints

Each thermal cycle brings the material closer to its decomposition threshold. This cumulative thermal damage is why materials with higher Td values (340°C+) are often specified for complex assemblies. Research indicates that materials with Td around 300°C frequently experience problems with lead-free soldering, while materials with Td of 400°C typically perform without issues.

Delamination Prevention

Delamination is one of the most common failure modes in PCB manufacturing, and Td plays a significant role. As the PCB approaches its decomposition temperature during soldering, the resin begins to break down and release volatile gases. This creates internal pressure between layers, which can cause separation at the weakest interfaces.

The reflow oven temperatures of 240-250°C required for lead-free solder can generate enough internal pressure to separate layers when combined with trapped moisture. This is why pre-baking boards before assembly and selecting appropriate Td materials is essential for preventing delamination.

Td Values for Common PCB Materials

Not all PCB materials are created equal when it comes to thermal decomposition resistance. Here’s a comparison of common materials and their typical Td values:

Material Type	Tg (°C)	Td (°C)	Typical Applications
Standard FR-4	130-140	300-310	Consumer electronics, low-cost applications
Mid-Tg FR-4	150-160	320-330	General purpose, moderate thermal demands
High-Tg FR-4	170-180	340-360	Lead-free assembly, automotive, industrial
Polyimide (Kapton)	250-280	360-400	Flex PCBs, aerospace, high-temp environments
BT Resin/Epoxy	180-210	350-380	IC substrates, high-density interconnects
Ceramic (Al2O3)	N/A	>800	Power electronics, extreme environments

Important Note: Interestingly, a higher Tg doesn’t always mean a higher Td. Some low-end FR-4 materials with Tg of 140°C may actually have higher Td values than certain high-Tg (170°C) versions. Always check both specifications when selecting materials.

How to Select PCB Materials Based on Thermal Decomposition (Td)

Selecting the right material involves considering several factors beyond just the Td value itself. Here’s my recommended approach:

Assess Your Assembly Process

1. Identify your soldering method: Lead-free reflow requires higher Td materials than traditional tin-lead processes.
2. Count thermal cycles: More reflow passes and rework potential means you need higher Td margins.
3. Consider peak temperatures: Your Td should be at least 50-60°C above your maximum process temperature.

Evaluate Operating Conditions

• High ambient temperature applications (automotive, industrial) need higher Td margins
• Power electronics with high heat dissipation require careful thermal analysis
• Products with long service life need extra reliability margin

Balance Cost vs Performance

Higher Td materials typically cost more and may require modified fabrication parameters. High-Tg substrates are generally more rigid and brittle, which can affect drilling efficiency and increase drill bit wear. Find the sweet spot that meets your reliability requirements without unnecessarily inflating costs.

Read more about raw material’s Parameter

What Happens When You Exceed Thermal Decomposition Temperature

Exceeding the Td threshold, even briefly, can cause several serious problems:

Delamination and Layer Separation

As the resin decomposes, volatile compounds are released creating internal pressure. This pressure can force layers apart, especially at interfaces between different materials (copper-to-laminate, prepreg-to-core). Delamination can occur between surface layers (visible as blistering) or internally (detectable only through cross-sectioning or acoustic microscopy).

Degraded Signal Integrity

The loss of resin material changes the dielectric constant and impedance of controlled impedance traces. For high-speed designs, even small changes in dielectric properties can cause signal integrity issues, including impedance discontinuities, increased crosstalk, and higher insertion loss.

Structural and Mechanical Weakness

Decomposition weakens the bond between glass fibers and resin matrix, reducing overall mechanical strength. This makes the board more susceptible to warping, cracking, and handling damage. In severe cases, the board may become too fragile to survive automated assembly equipment.

Outgassing and Contamination

Volatile compounds released during decomposition can contaminate solder joints and component surfaces. This is particularly problematic for fine-pitch components and ball grid arrays (BGAs), where outgassing products can prevent proper solder wetting or create voids in solder joints.

Best Practices for Managing Thermal Decomposition (Td) in PCB Design

Based on years of experience and industry best practices, here are my top recommendations for managing Td in your pcb designs:

Material Selection Guidelines

• For lead-free assembly, specify materials with Td ≥340°C minimum
• For high-reliability applications (automotive, aerospace, medical), target Td ≥360°C
• Always consider Td alongside Tg, CTE, and moisture absorption for a complete picture
• Request material certificates that include both Td (2%) and Td (5%) values

Manufacturing Process Control

• Pre-bake boards before assembly to remove absorbed moisture (typically 4-6 hours at 105-125°C)
• Optimize reflow profiles to minimize time above liquid temperature
• Control peak temperatures within ±5°C of target
• Monitor and document thermal profiles for each board design

Design Considerations

• Distribute heat-generating components to avoid thermal hotspots
• Use thermal vias to help conduct heat away from sensitive areas
• Consider the cumulative effect of multiple reflow cycles in your design margin
• For thick boards or high layer counts, increase Td margin to account for longer heat-up times

Testing and Quality Control for Thermal Decomposition

Several test methods can help verify that your PCB materials and processes are compatible with your thermal requirements:

Test Method	IPC Standard	Purpose
TGA	IPC-TM-650 2.4.24.6	Determine decomposition temperature (Td)
T260/T288	IPC-TM-650 2.4.24.1	Time to delamination at 260°C or 288°C
Thermal Stress	IPC-TM-650 2.6.8	Solder float testing for PTH reliability
Reflow Simulation	IPC-TM-650 2.6.27	Convection reflow assembly simulation
DSC	IPC-TM-650 2.4.25	Glass transition temperature and cure factor

Useful Resources and Reference Materials

Here are some valuable resources for further research on thermal decomposition and PCB materials:

Industry Standards and Specifications

• IPC-TM-650 Test Methods Manual – Available at electronics.org/test-methods
• IPC-4101E – Specification for Base Materials for Rigid and Multilayer Printed Boards
• IEC 61189-2-807 – Decomposition Temperature (Td) using TGA

Material Databases and Manufacturer Resources

• IPC Shop (shop.ipc.org) – Purchase official IPC standards
• Isola Laminate Materials – Detailed datasheets with Td values
• Rogers Corporation – High-frequency material specifications
• Panasonic Electronic Materials – Megtron series datasheets
• DuPont (Kapton) – Polyimide material data

Technical Documentation

• Sierra Circuits Blog – PCB substrate and material guides
• Altium Resources – IPC slash sheet explanations
• I-Connect007 – Industry news and technical articles

Frequently Asked Questions About Thermal Decomposition (Td)

1. What is a good Td value for lead-free PCB assembly?

For lead-free assembly with peak reflow temperatures of 240-270°C, you should select materials with Td values of at least 340°C. For high-reliability applications or boards requiring multiple reflow cycles, target Td values of 360°C or higher. Materials with Td around 300°C have been shown to experience problems during lead-free soldering, while materials at 400°C typically perform without issues.

2. Can I use standard FR-4 for lead-free soldering?

Standard FR-4 (Tg 130-140°C) with Td around 300-310°C can technically survive lead-free reflow, but it’s not recommended for production use. The margin is too thin, and you risk delamination, especially with multiple thermal cycles. For lead-free assembly, always specify high-Tg FR-4 (Tg 170°C+, Td 340°C+) or better materials. The small additional cost is insignificant compared to the reliability improvement.

3. How does moisture affect thermal decomposition?

Moisture trapped in the PCB laminate can significantly increase the risk of delamination during thermal exposure. When the board is heated above 100°C, absorbed moisture turns to steam and creates internal pressure. This pressure, combined with weakened resin bonds as the temperature approaches Td, can cause catastrophic layer separation. Always pre-bake boards that have been stored in humid conditions (4-6 hours at 105-125°C) before assembly.

4. What’s the relationship between Tg and Td?

Tg and Td are independent material properties, though they’re both determined by the resin chemistry. A higher Tg doesn’t automatically mean a higher Td. Some materials achieve high Tg through additives that actually lower Td. Conversely, some standard Tg materials have excellent Td performance. Always check both specifications and consider them together with CTE when selecting materials for your application.

5. How many reflow cycles can my PCB survive?

The number of safe reflow cycles depends on your material’s Td, T260/T288 values, and the actual thermal profile used. Industry guidelines suggest that materials with T260 of 60 minutes and T288 greater than 10 minutes can withstand at least 12 solder cycles without delamination. However, each cycle uses some of the material’s thermal budget, and cumulative damage builds up. For safety, design your process to minimize thermal cycles and always maintain proper moisture control.

Conclusion

Thermal Decomposition (Td) is a critical parameter that every PCB engineer and designer needs to understand. With the widespread adoption of lead-free soldering and increasing demands for reliability in automotive, aerospace, and industrial applications, selecting materials with appropriate Td values has never been more important.

The key points to remember are: Td represents an irreversible chemical breakdown of the material, unlike Tg which is a reversible physical transition. For lead-free assembly, always specify materials with Td of 340°C or higher. Consider the cumulative effect of multiple thermal cycles on your material. And always evaluate Td alongside Tg, CTE, and moisture absorption for a complete thermal reliability picture.

By following the guidelines and best practices outlined in this article, you’ll be well-equipped to select appropriate materials and design robust manufacturing processes that avoid thermal decomposition-related failures. Your boards will thank you, and so will your customers.