FR4 PCB Material Full Analysis: Properties, Grades (TG130/TG150/TG170) and Applications Explained
In PCB (Printed Circuit Board) design and manufacturing, FR4 is undoubtedly the most common and widely used substrate. Whether you are a hardware engineer, PCB procurement specialist, or electronics enthusiast, gaining an in-depth understanding of FR4 material properties and grade classification is an important prerequisite for ensuring product reliability and optimizing cost. This article provides a comprehensive interpretation of this “universal” PCB material from three dimensions: FR4 fundamental properties, Tg value classification, and typical application scenarios.
1. What Is FR4 PCB Substrate?
FR4 stands for Flame Retardant Grade 4, a glass fiber epoxy copper-clad laminate. It accounts for more than 75% of global PCB substrate usage and follows the IPC-4101 international laminate specification.
Three-layer core structure
- Reinforcement substrate: E-grade electronic glass fiber fabric, providing mechanical rigidity and bending resistance
- Bonding resin: Brominated flame-retardant epoxy resin, achieving UL94 V-0 self-extinguishing fire resistance and insulation
- Conductive layer: Electrolytic copper foil (0.5oz~4oz), laminated on both sides of the board to form conductive circuit pathways
Meaning breakdown of the name
- FR = Flame Retardant: Self-extinguishing within 10 seconds under flame, no dripping ignition risk
- 4 = Flame retardant grade standard: Different from paper-based FR1/FR2 and cotton-paper FR3 low-end laminates, suitable for industrial long-term use
2. Core Comprehensive Properties of FR4 PCB (Electrical / Thermal / Mechanical / Chemical)
2.1. Electrical properties (core of circuit stability)
- Dielectric constant Dk: 4.2~4.7 (at 1GHz), stable signal propagation delay, suitable for IoT modules such as Wi-Fi, LoRaWAN, Bluetooth (ESP32 main control boards commonly use this substrate)
- Dielectric loss Df: ≤0.02, low loss for low-to-mid frequency signals, sufficient for consumer and industrial control applications
- Volume resistivity: >10¹³ Ω·cm, dielectric breakdown strength 20~50 kV/mm, ensuring reliable insulation safety under high and low voltage
- Impedance control: Supports precise differential impedance design, suitable for high-speed digital boards and RF sensor PCBs
2.2. Thermal properties (Tg, CTE, Td — three key parameters)
- Tg glass transition temperature: The core classification parameter. It is the critical temperature at which resin transitions from a rigid glass state to a rubber-like elastic state. Beyond Tg, the board expands, softens, strength drops sharply, and failures such as delamination, via fracture, and layer separation may occur
- CTE (thermal expansion coefficient): Includes XY-plane and Z-axis expansion. Z-axis expansion directly affects via and microvia reliability. Higher Tg results in lower Z-axis CTE and better dimensional stability at high temperature
- Td thermal decomposition temperature: FR4 typically 300–350°C, meeting lead-free reflow peak temperature (260°C) requirements
- Thermal conductivity: 0.3~0.4 W/m·K, relatively poor heat dissipation capability. High-power boards require thermal vias, thick copper, or aluminum substrates for assistance
2.3. Mechanical and processing properties
- Flexural strength: 400~600 MPa, resistant to deformation and vibration, suitable for automotive and outdoor industrial equipment
- Water absorption: only 0.1%~0.2%, minimal degradation of insulation in humid environments
- Strong processing compatibility: supports drilling, laser microvias, lamination, etching, ENIG, HASL and full manufacturing processes; 2~30 layer multilayer boards can be produced
2.4. Chemical and safety properties
- UL94 V-0 flame retardant
- Resistant to weak acids, weak alkalis, and flux corrosion
- Conventional brominated FR4 has lower cost; halogen-free FR4 is used for new energy and medical environmental compliance requirements
3. FR4 Three Tg Grades: TG130 / TG150 / TG170 Full Comparison
Industry standard classification: Standard Tg (TG130), Mid Tg (TG150), High Tg (TG170). The core differences lie in thermal resistance, thermal expansion, multilayer capability, and cost.
Table
| Parameter | FR4 TG130 (Standard Tg) | FR4 TG150 (Mid Tg) | FR4 TG170 (High Tg) |
|---|---|---|---|
| Actual Tg range | 125~135°C | 145~160°C | ≥170°C (170~180°C) |
| Z-axis CTE (above Tg) | 60~80 ppm/°C | 40~50 ppm/°C | <30~40 ppm/°C |
| Reflow cycles | 1~2 (leaded soldering) | 2~3 (basic lead-free) | ≥4 (multiple rework, lamination cycles) |
| Suitable PCB layers | 2~8 layers simple boards | 4~12 layers general multilayer | 8~30 layers high-layer / HDI microvia boards |
| High temperature deformation risk | High, prone to warping | Medium | Very low |
| Cost premium | Baseline (lowest) | +10%~15% | +18%~25% |
| Main drawback | Fails in lead-free process | Insufficient long-term 120°C+ stability | Higher material and process cost |
Detailed grade breakdown
1. FR4 TG130 Standard FR4
The most basic general-purpose substrate, with the largest market share, fastest lead time, and highest cost performance.
- Thermal limitation: above 130°C resin softens rapidly, high Z-axis expansion
- Only suitable for traditional leaded soldering (peak 230°C)
- Not suitable for multiple rework, high-layer boards, or long-term high-temperature operation
- Lead-free reflow easily causes via cracking and delamination
2. FR4 TG150 Mid Tg balanced substrate
Currently the mainstream choice for consumer electronics and IoT devices, balancing cost and thermal reliability.
- Performance improvement: Tg increased by ~20°C, significantly reduced Z-axis expansion
- Stable for lead-free reflow (255~260°C peak)
- Suitable for limited rework and 4~10 layer multilayer boards
- Ideal for ESP32, LoRa modules, Wi-Fi routers, smart home PCBs
- Limitation: long-term operation above 125°C still risks aging and delamination
3. FR4 TG170 High Tg high reliability substrate
Industrial-grade high-temperature laminate defined by IPC. High cross-link density resin provides excellent thermal stability, widely used in new energy and automotive electronics.
- Core advantage: maintains rigidity below 170°C, excellent dimensional stability
- Low Z-axis expansion, supports multiple reflow cycles and thermal shock resistance
- T288 delamination time >15 min
- Reliable for HDI microvias and fine-pitch BGA soldering without warpage
- Suitable for high-layer boards, thick copper power boards, and harsh thermal environments
- Meets IATF16949 automotive and military-grade reliability requirements
4. FR4 vs Other Common PCB Materials (Key Selection Reference)
In actual PCB design, FR4 is not the only option. Different application scenarios (high frequency, heat dissipation, flexibility, high reliability) require different substrates.
4.1. FR4 vs Rogers high-frequency materials (RO4003 / RO4350)
| Item | FR4 | Rogers |
|---|---|---|
| Dielectric constant Dk | 4.2~4.7 (unstable) | 2.2~3.5 (stable) |
| Loss factor Df | ≤0.02 | 0.001~0.004 |
| High frequency performance | Significant loss above 1GHz | Suitable for 10GHz~100GHz |
| Cost | Low | High (3~10×) |
| Application | General electronics | 5G, radar, RF antennas |
Conclusion: FR4 is suitable for low/mid-frequency circuits; Rogers is used for RF and microwave applications.
4.2. FR4 vs CEM-1 / CEM-3 (low-cost boards)
| Item | FR4 | CEM-1 / CEM-3 |
|---|---|---|
| Substrate | Glass epoxy | Paper + glass composite |
| Strength | High | Medium-low |
| Layer capability | 2~30 layers | Single/double layer |
| Cost | Medium | Lower |
| Thermal resistance | Good | Poor |
Conclusion: CEM materials are used for low-end electronics (toys, LED lights); FR4 is industrial-grade mainstream.
4.3. FR4 vs Polyimide (PI)
| Item | FR4 | PI |
|---|---|---|
| Flexibility | Rigid | Flexible |
| Temperature resistance | ≤170°C (high Tg) | 200~400°C |
| Application | Rigid PCB | Flexible circuits (FPC) |
| Cost | Low–medium | High |
Conclusion: FR4 is for rigid structures; PI is for flexible circuits or extreme high-temperature environments.
4.4. FR4 vs Aluminum substrate (MCPCB)
| Item | FR4 | Aluminum PCB |
|---|---|---|
| Thermal conductivity | 0.3~0.4 W/m·K | 1~10 W/m·K |
| Heat dissipation | Weak | Strong |
| Application | Signal/control boards | LED power, high-power drivers |
| Cost | Low | Medium |
Conclusion: FR4 is for signal boards; aluminum substrates are for high-power heat-generating devices.
5. Application Guidelines by Tg Grade
5.1. TG130 applications (low-cost room temperature devices)
- Low-end toy PCBs
- Simple power adapters
- Door control switch boards
- Prototype boards, student labs
- Simple LED controller boards
Restrictions: not allowed for lead-free, multilayer, or high-temperature environments.
5.2. TG150 applications (consumer electronics / IoT mainstream)
Used in about 80% of consumer smart devices:
- IoT: ESP32 boards, LoRaWAN nodes, Wi-Fi/Bluetooth sensors
- Smart home: smart plugs, cameras, routers, sensors
- Consumer electronics: Bluetooth earphones, power banks, set-top boxes
- Light industrial: PLC modules, sensors, small inverters
5.3. TG170 applications (high reliability / automotive / industrial)
(1) Automotive electronics (mandatory high Tg)
- ECU engine controllers
- EV battery BMS
- Vehicle DC-DC power boards
- ADAS systems, dashboards, sensors (-40°C ~ 125°C cycles)
(2) Industrial automation & power systems
- Inverters, servo drives
- Furnace controllers
- Solar inverters, 5G base stations
- Thick copper high-power multilayer boards
(3) High-end communication & HDI boards
- 8+ layer high-speed signal boards
- HDI microvia boards
- Server control boards
- BGA precision boards
(4) New energy, medical, aerospace low-mid systems
- Energy storage systems
- Medical instruments
- Aerospace control boards
6. PCB Engineer FR4 Tg Selection Pitfall Guide
Lead-free reflow soldering must not use TG130:
The peak temperature of lead-free reflow soldering (260°C) is far above the Tg of 130°C. In mass production, this will cause large-scale board bursting and scrap failures. For limited budgets, TG150 should be prioritized; for high-temperature conditions, TG170 should be used directly.
Layer count > 8 must use TG170:
Multilayer boards undergo multiple lamination cycles and repeated high-temperature processes. Low Tg materials have large interlayer stress differences and are prone to delamination.
Automotive and outdoor sealed devices require minimum TG170:
Environmental temperature fluctuations are large. Low Tg materials may suffer via cracking and circuit open failures after long-term aging.
LoRaWAN / ESP32 IoT terminals generally use TG150:
For indoor constant-temperature environments and 2–4 layer boards, TG150 balances cost and soldering yield. Outdoor solar-powered LoRa data acquisition boards should be upgraded to TG170.
Cost optimization logic:
Non-high-temperature consumer products should use TG150 and avoid blindly upgrading to TG170 to increase material cost. Automotive-grade, power electronics, and multilayer projects must not downgrade material selection to save cost.
7. Limitations of FR4
Although FR4 is widely used, it is not a universal material and has clear limitations in certain scenarios:
High-frequency / high-speed application limitations:
FR4 has relatively high and unstable dielectric constant and relatively large loss tangent. In GHz-level high-frequency signal transmission, signal attenuation is severe. High-speed signals above 10Gbps typically require low-loss substrate materials instead.
Limited heat dissipation capability:
The thermal conductivity of FR4 is only about 0.25 W/m·K, making it insufficient for high-power device heat dissipation requirements.
Long-term high-temperature reliability:
Although high-Tg FR4 improves thermal resistance, FR4 is still an organic material. In extreme high-temperature environments, its long-term reliability is inferior to ceramic substrates or polyimide materials.
8. FR4 PCB Cost and Pricing Influence Factor Analysis
Although FR4 is a standardized PCB material, its price is affected by multiple factors. Different Tg grades and structural designs lead to significant cost differences.
8.1. Price range of different FR4 Tg grades
(Based on common industry 2–4 layer PCBs)
| Material grade | Relative price | Cost characteristics |
|---|---|---|
| TG130 | Baseline (lowest) | Simple process, mature material |
| TG150 | +10% ~ +15% | Mainstream consumer electronics standard |
| TG170 | +18% ~ +30% | High reliability, high-temperature resin system |
Note:
The higher the Tg, the more complex the resin system, the narrower the lamination process window, and the higher the yield control cost.
8.2. Key factors affecting FR4 PCB cost
(1) Tg grade
Tg is one of the core cost drivers:
Higher Tg → higher resin cross-link density → increased material cost
At the same time, higher lamination temperature → increased manufacturing cost
(2) Copper thickness (Copper Weight)
Common specifications:
- 0.5 oz (low-cost signal boards)
- 1 oz (standard mainstream)
- 2 oz / 3 oz (power boards)
The thicker the copper:
- Higher cost
- More difficult etching
- Better thermal dissipation capability
(3) Layer count
| Layer count | Cost impact |
|---|---|
| 2 layers | Baseline |
| 4 layers | +30% ~ +50% |
| 6–8 layers | +80% ~ +150% |
| 10+ layers | Exponential increase |
Reasons:
- More layers → more lamination cycles
- Higher alignment precision required
- Increased defect rate
(4) Via technology (HDI / microvias)
- Standard through-hole: low cost
- Buried/blind vias: medium to high cost
- HDI laser microvias: high cost (+30% ~ +100%)
(5) Surface finish process
| Process | Cost | Application |
|---|---|---|
| HASL (hot air solder leveling) | Low | General boards |
| ENIG (electroless nickel immersion gold) | Medium–high | BGA / high reliability |
| OSP | Low | One-time assembly |
8.3. FR4 PCB cost structure breakdown
Typical PCB cost composition:
- Material cost (FR4 + copper foil): 30% ~ 50%
- Manufacturing (drilling / etching / lamination): 30% ~ 40%
- Surface finishing: 10% ~ 20%
- Testing and yield loss: 5% ~ 15%
8.4. Cost optimization selection recommendations
- Non-high-temperature products → prioritize TG150 (best cost-performance ratio)
- Do not blindly choose TG170 (unless automotive / industrial requirement)
- Low-cost consumer electronics → TG130 + 1 oz copper is sufficient
- High-frequency / high-speed signals → do NOT try to save cost using FR4 (losses will be greater)
9. Summary
As the “main substrate” of the PCB industry, FR4 occupies an irreplaceable position in consumer electronics, industrial control, automotive electronics, and communication equipment due to its excellent comprehensive performance, mature manufacturing process, and good cost efficiency.
The Tg value is the core parameter for FR4 classification, directly determining thermal resistance, dimensional stability, and reliability. Understanding the differences and application scenarios of TG130, TG150, and TG170 is fundamental knowledge for every PCB engineer.
FAQ
Q1: Is higher FR4 Tg always better?
A: No. Higher Tg increases material procurement cost. TG150 is sufficient for standard indoor consumer electronics; only high-temperature, multilayer, and automotive applications require TG170. Blindly selecting high Tg increases unnecessary cost.
Q2: Should ESP32 LoRaWAN gateway PCBs use TG150 or TG170?
A: Indoor constant-temperature gateways should use TG150. Outdoor solar-powered, sealed metal enclosure, poor heat dissipation, or long-term sun-exposed LoRa acquisition nodes should use TG170 FR4.
Q3: Can TG130 be used for lead-free soldering?
A: Not recommended. Lead-free reflow peak temperature (260°C) is far above 130°C glass transition temperature. Under high temperature, Z-axis expansion is severe, and mass production defect rate (delamination, board bursting) exceeds 30%.
Q4: What are the main advantages of high-Tg FR4 (TG170) compared to standard materials?
A: Lower Z-axis thermal expansion coefficient, higher thermal resistance threshold, ability to withstand multiple reflow cycles, improved resistance to thermal cycling warpage and delamination, and significantly improved HDI and BGA soldering reliability. It is suitable for harsh industrial and automotive environments.
Q5: How to choose between FR4, aluminum substrate, and polyimide (PI)?
A: FR4 is used for general signal and low-to-medium power circuits; aluminum substrates are used for high-power LED and heat-intensive power boards; PI materials are used for flexible circuits and ultra-high-temperature military-grade applications.
