The most widely used PCB laminate in the electronics industry. Understand FR-4 dielectric properties, glass weave effects, and when to upgrade to specialty materials for high-speed designs.
FR-4 (Flame Retardant 4) is an epoxy-glass laminate that forms the foundation of over 90% of all PCBs manufactured worldwide. It consists of woven fiberglass cloth impregnated with brominated epoxy resin.
The "FR" designation indicates it meets UL 94 V-0 flame retardancy requirements. FR-4 offers an excellent balance of electrical, mechanical, and thermal properties at a low cost, making it the default choice for most applications.
However, FR-4's inconsistent dielectric properties and relatively high loss tangent make it less suitable for high-frequency and high-speed digital applications, where specialty laminates provide better performance.
Woven fabric provides mechanical strength. Dk ≈ 6.2
Fills gaps and bonds layers. Dk ≈ 3.2. Higher resin = lower overall Dk.
Typically bromine compounds. Adds to Df (loss).
Different glass fabric styles have varying resin content, which directly affects the dielectric constant. Understanding this is crucial for accurate impedance prediction.
| Glass Style | Thickness | Resin Content | Typical Dk | Common Use |
|---|---|---|---|---|
| 106 | 1.3 mil | ~75% | 3.6-3.8 | Very thin prepreg, filling |
| 1080 | 2.8 mil | ~65% | 3.8-4.0 | Standard thin prepreg |
| 2116 | 4.5 mil | ~52% | 4.0-4.2 | Most common, good fill |
| 1506 | 5.5 mil | ~50% | 4.1-4.3 | Medium thickness |
| 7628 | 7.0 mil | ~42% | 4.3-4.5 | Thick, stiff prepreg |
The glass bundles and resin pockets create localized Dk variations of ±10%. For differential pairs, this causes intra-pair skew. Mitigate by: routing traces at angles to the weave, using spread-glass laminates, or specifying high-resin prepregs.
FR-4 comes in different grades with varying thermal and electrical properties. Choose based on your assembly process and operating environment.
| Grade | Tg | Td | Dk | Df | Cost | Application |
|---|---|---|---|---|---|---|
| Standard FR-4 | 130-140°C | 310°C | 4.3-4.5 | 0.020-0.025 | $ | General purpose, low speed |
| Mid-Tg FR-4 | 150-160°C | 330°C | 4.2-4.4 | 0.018-0.022 | $ | Lead-free compatible |
| High-Tg FR-4 | 170-180°C | 340°C | 4.1-4.3 | 0.016-0.020 | $$ | Automotive, industrial |
| FR-4 IT180A | 180°C | 350°C | 4.0-4.2 | 0.015-0.018 | $$ | High reliability |
Consumer electronics, IoT devices, simple controllers
GPIO, SPI, I2C, UART, standard buses
Volume production where material cost matters
Sub-GHz wireless, GPS L1, basic RF
Use mid-loss (Df <0.01) for USB 3.0, PCIe Gen3+
Rogers, Taconic for frequencies >10 GHz
FR-4 Dk variation may exceed ±10%
High Df causes excessive insertion loss
Don't use the "4.5" default. Ask your fab for actual Dk at your frequency. Typical values: 4.0-4.2 at 1 GHz for mid-resin prepregs.
For consistent impedance, specify glass style (e.g., "2 × 2116" or "1 × 1080 + 1 × 2116"). Different combos have different Dk.
For lead-free assembly (260°C reflow), use Tg ≥ 170°C. Standard Tg may cause z-axis expansion and barrel cracking.
For differential pairs >3 Gbps, specify spread-glass or NE-glass to reduce intra-pair skew from fiber weave effect.
Use standard FR-4 for inner layers where speed doesn't matter, and upgrade only high-speed signal layers if needed (hybrid stackup).
Always get the fabricator's impedance report before production. They have precise Dk/Df for their specific materials.
FR-4 Dk varies from 3.8 to 4.8 depending on resin content, glass style, and measurement frequency. High-resin prepregs (106, 1080) have lower Dk (~3.8-4.0), while glass-rich styles (7628) have higher Dk (~4.3-4.5). Most CAD tools use 4.0-4.2 as a 'typical' value, but always get specific values from your fabricator.
FR-4 Dk decreases slightly as frequency increases (dispersion). At 1 MHz it might measure 4.5, but at 1 GHz it's typically 4.0-4.2, and at 10 GHz around 3.9-4.0. This happens because polarization mechanisms can't keep up at higher frequencies. For designs above 1 GHz, use frequency-appropriate Dk values.
Glass weave effect occurs because glass fiber (Dk ≈ 6.2) and epoxy resin (Dk ≈ 3.2) have different dielectric constants. A trace running parallel to glass bundles sees different Dk than one crossing them, causing up to ±5% impedance variation and intra-pair skew in differential signals. Mitigation: angle traces, use spread glass, or higher resin content.
Use High-Tg (170°C+) FR-4 for: lead-free soldering (peak temps up to 260°C), multiple reflow cycles, high operating temperatures, or when z-axis expansion during reflow could damage plated holes. Standard Tg (130°C) is fine for leaded solder and room temperature operation.
FR-4 works well up to about 3 Gbps. Beyond that, the high Df (0.020) causes significant signal attenuation. For 5-10 Gbps, consider mid-loss materials (Df ~0.010). For 10-28 Gbps, use low-loss laminates (Df <0.005). FR-4 also has inconsistent Dk which causes impedance variation in sensitive designs.