What is FR-4?
FR-4 (Flame Retardant 4) is a composite material composed of woven fiberglass cloth with an epoxy resin binder. It is a popular choice for PCBs due to its excellent mechanical, electrical, and thermal properties. Some key characteristics of FR-4 include:
- High strength and stiffness
- Good electrical insulation
- Flame retardancy
- Relatively low cost
- Acceptable Thermal conductivity for many applications
FR-4 is available in various grades with slightly different compositions and properties tailored for specific applications and environments.
Thermal Conductivity Basics
Before diving into the specifics of FR-4, let’s review some fundamentals of thermal conductivity:
Thermal conductivity (k) is a measure of a material’s ability to conduct heat. It is defined as the rate of heat transfer through a material per unit thickness per unit temperature difference. The units of thermal conductivity are typically watts per meter-kelvin (W/mK).
Fourier’s law of heat conduction states that the rate of heat transfer through a material is proportional to the negative temperature gradient and the area perpendicular to that gradient:
q = -kA dT/dx
where:
q = heat transfer rate (W)
k = thermal conductivity (W/mK)
A = cross-sectional area (m2)
dT/dx = temperature gradient (K/m)
In simple terms, materials with higher thermal conductivity will transfer heat more readily than materials with lower conductivity. For PCBs, the substrate’s thermal conductivity impacts how effectively it can dissipate heat from components into the environment.
FR-4 Thermal Conductivity Values
The thermal conductivity of FR-4 is relatively low compared to metals like copper or aluminum, but acceptable for many PCB applications. The exact value can vary somewhat depending on the specific grade and manufacturer.
Typical FR-4 thermal conductivity values fall in this range:
| Direction | Thermal Conductivity (W/mK) |
|---|---|
| In-plane (x-y) | 0.25 – 0.4 |
| Through-plane (z) | 0.15 – 0.3 |
As seen in the table, FR-4 exhibits anisotropic thermal conductivity, meaning it conducts heat differently in different directions. The in-plane conductivity (along the x and y axes) is generally higher than the through-plane conductivity (along the z-axis). This is due to the orientation of the fiberglass weave within the composite.
For comparison, here are some thermal conductivity values for other common materials:
| Material | Thermal Conductivity (W/mK) |
|---|---|
| Copper | 385 |
| Aluminum | 205 |
| Stainless steel | 15 |
| Glass | 1.05 |
| Air | 0.024 |
Clearly, FR-4 is a poor thermal conductor relative to metals but still far better than air, making it suitable as an insulating substrate that can still dissipate some heat.
Factors Affecting FR-4 Thermal Conductivity
Several factors can influence the effective thermal conductivity of FR-4 in a PCB:
Resin and Glass Content
The ratio of epoxy resin to glass fiber in FR-4 affects its thermal properties. Higher resin content generally reduces thermal conductivity, while higher glass content increases it. The type and quality of the resin and glass used also play a role.
Glass Style and Orientation
The style of fiberglass weave and its orientation within the FR-4 impact conductivity. Tighter weaves with finer fibers tend to have slightly higher in-plane conductivity. Some specialized FR-4 grades use altered weave patterns to enhance or equalize in-plane vs through-plane conductivity.
Fillers and Additives
Some FR-4 formulations incorporate fillers or additives to modify properties like thermal conductivity. For example, adding ceramic fillers can boost conductivity at the cost of other properties like drillability.
Copper Layers and Vias
While not intrinsic to the FR-4 itself, the configuration of copper traces and planes on the PCB greatly affects overall thermal performance. Copper’s high conductivity (400x that of FR-4) allows it to efficiently spread heat laterally across the board. Through-hole vias lined with copper act as “thermal vias” to conduct heat through the thickness of the board.
Moisture Content
FR-4 is somewhat hygroscopic and will absorb small amounts of moisture from the environment. Higher moisture content decreases thermal conductivity. Baking FR-4 PCBs prior to assembly can help drive out moisture and ensure optimal properties.
Impact of FR-4 Thermal Conductivity on PCB Design
The relatively low thermal conductivity of FR-4 presents challenges and constraints for PCB designers dealing with high-power, high-density, or thermally sensitive components. Some implications include:
Component Placement
Components that generate significant heat, like power transistors, voltage regulators, or processors, must be placed strategically on the board. They are often located near edges or corners to minimize conducted heat into the board and placed on top or bottom layers for better air flow.
Copper Pour and Planes
Designers often add generous copper pours or dedicated copper planes on PCB Layers to act as heat spreaders. These large copper areas efficiently conduct heat laterally away from hot components across the FR-4 substrate.
Thermal Vias
Arrays of thermal vias are used under or around high-heat components to provide low-resistance paths for heat to flow through the FR-4 to copper layers on the opposite side of the board. Thermal vias may be filled with thermally conductive epoxy for even better performance.
External Heat Sinking
For components with extreme power dissipation, the FR-4 substrate alone may be insufficient to prevent overheating, even with thermal vias and copper pours. In these cases, external heat sinks, fans, or other cooling solutions may be required to sufficiently cool the PCB.
Advanced FR-4 Variants for Thermal Management
While standard FR-4 thermal conductivity is adequate for most applications, some specialized grades are available with enhanced thermal properties:
High Tg FR-4
FR-4 grades with higher glass transition temperatures (Tg) around 180°C are sometimes used for improved thermal reliability. These substrates maintain mechanical and insulating properties at higher temperatures than standard FR-4 (130-140°C Tg).
Thermally Conductive FR-4
Some manufacturers offer FR-4 substrates with thermally conductive fillers added to the epoxy resin. These fillers can boost thermal conductivity to 1-2 W/mK, though at higher cost and with some impact on other properties like CTE and drillability.
Ceramic-Filled FR-4
Similar to thermally conductive grades, ceramic-filled FR-4 uses ceramic particles to enhance conductivity. The ceramic fillers also reduce CTE for better thermal cycle reliability but make the material more brittle and abrasive on drill bits and cutting tools.
Metal-Core or Insulated Metal Substrates
For the most extreme thermal demands, some PCBs use metal cores (usually aluminum) or insulated metal substrates (IMS) instead of FR-4. These metal bases offer thermal conductivity 100x better than FR-4 but require special design considerations for electrical insulation and thermal mismatch stresses.
FR-4 Thermal Conductivity Testing and Measurement
Accurate thermal conductivity data is crucial for PCB thermal modeling and design. Several methods are used to measure the thermal conductivity of FR-4 and other insulating materials:
Guarded Heat Flow Meter Method
This steady-state technique sandwiches a sample between two plates at different temperatures. The heat flow through the sample is measured along with the temperature gradient to calculate thermal conductivity per Fourier’s Law. Laser flash or transient plane source methods are also used.
Thermal Imaging
Infrared cameras can be used to map the surface temperature profile of an FR-4 sample as heat flows through it. The thermal image can be used to infer the rate of heat flow and estimate thermal conductivity. This method is less precise than heat flow meters but offers fast, visual results.
Standardized Test Methods
ASTM and IPC publish several standardized test methods for measuring the thermal conductivity of PCB substrates and laminates. Examples include:
- ASTM D5470 – Thermal Transmission Properties of Thin Thermally Conductive Solid Electrical Insulation Materials
- ASTM E1461 – Thermal Diffusivity by the Flash Method
- IPC-TM-650 2.4.50 – Thermal Conductivity of Dielectric Materials
Following standardized methods ensures reproducible, comparable results between different labs and manufacturers. Always be sure to review thermal conductivity data in the context of the specific test method used.
FAQ
What is a typical value for FR-4 thermal conductivity?
FR-4 thermal conductivity is typically in the range of 0.25-0.4 W/mK in-plane and 0.15-0.3 W/mK through-plane at room temperature.
Can FR-4 thermal conductivity be increased?
Yes, the thermal conductivity of FR-4 can be increased by using specialized grades with thermally conductive fillers added to the epoxy resin. However, these grades are more expensive and may have other tradeoffs in mechanical or electrical properties.
How does FR-4 thermal conductivity compare to other PCB materials?
FR-4 has lower thermal conductivity than metal-core or insulated metal substrates, which can have conductivity over 100x higher. However, FR-4 is much less expensive and has better electrical insulating properties. Some specialized non-FR-4 laminates like polyimide or BT epoxy may have higher conductivity than FR-4.
Does FR-4 thermal conductivity change with temperature?
Yes, the thermal conductivity of FR-4 generally increases slightly with increasing temperature, though the effect is relatively small over typical operating temperature ranges. Thermal conductivity may increase 20-30% from room temperature up to the glass transition temperature around 130-180°C.
How does moisture affect FR-4 thermal conductivity?
Moisture absorbed into the FR-4 substrate decreases its effective thermal conductivity. Moisture’s thermal conductivity is lower than FR-4, so it acts as an insulating filler. Baking PCBs prior to assembly can help drive out moisture and ensure optimal thermal performance.
Conclusion
The thermal conductivity of PCB FR-4 substrates is a critical property affecting the thermal management and reliability of electronic circuits. While FR-4 is not a great thermal conductor compared to metals, its conductivity is sufficient for many applications when used with proper heat spreading and sinking techniques. Designers must carefully consider the thermal limitations of FR-4 and leverage copper planes, thermal vias, and component placement to ensure adequate cooling of high-power and high-density PCBs. For extreme thermal demands, specialized grades of FR-4 with enhanced conductivity or alternative metal-based substrates may be required. By understanding the thermal conductivity of FR-4 and its impact on PCB performance, designers can make informed material selections and optimize layouts for thermal efficiency and reliability.
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