FR4 PCB: The FR4 thermal conductivity guide for your circuit board

FR4 PCB: The FR4 thermal conductivity guide for your circuit board
FR4 thermal conductivity

Just like the light-emitting diode, which served exclusively as an indicator lamp for decades, PCB has also left its shadowy existence and has rapidly advanced to a multifunctional element within an electronic system. However, along with the development of integration technology, the total power density of electronic components continues to increase, but the physical size of electronic components and electronic devices is designed to be smaller and smaller that would cause the  increased heat flux density around the device, which would affect the performance of electronic components, so it is necessary to find a more efficient way to manage the thermal conductivity.  In this blog, we will focus on the FR4 PCB thermal conductivity as it is one of the most widely used PCBs.

Technical Characteristics of FR4 PCB Thermal Conductivity

Before we introduce the technical characteristics of FR4 thermal conductivity, it would be better to figure out some base questions. First, we should understand some features about FR4: The material is stable, the insulation is reliable, the dielectric behavior is usable, the costs are acceptable, the processing is established and the heat resistance is tolerable. Its derivatives are essentially generated by modifying the epoxy resin matrix. In the wake of the switch to lead-free electronics (~ RoHS), it has become common to replace part of the resin volume with mineral fillers. This delays the temperature-related increased Z-axis expansion of the circuit board (parameter CTE (z) = “Coefficient of thermal expansion”).

Another question: What is thermal conductivity? Thermal Conductivity (Κ, λ, or κ.) is a physical quantity in PCB measures, which can be defined as the rate at which heat is transferred from a heat source to cooler regions of the PCB. Talking about this, we need to know another term “Tg”, which is always misleadingly translated as “glass transition temperature. However, it actually means that the epoxy resin structure becomes soft and elastic as soon as the Tg value is reached and that the Z-axis expansion consequently increases considerably. The standard Tg value for FR4 is between 130-140 °, higher Tg values are 150 ° or 175 °, depending on the material manufacturer. Here is one thing that we should note, even though a higher Tg value does not increase the continuous operating temperature of a module, the continuous operating temperature of a module with FR4 material should not exceed 95 to 100 Celsius. Another parameter is the Td value. “Td” stands for “Time to Decomposition” and describes the temperature value at which the material has lost 5% of its mass due to outgassing/evaporation.

fr4 thermal property

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What Factors Affect FR4 PCB Thermal Conductivity?

PCB thermal conductivity requires much attention from manufacturers, as it decides how the circuit board can transfer heat to other components. As we all know that a printed circuit board consists of electronic components, insulators and conductive materials, and different components and materials have different thermal conductivity performances. In addition, there are many factors that would affect the FR4 PCB thermal conductivity:

Thermal vias

Thermal vias are holes that are placed on printed circuit boards, which play a vital role to dissipate heat. Generally speaking, more thermal vias in a circuit board can improve the thermal conductive performance as these vias provide more rooms to discharge the heat of the PCB and components.

Copper Traces in PCB

Copper traces is another important factor that would influence the thermal conductivity. The thermal conductive performance is actually dependent on if the traces are complete, that is, the traces connect from one end to another end. The thermal conductivity would be high if the traces are complete, and it would be low if the traces discontinue.

Internal layers

The internal layer is a factor that would affect the heat dissipation of the circuit boards. The thermal conductivity would be decreased if there are many inner layers and vice versa.

FR4 PCB Thermal Conductivity Management

Thermal Conductivity Management is crucial for FR4 PCB that would affect their performance, reliability, and longevity. Without heat management, the printed circuit boards may have problems of delamination, damage, or device failure. Luckily, there are several methods to manage the thermal conductivity effectively. In this blog, we would explain them in two aspects:

Design PCB Better

Thermal conductivity is a factor that must be considered when designing a PCB, below are some tips for better PCB design:

First, when designing a printed circuit board, it would be better to separate high power and signal conductors. And we can insert more thermal vias along the thermal path. The thermal vias can both be plated or not plated, which enables the circulation of air and dissipate heat. In addition, the reasonable thermal via array is very helpful to reduce the thermal resistance and enhance the performance of thermal dissipation.

Second, we suggest increasing the distance between tracks to get a more uniform heat distribution in the layers, which can reduce the risk of generating hot spots. But we should notice that this method is not suitable for the PCB with small sizes.

Third, the geometry of tracks is also an important factor need to be considered during design. The tracks that connect components should be as short and wide as possible, and tracks that conversed high currents should use copper with thick heights. If the tracks are too small, then there is a possibility that the electronic components would fail.

Embed Copper Wire into FR4 PCB

Moko Technology takes a different approach with »HSMtec«. The technology, which is qualified in accordance with DINEN60068-2-14 and JEDECA101-A and audited for aviation and automotive, is selective: only where high currents are supposed to flow through the printed circuit board does thick copper.

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Currently, 500µm high profiles with widths from 2.0mm to 12mm are available in variable lengths, with wires a diameter of 500µm has become established. The solid copper elements that are firmly bonded to the conductor patterns can be applied directly to the base copper using ultrasound connection technology and integrated into any layer of a multilayer using FR4 base material. There are several reasons why copper is used: It has twice the thermal conductivity compared to aluminum and thus ensures rapid heat dissipation without insulating intermediate layers underneath the LED heat pad.

Material Thermal conductivity λ [W / mk]
Copper RA 300
aluminum alloy 150
solder 51
Ceramic (LED) 24
FR4 0.25
Air (resting) 0.026

Table 1: Thermal conductivity of the materials involved
Another advantage of copper and the circuit board base material FR4 are the thermal expansion properties (Table 2): Especially in connection with ceramic LEDs, circuit boards based on copper or FR4 have a high resistance to thermal stresses, which depend on environmental or operating conditions and others Temperature cycles, such as for “intelligent” lighting controls.

Material Expansion coefficient [ppm / K]
aluminum 24
solder approx. 22
copper 16
FR4 13-17
Al2O3 (LED) 7
AlN (LED) 4

Table 2: Thermal expansion coefficient in the X / Y direction
In this way, the lifespan and reliability of the entire lighting unit can be significantly increased compared to conventional metal core PCB based on aluminum.

Advantages of FR4 PCB with Embedded Copper

Compared to thermovias, which are placed directly under heat pads, this method makes it possible to solder filled microvias without any problems. The main advantage of HSMtec compared to alternative solutions is the use of inexpensive standard FR4 material in addition to manufacturing in the standard manufacturing process. It is also possible to use this process to construct self-supporting, multi-dimensional printed circuit boards with copper wire.

With the help of notch milling at the predetermined bending points, individual segments can be brought into the desired orientation by any adjustment of the inclination angle. The thick copper integrated with profiles and wires withstands currents of up to 500 A. This represents a sensible alternative to circuit board solutions that provide full-area copper layers up to 500 µm thick, or to cost-intensive IMS solutions that use massive aluminum cores as heat carriers instead of the usual base material deploy.

 

FR4 thermal conductivity

With HSMtec®, the thermal conductivity of such an FR4 surface would be increased to 30 W/m-K. which is 100 times more thermally conductive than regular RF4 and 10 times higher than even the best thermally conductive substrates. In addition, this tech uses standard FR4 material and can be achieved by standard manufacturing processes, moreover, it allows the FR4 to be further processed in the assembly and soldering process. In a word, it is a cost-saving method that can effectively improve the thermal conductivity performance of FR4 PCB.

Conclusion

FR4 is a commonly used material for PCB manufacturing as it is economical and has great properties that can be utilized in different applications. But compared to other materials, it has poorer performance in the thermal conductivity. Thus, it is necessary for manufacturers to understand the thermal conductivity features of FR4 and learn how to manage it, which can not only help them reduce cost, but also improve the quality of their products. If you still have questions about the FR4 PCB thermal management, you can go to MOKO to get the answer.

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