We will compare 370HR vs. FR4 in this article.
Like in any circuit, a PCB must have conductors and insulators.
And material selection is one of the vital steps in the circuit board design process.
The conductive layer is usually copper or silver, but you can use several materials for the core, laminate, or prepreg. 370HR vs. FR4 are two of the most typical PCB laminate materials.
Read on to learn more!
What Is 370HR?
370HR is a high-performance PCB material that consists of epoxy resin and glass fabric.
Built for prepreg and lamination purposes, 370HR has good electrical insulation and remarkable thermal performance.
The material’s thermal resistance properties are due to the included flame-retardant additive in the formulation, which helps enhance safety.
So the laminate can withstand high temperatures without compromising its electrical performance or structural integrity.
Additionally, the material exhibits sound mechanical strength, dimensional stability, and high resistance to moisture and chemicals.
These characteristics, combined, make 370HR the ideal material for circuit boards that generate lots of heat.
They include high-frequency circuits and power electronics.
A high-power radio frequency PCB
What Is FR4?
Although composed of almost the same materials (glass fabric and epoxy resin), FR4 is a standard PCB material.
It lacks the high-performance aspect, making the material affordable and widely available for high-volume circuit board projects.
Clear epoxy resin
FR stands for Fire Retardant, a property made possible by the epoxy resin binder.
FR4 is the most typical FR material, and these woven fiberglass structures are known for having these properties.
- High tensile and dielectric strength
- Zero water absorption
- Good flexibility
FR4 is also known to have superb electrical insulation and dimensional stability.
Factors to Consider When Picking PCB Laminates
Consider these thermal and electrical properties before choosing a PCB laminate.
Glass Transition Temperature (Tg)
Tg is the temperature at which a PCB substrate changes state (glassy and solid to soft and deformable).
The change of state occurs because the polymer chains become more mobile.
Expressed in degrees Celsius, the Tg for the two materials are:
- 180°C for 370HR
- 130°C for FR4
So FR4 melts at a lower temperature than 370HR.
Therefore, it is not the best for hot components.
A circuit board with a hot chip
Decomposition Temperature (Td)
This property refers to the temperature at which a material chemically decomposes.
The materials can transition back to solid if you exceed the Tg.
But once you exceed this Td temperature, the material loses its resin quality.
370HR has a flame-retardant additive, making it capable of withstanding more heat.
Here are the numbers for the two laminates.
- 340°C for 370HR
- 240-300°C for FR4
A PCB withstanding high temperatures despite having burning components
Coefficient of Thermal Expansion (CTE)
This property defines the rate of expansion of a PCB laminate when heated, and its SI unit for CTE is parts per million/°C.
But the material extension occurs at different rates on the three axes.
And the substrate’s CTE is way higher than that of PCB traces.
So the unequal expansion can result in interconnection issues if you heat a PCB.
And as the temperature rises above the Tg, the CTE will increase significantly.
Take a look at these figures.
|370HR||13 ppm/°C (X-axis), 14 ppm/°C (Y-axis), and 45 ppm/°C (Z-axis)|
|FR4||16 ppm/°C (X-axis), 20 ppm/°C (Y-axis), and 60 ppm/°C (Z-axis)|
You can note the expansion along the X and Y axes is relatively low compared to the Z axis.
The woven fiberglass helps restrict extension in these directions to avoid interconnection issues caused by thermal stress.
And the CTE doesn’t change much even as the temperature rises above the Tg.
But the material must expand, so most of the lengthening occurs in the Z-axis direction.
However, you should pick a substrate with at most 70 ppm/°C to maintain structural integrity.
And in this direction, the CTE will increase when you exceed the Tg.
Thermal conductivity refers to a material’s ability to conduct heat.
So it describes how fast or slow heat moves from hot to cold areas on a circuit board. And its unit is watts/meter Kelvin.
If a material has a k value of one, it implies a meter cubed of the material will transfer heat at a rate of one watt for every temperature degree difference.
So low thermal conductivity means slow heat transfer, while high thermal conductivity implies quick heat transfer.
370HR has better thermal conductivity than FR4 because it conducts at 0.4W/mK.
FR4 transmits at almost half this value (0.25W/mK).
Dissipation Factor (Df) or Loss Tangent (tanδ)
This property describes the phase angle tangent between the resistive and reactive dielectric currents.
And the dielectric loss increases the loss-tangent values.
So low Df values create a fast substrate, while high Df values create a slow substrate.
Also, it is vital to note that loss tangent increases marginally with frequency.
So high-frequency materials with low tanδ values (fast substrate) have a low-frequency variation.
Here’s a comparison of the two materials at 10GHz.
- 0.025 for 370HR
- 0.0398 for FR4
In simple terms, FR4 has a higher loss tangent, meaning it absorbs more of the transmitted signal.
So the material is not ideal for building high-frequency circuits.
There will be a higher signal loss at the end of the transmission line.
A radio frequency amplifier PCB
Dielectric Constant (Dk)
Also known as relative permeability (Er), the dielectric constant is the ratio of the material’s electric permittivity to permittivity in a vacuum.
In simpler terms, Dk defines a material’s ability to store electrical energy when placed between two metal traces or plates.
So a low dielectric constant is better because it minimizes power losses for high-power or high-frequency applications.
But materials with high dielectric constants have unique applications, as well.
The high values are ideal for capacitance applications in tiny sizes.
The two materials have the following dielectric constants.
- 370HR: 4 (3.92 at 50% resin content)
- FR4: 3.8 – 4.8 (average 4.3)
370HR vs. FR4: Thermal Stability
Considering thermal stability, 370HR is the victor because it has a higher decomposition and glass transition temperature than FR4.
So it can withstand higher temperatures without losing its electrical properties and structural integrity.
A motherboard with an overheating CPU
370HR vs. FR4: Electrical Insulation
370HR wins in this category again because it has a lower dielectric constant and loss tangent.
So it is a better substrate and laminate for high-frequency PCBs.
But you’d better use FR4 for applications requiring capacitance.
370HR vs. FR4: Mechanical Stability
Because it can withstand more heat and mechanical stress, 370HR is stronger mechanically and more durable than FR4.
Also, the material has better dimensional stability.
370HR vs. FR4: Cost
The only area where FR4 beats 370HR is the cost. FR4 is cheaper, making it more common in circuit boards.
Which Laminate Is Better? – Wrap Up
As you can see, 370HR is a better PCB laminate material.
But that does not mean FR4 is not usable; it is ideal for low-frequency circuits.
So FR4 still has tons of applications in modern circuit boards.
And its low cost gives it an upper hand because the factor affects the overall product price.
That’s it for this article. Comment below to share your thoughts and sentiments. We appreciate your feedback.