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What is Substrate in a PCB?

Every building needs a “foundation” or a “platform” upon which the structure will stand. The absence of such makes it impossible for such a building to stand “the test of time,” as it is susceptible to collapsing after some time. The same applies to the production of Printed Circuit Boards (PCBs). There is a need to have a substrate in the PCB and that is what we will discuss in this article.

Why a Substrate is the “Foundation” of a PCB

A Printed Circuit Board (PCB) is merely a “flat or bare board,” which wouldn’t stand on its own and perform excellently, except if it has some properties. This is why the board is “equipped” with a non-conductive substrate material that has both conductive pathways and electronic components attached to it.

The substrate is aptly described as the “foundation” or “base material” of the circuit board, because it is both a non-conductive material and a dielectric composite structure made from epoxy resin and either paper or glass fabric.

As such, the substrate provides the foundation or platform that “holds” the electronic components and traces in place. Therefore, you can leverage the substrate used in PCB to securely hold the foundational parts and electronic components on the circuit board.

The Types of PCB Substrates

Fr4 Laminate
Fr4 Laminate

You will find different kinds of base materials used in the production of Printed Circuit Boards (PCBs). Of course, the properties and or functionalities of these materials differ.

Here is a list of some of the popular/common PCB substrate materials:

1.    FR-4

This is not just the most popular PCB substrate material, but also the one that offers the best performance.

The Flame-Retardant 4 (FR-4) circuit board substrate is commonly made out of a woven glass reinforced by epoxy resin. It is also said to be fire-resistant, because of the properties of the epoxy resin used:

  • The epoxy resin is always Flame-Retardant Level 4.
  • The epoxy resin doesn’t absorb moisture and;
  • It is water-resistant.

a.    Features of FR-4 PCB Substrates

Besides offering the best performance and being used in a vast majority of circuit boards, the FR-4 substrate has these characteristics:

  • High electrical insulation, hence, the fireproof.
  • Lightweight design
  • Higher moisture resistance
  • Lower cost
  • Excellent performance
  • Higher electric strength

b.    Applications

The use cases of the FR-4 substrate are often prioritized for the low-speed digital circuits. You can also use this substrate with the circuit boards that have a complex structure, as well as the PCBs requiring several layers.

It is true that this substrate can be used for a wide range of circuit boards and consumer electronics, it does have a higher dielectric loss. This is the reason why FR-4 substrates are not always ideal for either the high-frequency or high-speed applications.

c.     Different Variants

You will find different variations and or versions of the FR-4 PCB substrates. Of course, they differ in terms of the properties and performances.

These are some of the variants:

  • Halogen-Free FR-4: this variant contains less halogens and these are one of the fireproof elements.
  • Standard FR-4 Substrate: this type of substrate is considered to be the “standard” or the format for others, because of the economic undertone. It also has a higher heat resistance, of up to 150-degree Celsius.
  • High Tg FR-4 Substrate: this type of PCB substrate is prioritized for the higher temperatures. It is noted for the higher value of Glass Transition Temperature (Tg) – an attribute that supports the substrate’s clocking of higher temperatures, of up to 1180˚C.

2.    Teflon

Teflon is a brand name for Polytetrafluoroethylene (PTFE), which has a Glass Transition Temperature (Tg) of 160. It is a type of plastic that doesn’t offer resistance, which is why it is always used with high-frequency and high-speed signals.

PTFE is highly-recommended for the high-frequency, microwave and high-power PCBs.


The features include:

  • Excellent high-frequency properties that make it ideal for electronic devices that handle higher signals, ranging from a few hundred MHZs to tens of GHZs.
  • The flame-resistance, flexible and lightweight design makes it ideal for different kinds applications.
  • The low dissipation and excellent temperature stability makes it usable in aerospace telemetry, cruise control, antennas and power amplifiers.

3.    FR-2

This is the Flame-Retardant Level 2 (FR-2) substrate. Although it has fireproof properties, it offers a lower performance, when compared to the FR-4 substrate.

It is made of a phenolic material and is commonly used in economical and low-cost consumer electronics, an example being low-cost transistor radios.

4.    Flexible Substrates

These substrates are not only flexible, but also offering the best performances, without compromising the circuit board’s electrical continuity.

The major attributes are wrapping, bendability and folding – making it easier to optimize the boards for different applications. Circuit board designers also leverage this to fit the substrate into the tiniest of spaces in the PCB.

5.    CEM-1

This is the type of substrate used in producing the single-sided circuit boards. The CEM-1 substrate has a Dielectric Constant (Dk) of up to 5.4 and is recommended for use with the high-density PCB.

If you cannot afford the FR-4 substrate, then using the Composite Epoxy Material 1 (CEM-1) is an ideal alternative.

6.    Polyimide

flexible printed circuit

This type of PCB substrate is higher Tg of up to 250-degree Celsius, which makes it ideal for use with the high-temperature applications.

Unlike the CEM-1, which is a cost-effective variant to the FR-4 substrate; the Polyimide substrate is costlier than the FR-4.

The attributes include:

  • Solid conductive properties
  • Flexible use to develop both Rigid-Flex and Flexible PCBs.
  • Good temperature range and;
  • Higher chemical resistance.

7.    Rigid PCB Substrates

These are the opposites of the flexible substrates. The rigid PCB substrates are rigid or “immobile,” hence, they cannot be relied on to offer a similar bendability and or folding, as the flexible PCB substrates offer.


The different PCB substrate types offer you a plethora of options to choose from. Rest assured that by using the FR-4 substrate, you will be able to make different kinds of circuit boards.

If you are not cool with any of the options on this list, you can always contact your substrate PCB manufacturer for recommendations.




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