PCBs Thickness: Understanding Thickness Variations

Introduction 

Thickness is a crucial factor for printed circuit boards, as it is significant for board conductivity and resistance. PCBs were originally single-sided and plywood sheets with a thickness of 0.062 inches were used as substrates for the boards.

When multilayer PCBs started to develop, the thickness of the connectors between the boards had to correspond. Therefore, the level of thickness consistency became a critical variable, and there was a need for a standard layer of copper used on the boards. Hence being the known value, 0.062 inches was retained as the Standard PCB Thickness.

Nevertheless, there is a broad thickness range of 0.008 inches to 0.240 inches, that manufacturers select, according to the required application. Apart from the usage needs, there are design and manufacturing factors that affect the thickness of circuit boards, all of which must be considered for quality and high-performing PCB fabrication.

What is Standard PCB Thickness?

PCB thickness refers to the thickness of a completed printed circuit board. The PCB thickness is determined by factors including board function, copper thickness or weight, the number of layers, the mechanical load of assembly components, the operational environment, and substrate.

The most common and widely used PCB thickness in the electronics industry is 1.57 mm (0.062 inches), as is found in diverse electronic products and equipment, hence it is regarded as the default thickness. However, technological advancements and increased circuit complexity have induced an increase in the possible number of layers and copper weight of PCBs. Consequently, a few more common PCB thickness selections emerged, including 2.36mm (93 mils) and 3.17mm (125 mils). These values, among others, constitute the range of standardized PCB thickness. 

PCB Board Thickness

The thickness of the printed circuit board is one of the most pertinent factors to consider in PCB design. The overall thickness of the PCB is typically determined through the layers of the PCB, which can be in multiples of 2 to 32 layers. This size versatility makes it difficult to specify a standard PCB thickness, so there is a range of possible measurements, determined by the layers of particular PCBs.

Using these, thickness can be estimated ensuring that there is no adverse impact on the mechanical weight, conductivity, and resistivity of the PCB. Hence PCB thickness affects the functionality of circuit boards.

PCB thickness was specified in the 20th century, having a standard thickness of 0.062 inches, since PCBs were characterized by a single panel with several layers placed together. This was determined through the substrate used at the time - the plywood substrate - which had a thickness of 0.062 inches. It, therefore, became the standard thickness.

In modern times, however, PCBs are either of a single copper layer or a multilayer that has substrates between them. This results in board thicknesses that range from 0.008 inches to 0.240 inches. Moreover, some factors are used to determine the overall thickness of the board, which are board thickness, copper thickness, and trace thickness.

The board thickness depends on the material used in the boards. The standard thickness of earlier times was about 0.0062 inches because of the plywood substrate used between the copper foil. With the introduction of lighter substrates like epoxy, resin, etc, PCBs have seen a size drop that makes it possible for board thickness to be below  0.0062 inches. [1]

 PCB Copper Thickness

The copper thickness used in PCB production is generally measured in ounces. The standard copper thickness is 1 oz. To determine the thickness, a specific weight of copper is spread out evenly on a board to cover 1 square foot. Therefore, the resulting foil thickness of 1 oz of copper rolled out over an area of 1 square foot will be 0.0348 mm or 1.37 mils. 

Copper thickness plays an essential role in the overall thickness of the PCB. The current carrying capacity of a PCB is determined by the thickness of its copper traces and, therefore, of the copper layer.

For boards with impedance control, the required trace width is a significant determinant of how thick the copper should be. Other important factors include the PCB weight, component compatibility requirements, and the type of connectors that will be assembled onto the board.

In circuits where the current requires more thickness, copper would be added as required. It is worth noting, however, that this process is quite complex and time-consuming. [2]

PCB with Copper Layers

PCB Trace Thickness

All modern electronic devices fundamentally require the PCB, and the thickness of the circuits is vital in the overall design process.

Different types of PCBs abound in the electronics market today, such as single-sided, double-sided, and multi-layered. Therefore, the thickness of the board varies according to type requirements, and common PCB trace thickness ranges from 0.008 inches to 0.240 inches.

Recommended reading: Trace PCB: A Comprehensive Guide.

Circuit board designers engage in many complex calculations to accurately determine the correct trace thickness for a PCB, during the design stage. This is pertinent because mistakes could have damaging effects on the components attached to the board, eliciting additional costs with expensive repairs and generally hiking the manufacturing budget. More so, miscalculations can make the board fall short of expected functionality. [3]

Prepreg Thickness

Prepreg is a dielectric material that provides insulation between two cores or between a core and a copper foil in a PCB. It is also called a binding material, as an insulation layer that either binds two cores or a core and a copper foil.

Prepreg is a major element of multilayer PCBs. The basic functions of prepregs include binding a core to another core, binding a core to a layer, providing insulation, and protecting a multilayer board from short-circuiting.

Prepreg is made with a combination of fiberglass and a resin bonding agent. The fiberglass is pre-impregnated with the resin bonding agent, which is how the name 'prepreg' came about. This impregnation is usually done with epoxy-based materials or partially cured polyimides, to create prepreg materials. 

There are three main types of prepreg based on the amount of resin present: high resin (HR), medium resin (MR), and standard resin (SR). The thickness of a Prepreg material depends on the amount of resin content bound with the glass fiber. Therefore, high resin makes for thicker and more expensive prepreg materials.

Moreover, prepreg thickness and resin content regulate the impedance, drilling and etching quality, dielectric constant, and Coefficient of Thermal Expansion (CTE).

In PCB fabrication, the prepreg is sandwiched between two copper layers and then stacked, heated, and pressed together to create a laminate. Laminates are regarded as the core of a PCB which serves to bind, strengthen and protect the layers of a PCB. The thickness of the prepreg used in a multilayer board varies according to the overall thickness of the board. They can be pp-106 (2 mil), pp-1080 (3 mil), pp-2113(4 mil), and pp-2116 (4.7 mil). [4]

Internal Layers PCB Thickness

Internal layers are also known as the inner layers of a multilayer PCB. It is the vertical distance between the copper-tracked layers of a PCB. The thickness of internal layers is determined by the specific construction sequences of the circuit board. This means that the copper weight and thickness are customizable according to the application requirements. The copper weight in the layer determines the minimum trace width and spacing that can be produced. [5]

Multilayer PCB Thickness

A multilayer PCB has more than two layers in its stack-up. An accurately stacked board will improve signal integrity as well as reduce electromagnetic emissions and crosstalk. The thickness of a PCB largely depends on factors such as copper thickness, materials used, and the number of layers. The standard thickness of a regular board is around 62 mils (1.57mm). 

In modern times, electronic devices are growing more complex and contain a higher number of components and circuits. Accommodating these complex circuits in a single-side PCB has become difficult. So manufacturers turn to the solution of adding layers to the stack-up.

Today, PCBs have become more complex as the copper layer weight and the layer count have increased for various applications. Resultantly, PCBs are becoming thicker as manufacturers produce standards such as 93 mils (2.36mm) and 125 mils (3.17mm). [6]

Standard Two-Layer PCB Thickness

In two-layer printed circuit boards, the central substrate usually has two copper layers known as the top and bottom layers.

As mentioned in earlier sections of this article, the layers of a PCB affect its thickness. 2-6 layers PCB may have the standard thickness of 1.57mm, but eight or more layers of PCB may not be able to maintain the standard thickness of a PCB. 

Substrates are usually made in standard thicknesses of approximately 1.6 mm for 2- layer, 4-layer, and 6-layer boards. But the thickness may be reduced when there is a higher number of layers in a board. 

However, for a PCB that requires more layers, it is impractical and counterproductive to use thinner layers to achieve standard PCB thickness, so the thickness should be adjusted accordingly. [7]

Four-Layer PCB Thickness Standard

Generally, the layout of the 4-layer PCB consists of the top layer, the bottom layer, and two middle layers. The middle 2 layers are called the power layer, represented as VCC, and the ground layer represented as GND. The power layer and the ground layer can be isolated in the middle of the stackup to reduce interference.

The four-layer printed circuit board comprises five different thicknesses, which are: 0.5mm, 0.8mm, 1.0mm, 1.2mm, and 1.6mm. This is pressed based on the double-sided PCB.

The lamination process involves prepreg and copper foil added on both sides of the double-sided board, and then pressed into a multi-layer board using both high pressure and temperature. This means that the 4 layer-printed circuit board has an inner layer. In the course of the process, some traces will be etched through the inner layer and then pressed together. [8]

What Affects a PCB’s Thickness?

As earlier established, thickness is a critical factor in PCB design and manufacturing. The standard PCBs are designed and fabricated using standardized thickness values. However, there are customized PCBs designed for various thicknesses, weights, and layer counts.

Thickness is a design parameter for PCBs but it is dependent on other parameters that determine its range and final value. These parameters are majorly subsumed under design and manufacturing factors that affect PCB thickness, which include the number of layers, weight, and many others that will be discussed in detail subsequently.

Design Factors that Impact PCB Thickness

Design factors are considered during the PCB design phase. These factors are mostly concerned with the functionality and intent of the board, rather than practical concerns that arise during manufacture. Some of the significant design factors that impact PCB thickness include the following:

Copper Thickness

Copper thickness is significant for the overall thickness of the PCB. The thickness of the copper layer used is usually determined by the current that needs to pass through the PCB. A copper trace that is required to carry a high-power signal will need to be thicker than one that only needs to carry a low-power signal.

Standard copper thickness is about 1.4 to 2.8 mils, but this thickness can be modified according to the particular requirements of the board. Thicker copper will make a thicker board that is more expensive due to material needs and processing complications.

To get a custom copper thickness, a PCB manufacturer will try to adjust the thickness of the prepreg and core until they meet their target PCB thickness.

Board Materials

The selection of materials is one vital factor that affects PCB thickness. Typically, board fabrication consists of materials for the substrate, solder mask, laminate, and silkscreen. The most significant of these for board thickness is the laminate and substrate materials, as they form the board's structure.

Usually, the substrate material is epoxy resin, glass weave, or ceramic, while the laminate material is thermoset resin and either paper or cloth layers. The material choices for laminates and substrates determine the thermal, electrical, and mechanical properties of the PCB and also determine the circuit board's overall thickness.

Number of PCB Layers

The number of PCB layers undoubtedly influences the board thickness. The layer numbers vary according to the PCB application requirement, and the thickness becomes greater as the layers increase in number. 2 to 6 layers fall within the standard boundary of PCB thickness (1.57 mm) but upwards from 8 layers would produce more thickness.

Manufacturers may be able to apply thinner PCB layers to attain standard thickness, but this becomes impractical in the long run. 

The better strategy for a multi-layered PCB design is to allow for greater PCB thickness. On the other hand, reducing the number of layers is the best move for PCB designs that do not require multiple layers but specify certain thickness values.

Signal Types

Circuit boards possess a broad selection of signal types. The materials needed for the board are determined by the type of signal it carries, and this can in turn influence the thickness of the board. To illustrate this influence, a board with high power signal capacity requires thicker copper and wider traces, hence the finished board will be considerably thicker than boards carrying low power. However, it is worthy of note that high-density PCBs with complex signals usually use laser microbial, thin travels, and high-capacity materials therefore they are usually thinner than other board types.

Types of Vias

Modern electronics require high-density compact PCBs and vias are used to achieve this. Vias route traces through circuit boards instead of on their surfaces, which makes it possible to create compact designs. PCB manufacturers use different via types such as micro via, buried via, through via, blind via and via-in-pad

The type and density of vias used are determinants of the required thickness of the board. The board thickness must be able to accommodate vias and fabrication.

Recommended Reading: What is a Via: A Comprehensive Guide.

Operating Environment

The operating conditions must be assessed before selecting board thickness. A thin board, for instance, may not be able to withstand rugged operating environments, while thick PCBs may provide optimal flexibility for modern electronic operations.

A thin or flexible board may not be the optimal choice for rugged operating environments. Also, thicker copper traces are less thermally stable with a high current, making them suboptimal for thermally variable or high-current environments. More so, the components and connectors on the board are vital for having performance and material requirements that may influence board thickness. [9]

Manufacturing Factors that Impact PCB Thickness

Manufacturing factors that are no less significant than design factors also impact PCB thickness. Some of these factors include the following:

Drill Hole Equipment

While the emphasis is often placed on drill hole size and spacing for performance objectives, drill hole manufacturing processes bring about some complexity that cannot be ignored. When drilling any type of drill hole, fabricators are limited by the thickness of the board as well as the diameter and depth aptitudes of milling machines and lasers.

This limitation is illustrated by an aspect ratio, which is the ratio between the depth of the hole and the drill hole diameter. For standard drilling, all manufacturers should aim to achieve a 7:1 aspect ratio. Some manufacturers can accomplish a greater aspect ratio, but this must be decided before the board design is finalized and usually comes at an additional cost. What this means for thicker boards, is that there is less possibility of fabricating small-diameter holes.

Copper Thickness

Copper traces are made by a process called etching, which is a crucial step in PCB manufacturing. Manufacturing processes that require etching or plating are dependent on the thickness of the internal copper layer. Because of this, thicker copper layers will influence manufacturability, which means it has the potential to impact the design and the cost of the final PCB product.

Layer Count

We earlier established that multiple count layers make it difficult to achieve standard PCB thickness. We also discussed that some manufacturers may negate this by stacking up thinner layers, so that they may meet a specific thickness, but this is a complex endeavor that also tends to be expensive.

Depanelization Method

Manufacturers produce PCBs in a large panel that contains several boards, and then the panels are separated into individual boards. The board thickness affects which depanelization methods may be used. Thicker boards may require careful depanelization using scoring, while thinner boards may be routed to form breakaway tabs. Depanelization methods and requirements must be carefully planned before PCB manufacturing begins. [10]

Key Takeaways

Printed circuit boards have a standard thickness of 0.062 inches or 1.57mm. This threshold is considered the standard because it is the most commonly used value for PCBs. However, because of design specifications for various applications as well as manufacturing factors, the thickness of PCBs can vary between the range of 0.008 inches to 0.240 inches.

References

1. PCB Directory. What is the Standard PCB Thickness? 2021. [Cited 2022 Nov 12] Available from: https://www.pcbdirectory.com/community/what-is-the-standard-pcb-thickness 

2. Camptech Circuits Inc. A Shortcut to Understanding PCB Copper Weight/Thickness. 2022. [Cited 2022 Nov 12] Available from: https://camptechii.com/a-shortcut-to-understanding-pcb-copper-weight-thickness/

3. PCBBUY.COM. What is a PCB Trace? 2021. [Cited 2022 Nov 12] Available from: https://www.pcbbuy.com/news/What-is-a-PCB-trace.html

4. PCB Directory. What is Prepreg in a PCB? 2022. [Cited 2022 Nov 13] Available from: https://www.pcbdirectory.com/community/what-is-prepreg

5. Twisted Traces. PCB Layers. 2022. [Cited 2022 Nov 13] Available from: https://www.twistedtraces.com/capabilities/pcb-layers 

6. Sierra Circuits. How to Build a Multilayer PCB Stack-up. 2021. [Cited 2022 Nov 13] Available from: https://www.protoexpress.com/blog/build-multilayer-pcb-stack-up/ 

7. PCBTOK. Standard PCB Thickness: The Ultimate Guide for You in 2022. 2022. [Cited 2022 Nov 13] Available from: https://www.pcbtok.com/standard-pcb-thickness 

8. AiPCBA. What is the Thickness of the 4-Layer PCB? 2022. [Cited 2022 Nov 13] Available from: https://www.aipcba.com/pcb/thickness-of-four-layers-pcb.html

9. Creative Hitech. Know About the Design Factors that Affect PCB Thickness. 2021. [Cited 2022 Nov 13] Available from: https://www.creativehitech.com/blog/know-about-the-design-factors-that-impact-pcb-thickness/ 

10. Millennium Circuits Limited. What Impacts PCB Thickness? 2022. [Cited 2022 Nov 14] Available from: https://www.mclpcb.com/blog/pcb-thickness/#:~:text=Copper%20thickness%20plays%20a%20role,unique%20requirements%20of%20the%20board.