Introduction

Printed circuit boards (PCBs) are essential components in nearly all modern electronics. As electronic devices become more complex and packed with features, PCBs must increase in layer count to accommodate all the necessary interconnections and circuitry. One of the most common high layer count PCBs is the 6 layer board. But how thick is a typical 6 layer stackup? In this article, we’ll look at the thickness of each layer type and total thickness for some common 6 layer constructions.

Overview of a PCB Stackup

A PCB is constructed by laminating together sheets of insulating and conductive materials. Here are the major elements in a multilayer PCB stackup:

Dielectric Layers

These are sheets of insulating material such as FR-4, polyimide, or other substrates. The copper layers are separated by dielectric layers. Common thicknesses are 0.002″, 0.004″, 0.006″, and 0.008″ per layer.

Copper Layers

Sheets of copper foil, typically 1 oz (35 micron) or 0.5 oz (18 micron), are etched to form the conductive traces, pads, and planes. There is usually a layer on the top and bottom, and additional internal layers.

Soldermask Layers

The outer layers are covered with soldermask, a coat of polymer materials, for protection and to prevent solder bridging. Typical thickness is 0.001″ per side.

Silkscreen Layers

Used for component designators, logos, board outlines. Typical thickness is <0.001″ per side.

6 Layer Stackup Configurations

Some common arrangements for 6 layer PCBs:

6 Layer with Blind and Buried Vias

• Top Soldermask
• Top Silkscreen
• Signal Layer 1
• Dielectric 1
• Signal Layer 2
• Dielectric 2
• Signal Layer 3
• Dielectric 3
• Signal Layer 4
• Dielectric 4
• Signal Layer 5
• Dielectric 5
• Signal Layer 6
• Bottom Dielectric
• Bottom Copper
• Bottom Soldermask
• Bottom Silkscreen

This stackup allows for greater routing flexibility by placing signal layers between each power and ground plane. Blind and buried vias can connect between layers.

6 Layer with Skip Vias

• Top Overlay
• Signal Layer 1
• Dielectric 1
• Signal Layer 2
• Dielectric 2
• Power Plane
• Dielectric 3
• Ground Plane
• Dielectric 4
• Signal Layer 5
• Dielectric 5
• Signal Layer 6
• Bottom Dielectric
• Bottom Copper

In this arrangement, skip vias can be used to jumper over the power and ground planes to route signals between the outer layers. This provides decoupling capacitance between the power and ground planes.

Typical Thickness of Each Layer Type

Now let’s look at the thickness of each layer in a typical 6 layer PCB stackup:

Layer	Thickness
Soldermask	0.001″ (1 mil)
Silkscreen	< 0.001″
Signal Layers	0.0028″ (1/2 oz copper)
Dielectric	0.004″
Power/Ground Layers	0.0014″ (1 oz copper)

• 0.001″ Soldermask on top and bottom
• <0.001″ Silkscreen on top and bottom
• 4 x 0.0028″ Internal Signal layers
• 5 x 0.004″ Dielectric layers
• 2 x 0.0014″ Power and Ground layers

Total 6 Layer Board Thickness

Adding up the thicknesses of each layer type above, we can determine the total thickness for a 6 layer FR-4 PCB:

• Soldermask: 0.001″ x 2 = 0.002″
• Silkscreen: < 0.001″ x 2 = <0.002″
• Signal Layers: 0.0028″ x 4 = 0.0112″
• Dielectric: 0.004″ x 5 = 0.020″
• PWR/GND: 0.0014″ x 2 = 0.0028″

Total: 0.035″ (35 mils)

So in summary, a typical 6 layer FR-4 PCB with 1 oz power/ground planes and 0.5 oz signal layers will be around 0.035″ (35 mils) thick. This provides a good combination of layer count, thickness, and manufacturability.

Of course, thicker dielectrics, more copper, and additional layers will increase the overall thickness. Key factors that influence thickness include:

• Dielectric material and thickness
• Copper weight of signal layers
• Use of 2 oz copper or thicker power/ground planes
• Signal vs power/ground layer ratio
• Number of layers
• Overall board size

Effect of Thickness on PCB Performance

The thickness of the PCB stackup can impact performance in several ways:

• Rigidity – Thicker boards are stiffer, which helps prevent warping or flexing under stress. A minimum of 60 mil thickness is recommended for reasonable stiffness.
• Signal integrity – Thicker dielectrics reduce capacitive coupling between traces but increase propagation delay. Adjusting dielectric thickness can help tune impedance.
• Manufacturability – Thinner PCBs can allow for finer features, spaces and traces. But too thin can lead to issues with handling, warping and snapping.
• Weight – Increasing thickness adds weight, which may matter in high density or portable electronics.
• Cost – Adding layers and thickness increases material usage, layer alignment complexity, and drill time – leading to higher fabrication costs.

So in summary, the PCB stackup thickness requires balancing the electrical, mechanical, and manufacturing needs of each design.

Frequently Asked Questions

Here are some common questions about 6 layer PCB stackups:

What is the typical dielectric thickness used in multilayer PCBs?

The most common dielectric thickness used is 0.004″ (4 mils). Other common options are 0.002″ (2 mils), 0.006″ (6 mils), and 0.008″ (8 mils). Thinner dielectrics allow for tighter trace spacing while thicker dielectrics reduce capacitive coupling.

What are the typical copper weights used in PCBs?

1 oz (1.4 mils) copper is standard for external layers and power/ground planes. 0.5 oz (0.7 mils) copper is commonly used for internal signal layers. 2 oz (2.8 mils) or 3 oz (4.2 mils) copper is sometimes used for power planes requiring high current.

How many routing layers are needed for a 6 layer board?

A 6 layer board would typically have 4 routing layers – the two internal signal layers and the external top and bottom copper layers. This provides sufficient trace routing resources for many designs.

What panel sizes are commonly used for PCB fabrication?

Some standard panel sizes are 18″x24″, 21″x24″, and 24″x24″. Maximum panel sizes can go up to 28″x36″ or larger. Smaller panels tend to be more expensive per unit area than larger panels.

Can flex PCBs have 6 layers?

Yes, flex PCBs made from polyimide can have 6 or more layers. However, each layer adds thickness and stiffness. For dynamic or high-flex applications, 4 layers is typically the practical limit before performance is impacted.

So in summary, a 6 layer PCB with standard FR-4 dielectric and 1oz copper planes will typically have a thickness around 0.035″ (35 mils). This provides a good balance of layer count, manufacturability, and cost while maintaining reasonable thickness and board performance. Careful stackup planning is required to optimize the construction for each unique design.