Introduction

Determining the right printed circuit board (PCB) thickness is an important aspect of the design process. The thickness impacts manufacturing feasibility, component clearances, stiffness, thermal performance, weight, and cost. With PCBs becoming more complex including embedding actives and passives, the number of layers increasing, and higher density designs with HDI, selecting appropriate board thicknesses is more nuanced than ever. This article will provide guidance on how to optimally design PCB thickness to meet mechanical, electrical, and fabrication requirements.

Key Factors In PCB Thickness Selection

Several interrelated considerations influence the choice of PCB thickness:

• Number of layers – More layers require greater thickness to accommodate inner layer spacing. High layer counts (>36) drive thicker designs.
• Component height – Clearance must be allowed between tallest components and outer layers for assembly and preventing shorts.
• Routing density – Compact routing of fine features needs thicker cores for trace impedances and layer separation.
• Stiffness – Thicker boards provide more rigidity, important for large PCBs and minimizing flex stress.
• Thermal dissipation – Thicker cores spread heat better laterally. But thicker PCBs hinder heat escape.
• ESD protection – For high voltage PCBs, thicker dielectrics help prevent electrostatic discharge between layers.
• Embedded components – Cavities and space for embedded actives or passives add depth.
• Weight – In mobile or aerospace apps, minimizing thickness reduces weight.
• Cost – Thicker PCBs require more materials driving up fabrication costs.

Weighing all factors culminates in an ideal thickness. Next we’ll examine how to determine thickness for specific requirements.

Calculating Minimum Thickness

The minimum workable PCB thickness is based on:

Copper Layer Stackup

• Number of copper layers
• Copper weight (oz/ft2) of outer layers
• Copper weight (oz/ft2) of inner layers
• Copper thickness (mils) of outer layers
• Copper thickness (mils) of inner layers

Dielectric Stackup

• Dielectric material between layers
• Dielectric thickness between layers

Component Height

• Height of tallest component(s) above board
• Minimum clearance required above/below components

Minimum Bend Radius

• Flexibility required for rigid-flex designs

Example:

For a 6-layer board with:

• Layer 1, 2, 5, 6: 1 oz copper (0.7 mils thick)
• Layer 3, 4: 0.5 oz copper (0.35 mils thick)
• Dielectric between layers: 3 mils
• Tall components: 0.120” height
• Minimum component clearance: 15 mils above/below
• Minimum bend radius: 10X board thickness

Copper Layer Stackup

• Layer 1: 0.7 mils
• Layer 2: 0.7 mils
• Layer 3: 0.35 mils
• Layer 4: 0.35 mils
• Layer 5: 0.7 mils
• Layer 6: 0.7 mils

Total Copper Thickness: 3.45 mils

Dielectric Stackup

• Layer 1 to 2: 3 mils
• Layer 2 to 3: 3 mils
• Layer 3 to 4: 3 mils
• Layer 4 to 5: 3 mils
• Layer 5 to 6: 3 mils

Total Dielectric Thickness: 15 mils

Component Clearance

• Component Height: 120 mils
• Add 15 mils clearance above/below
• Total Height Needed: 150 mils

Minimum Bend Radius

Flex portion minimum bend radius: 10X board thickness If 0.1” bend radius needed → 0.1/10 = 0.01” minimum thickness

Total Minimum Thickness

Copper: 3.45 mils Dielectric: 15 mils Clearance: 150 mils Bend (N/A here): 1 mil

Total: 168.45 mils (0.168”)

Round up: 0.170” minimum thickness

This methodology can be followed to calculate minimum workable thickness for any PCB stackup and design constraints.

Adjusting Thickness for Functionality

Beyond the bare minimums, PCB thickness is often increased to optimize performance or fabrication. Common reasons include:

Trace Impedance Control – Matching trace impedances requires specific dielectric spacing. Thicker material may be needed between layers to achieve target impedance.

Signal Integrity – Thicker dielectrics reduce capacitive coupling and crosstalk between layers.

Noise Isolation – Additional dielectric layers helps isolate sensitive analog signals from noisy digital routing.

Rigidity – For large boards (>15” edge), increasing thickness adds stiffness to counter flexing forces.

Warpage Reduction – Symmetric center-core construction with thicker dielectrics minimizes warpage from manufacturing stresses.

ESD Resistance – More dielectric buildup helps withstand electrostatic discharges in high voltage applications.

Thermal Management – Added inner layers enables lateral heat spreading while minimizing impact on thickness.

Embedded Components – Cavities for embedded actives and passives require extra thickness.

In each case, the cross-functional design team weighs the tradeoffs of increased thickness against other constraints to find the sweet spot.

Standard Thickness Increments

Rather than arbitrary values, there are common PCB thicknesses used across the industry:

These standard thicknesses align with typical layer counts and offer sufficient margins for most applications. They provide a good starting point when estimating initial thickness. As the design progresses, the thickness can be dialed in based on specific requirements.

Thickness Ranges By Application

Some useful thickness guidelines by application:

• Consumer electronics – 0.030” – 0.125”
• Networking/telecom – 0.062” – 0.250”
• Automotive – 0.093” – 0.125”
• Industrial controls – 0.125” – 0.250”
• High speed computing – 0.093” – 0.187”
• Aerospace/military – 0.025” – 0.200”
• Wearables – 0.005” – 0.030”
• IoT devices – 0.010” – 0.125”

Of course, specialized or high complexity designs may warrant going outside these general ranges. But they provide a reasonable starting point when estimating thickness by application.

PCB Stackup Configurations

There are several stackup arrangements that impact overall thickness:

Symmetric

Layers are distributed equally about the center core. This avoids mechanical stresses from asymmetric construction. Often used for >8 layer designs.

Asymmetric

Layers are grouped toward one side of the center core. Can cause bowing but uses less dielectric. Often used for simpler <8 layer boards.

Core Thickness

Alternative core thicknesses like 0.024”, 0.049”, or 0.081” can be specified when total thickness requirements differ from standard sizes.

Buried Cores

A buried core adds rigidity for ultra-thin boards. A thin core is laminated between outer buildup layers. Allows high layer count in minimal thickness.

Mixed Cores

Different core thicknesses can be combined for complex requirements. Thinner base cores reduce weight while thicker inserted cores add rigidity.

Copper Balancing

Equal thickness copper layers on extremes minimizes curl and wrinkling. Heavier internal planes provide planarity.

The right stacking arrangement contributes to optimizing the finished board thickness.

Panel Thickness vs Final Thickness

It helps to distinguish between:

Panel thickness – The initial PCB panel thickness through fabrication, often slightly thicker than final thickness.

Final thickness – The completed board thickness after processing. May involve post-fabrication steps like surface grinding to achieve final thickness target.

For example, a 0.100” final thickness board may use a 0.104” or thicker panel to allow for processing variances and finishing.

Thickness Tolerances

Standard PCB thickness tolerance is ±10% of the nominal value. However, tolerance can be reduced to ±5% or tighter when holding tight finished thickness is required.

Tighter tolerances often warrant steps like starting with oversized panels and grinding down precisely to achieve specified thickness.

Markings For Thin Boards

For rigid boards under 0.031” thick, fabrication notes indicating “Thin Board” alerts manufacturing processes must delicately handle the thinner material to avoid damage.

Flex Circuits

Flex PCBs involve separate considerations for minimum bend radius, flex layer thickness, stiffener thickness, and other unique constraints.

Consult the Fabricator

To ensure manufacturability and avoid surprises, always review your thickness design requirements with your PCB fabricator early in the design process. An experienced manufacturer can validate your design or suggest improvements.

Conclusion

Designing the optimal PCB thickness requires juggling mechanical, electrical, thermal, fabrication, and cost considerations. Following the guidance in this article will help you select appropriate thicknesses to meet your product needs while enabling efficient manufacturing. Partnering closely with your fabricator is key to optimizing thickness. PCB thickness may seem like a simple issue, but deserves thoughtful design consideration given how foundational it is in determining the quality, cost and performance of the end product.

Frequently Asked Questions

Q: At what point should PCB thickness be considered in the design process?

A: PCB thickness parameters should be estimated starting in the preliminary design stage based on likely layer count, component height, compliance requirements, etc. As the design progresses, thickness can be refined after spacing, stackup, embedded components, and other details are worked out. Consult the fabricator early to validate thickness feasibility.

Q: How does thickness impact the cost of PCB fabrication?

A: Generally, thicker PCBs cost more to fabricate than thinner ones due to increased materials usage. However, ultra-thin boards under 0.020” can also cost more due to additional handling care required. Moderate thicknesses between 0.062” – 0.125” are often the most cost effective.

Q: Can PCB thickness be increased by adding copper rather than dielectric?

A: Yes, thick copper up to 20 mils can be plated onto layers to add rigidity and thickness without impacting dielectric spacing. However, thermal performance diminishes as the board heats up faster due to higher copper content.

Q: What are recommended clearances between components and outer board surfaces?

A: For circuit boards under 24V, a minimum clearance of 0.015” (15 mils) between components and outer surfaces is typical. For higher voltages, greater clearances are advisable, sometimes up to 0.250” (250 mils) or more.

Q: How are very thick PCBs above 0.250” fabricated?

A: Extremely thick boards can be manufactured by laminating separate 0.250” cores together. The individual cores are bonded under high pressure and temperature to form a single thick board ready for further processing. But such thick boards can be challenging to fabricate and often unnecessary.