PCB Stack Up & Controlled Impedance Considerations

A well-designed PCB stackup is critical to manufacturability, dimensional stability, signal integrity and controlled impedance performance. Westak works with customers during the design stage to help develop manufacturable multilayer constructions based on board thickness, copper weight, material selection, impedance targets and fabrication capability.​

Use an Even Number of Layers
Multilayer PCBs are generally designed with an even number of layers to support balanced, symmetrical constructions.

Maintain Stackup Symmetry
Copper and dielectric thicknesses should be balanced around the centerline of the board whenever possible. A symmetrical construction helps reduce the risk of bow and twist during lamination and thermal processing.

Balance Power and Ground Planes
Plane layers should be positioned as symmetrically as possible relative to the board centerline.

Consider Copper Distribution
Large differences in copper density between opposing layers can create stress during processing. Copper balancing or thieving may be used where appropriate.

Select Core vs. Foil Construction Carefully
The optimum construction depends on layer count, dielectric requirements, copper weights, material availability, sequential lamination requirements and impedance targets.

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Material Selection Matters

​PCB materials influence dielectric thickness, dielectric constant, loss characteristics, thermal performance and overall manufacturability. Controlled impedance designs should be evaluated using the actual material system expected for production whenever possible.

Material considerations:

• Dielectric constant / Dk
• Dissipation factor / Df
• Tg
• Laminate availability
• Resin content
• Pressed dielectric thickness
• High-speed / low-loss materials
• Material substitutions
• UL requirements
• Halogen free requirements

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Controlled Impedance Design

Achieving controlled impedance requires careful coordination of material properties, dielectric thicknesses, copper weights, and trace geometry. Westak's engineering team works with customers to evaluate these factors and develop manufacturable stackups that meet the design's electrical performance requirements.

Trace widths, differential spacing, and dielectric thicknesses may need to be adjusted during impedance modeling to achieve the specified targets. Unless dimensions are critical to the design's functionality, we recommend allowing reasonable flexibility for manufacturing adjustments, subject to customer approval.
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A ±10% impedance tolerance is generally recommended for manufacturability. Tighter tolerances may be achievable depending on the construction and design requirements but should be reviewed with Westak engineering before the layout is finalized. Additional validation or pilot builds may be required.

Have Questions?

Westak engineers can review your proposed construction, model controlled impedance and recommend available materials before your design is released for production

Additional Resources:
Materials
Design Guidelines
PCB Tooling Packages

Transfer PCB Data
Request for Quote

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What Affects PCB Impedance?

• Dielectric constant decreases → Impedance increases 
• Dielectric constant increases → Impedance decreases
• Dielectric thickness increases → Impedance increases 
• Dielectric thickness decreases → Impedance decreases
• Trace width decreases → Impedance increases  
• Trace width increases → Impedance decreases
• Copper thickness decreases → Impedance increases  
• ​Copper thickness increases → Impedance decreases

​These are general relationships. Actual impedance depends on the complete transmission-line geometry and material properties.​

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Why Etch Factor Matters

PCB traces do not remain perfectly rectangular after etching. The finished trace profile is affected by copper thickness and the etching process, which can reduce the effective trace width and alter impedance. For this reason, impedance modeling should account for manufacturing etch compensation rather than relying only on nominal CAD trace dimensions.

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 HDI and Sequential Lamination Considerations

​HDI constructions involving blind vias, buried vias, stacked microvias or sequential lamination should be reviewed early in the design process. Via structure, dielectric thickness, material selection and lamination sequence can significantly affect manufacturability and cost.

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Information to Provide for Stackup Review

Providing complete design requirements early allows Westak's engineering team to recommend a manufacturable PCB stackup that meets your electrical, mechanical, and material requirements.

General Stackup Requirements

• Layer count and overall finished board thickness
• Copper weights for internal and external layers
• Preferred laminate material or required material properties
• Minimum dielectric thickness requirements, if applicable
• Special thermal, high-frequency, or RF requirements

Controlled Impedance Requirements (if applicable)

• Target impedance values and tolerances
• Single-ended and/or differential impedance requirements
• Applicable signal layers and reference planes
• Proposed trace widths and differential pair spacing
• Impedance coupon or testing requirements

Advanced Construction Requirements (if applicable)

• Blind and buried via structures
• Microvia configurations
• Sequential lamination requirements

​Providing a preliminary stackup, fabrication drawing, or PCB design files with these requirements helps Westak identify potential manufacturing constraints and recommend appropriate materials and constructions before the design is finalized.

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Frequently Asked Questions

What information does Westak need to develop a PCB stackup?
Westak typically needs the layer count, finished board thickness, copper weights, material requirements, and any controlled-impedance specifications. Additional design details may be requested depending on the complexity of the construction.

Can Westak recommend dielectric thicknesses?
​Yes. Westak's engineering team can recommend dielectric thicknesses based on material properties, overall board thickness, copper weights, and controlled-impedance requirements.

Can Westak adjust trace widths to achieve impedance?
Yes, with customer approval. Westak can recommend trace-width adjustments based on impedance modeling, material selection, and manufacturing requirements to achieve the specified impedance targets.

What impedance tolerances can Westak manufacture?
Westak typically supports controlled-impedance tolerances of ±10%. Tighter tolerances may be available following engineering review and may require a pilot build to validate the manufacturing process.

Does controlled impedance increase PCB cost?
Not necessarily. Westak generally does not apply an additional charge for standard ±10% controlled-impedance requirements. Tighter tolerances, specialized materials, or additional testing may increase costs.

When should I involve Westak in PCB stackup development?
Ideally, before finalizing your PCB layout. Early engineering collaboration helps confirm material availability, impedance requirements, and manufacturability while minimizing potential design revisions and production delays.

Can material availability affect my PCB stackup?
Yes. Laminate and prepreg availability can influence dielectric thicknesses, material selection, lead times, and cost. Westak can recommend alternative constructions or materials for customer approval when necessary.

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Need Help Developing a PCB Stackup?

​Design it right before you build it.
Westak can review your stackup, materials, impedance targets, and HDI structure early in the design process to help improve manufacturability and avoid unnecessary revisions later.