During the early stages of PCB development, hardware engineers sometimes specify a custom finished board thickness based primarily on mechanical clearance. Values such as 1.4 mm, 1.8 mm, or 2.2 mm may be selected even when the application does not require a non-standard thickness.

Although a custom thickness may appear to be a minor mechanical decision, it can increase material, engineering, and manufacturing complexity. A manufacturer may need to source less-common core materials, create a special prepreg combination, or use a less-established lamination recipe. These factors can affect procurement efficiency, production lead time, and process stability.

At the same time, simply changing a non-standard thickness to a commonly available standard thickness is not always safe. An uncontrolled change can modify dielectric thickness, impedance, mechanical stiffness, via aspect ratio, and assembly dimensions.

The key is PCB stackup optimization. By identifying which physical dimensions are electrically sensitive and which can be adjusted, engineers can often convert a custom board thickness to a standard construction while minimizing changes to the original design.

1. Why Non-Standard PCB Thickness Can Increase Cost

The finished thickness of a multilayer PCB is determined by the combined construction of cores, prepregs, copper foils, and the lamination process.

A standard thickness is generally easier for manufacturers to support because commonly used core and prepreg constructions may already be available in their material inventory. Established material combinations can also simplify production planning and process setup.

A non-standard thickness may require:

• Special core thicknesses
• Uncommon prepreg combinations
• Additional material sourcing
• New lamination setup or process verification
• Additional engineering review
• More complex inventory management
• Longer material preparation or procurement cycles

However, the actual cost impact varies by manufacturer, order volume, material system, layer count, copper construction, and production requirements. Therefore, a fixed percentage increase should not be treated as a universal industry rule.

For low-volume prototypes, special material procurement can have a more noticeable effect on unit cost. For mass production, the impact depends on whether the required materials can be efficiently stocked and whether the construction fits an established manufacturing process.

This is why engineers should consider manufacturability when defining the initial board thickness rather than selecting a custom value solely from mechanical dimensions.

2. Total Board Thickness Is Not the Same as Dielectric Thickness

One of the most important concepts in PCB stackup optimization is that finished board thickness and local dielectric thickness are different parameters.

For a controlled-impedance transmission line, impedance is primarily influenced by the electromagnetic geometry around the trace, including:

• Dielectric thickness
• Dielectric constant (Dk)
• Trace width
• Copper thickness
• Trace-to-reference-plane spacing
• Differential-pair spacing
• Reference-plane configuration

The overall finished thickness of the PCB is not itself the direct input to a basic microstrip or stripline impedance calculation.

This creates an opportunity during thickness conversion.

Instead of changing every dielectric layer proportionally, engineers can identify the dielectric structures that are critical to signal integrity and preserve them while modifying less-sensitive portions of the stackup.

For example, consider a four-layer PCB with:

• Original finished thickness: 1.4 mm
• 50 Ω outer-layer controlled impedance
• Signal-to-ground dielectric thickness: approximately 0.15 mm

If the target is a standard 1.6 mm construction, one possible strategy is to preserve the outer signal-to-reference-plane dielectric structure and adjust the internal core or other mechanically adjustable dielectric construction.

The actual result must be recalculated using the selected material’s Dk, copper thickness, pressed dielectric thickness, and manufacturer-specific lamination data.

If the critical signal geometry remains effectively unchanged, the original routing may require little or no modification. However, this should be demonstrated through impedance calculation and, for demanding designs, appropriate SI simulation and physical verification.

3. Two Main Stackup Reconstruction Strategies

Strategy 1: Preserve Critical Dielectric Layers

For high-speed, RF, and controlled-impedance PCBs, the preferred approach is often to identify and preserve the dielectric geometry surrounding critical signal layers.

The process can include:

1. Identify every controlled-impedance signal layer.
2. Determine its reference plane.
3. Record the corresponding dielectric thickness.
4. Confirm the material Dk and Df.
5. Identify stackup regions that can be modified.
6. Select alternative core and prepreg combinations.
7. Recalculate the complete stackup.
8. Verify impedance and manufacturing tolerances.

For example, the thickness between a high-speed signal layer and its reference plane may remain unchanged while the thickness of an internal power/ground structure is adjusted.

This method can minimize routing changes and reduce engineering workload.

However, it is important not to assume that a layer is electrically “non-critical” simply because it is not directly adjacent to a signal trace. Changes to internal dielectric structures can still affect power integrity, reference-plane behavior, via transitions, mechanical balance, and thermal characteristics.

Strategy 2: Recalculate the Entire Stackup and Adjust Trace Geometry

For boards without strict controlled-impedance requirements, a broader stackup change may be practical.

The engineer can select a standard material construction, recalculate the dielectric geometry, and modify trace width or spacing where necessary.

This approach may be appropriate for some low-speed digital circuits, control boards, and power-oriented designs.

However, the classification of a board as “low speed” should not be based only on clock frequency. Fast signal edges can contain significant high-frequency components even when the nominal data rate is relatively low.

Therefore, signal rise/fall time, trace length, interface type, and system timing requirements should also be considered before deciding that impedance effects are negligible.

4. Four Boundary Conditions That Must Be Checked

4.1 Mechanical Clearance and Assembly Compatibility

Changing finished PCB thickness changes the mechanical envelope.

Engineers should review:

• Enclosure slots
• Board guides
• Board-to-board connectors
• Card-edge interfaces
• Press-fit components
• Standoffs and mounting hardware
• Heat sinks and mechanical supports

Connector thickness requirements are application- and component-specific, so the actual connector manufacturer’s specification should be checked rather than assuming that one board thickness is universally compatible.

4.2 Mechanical Stiffness and Reflow Warpage

Board thickness affects mechanical stiffness. A thinner PCB can have significantly lower bending stiffness, particularly for large boards.

This can become important when the PCB carries:

• Large heat sinks
• Transformers
• Heavy connectors
• Relays
• Large inductors
• Other mechanically concentrated components

After changing the thickness, engineers should reassess stackup symmetry and copper balance.

A physically balanced stackup can help reduce thermomechanical deformation during lamination and SMT reflow.

4.3 Via Aspect Ratio

Changing board thickness also changes via geometry.

If board thickness increases while the drill diameter remains unchanged, the via aspect ratio increases.

Higher aspect ratios can make drilling and through-hole plating more demanding. Therefore, engineers should reassess:

• Finished hole diameter
• Drill diameter
• Via aspect ratio
• Copper plating requirements
• Hole-wall reliability
• Thermal cycling performance

For thick boards or small through-holes, increasing the drill diameter or changing the via structure may be necessary to maintain manufacturing capability.

4.4 Material Tg, CTE, and Electrical Properties

When changing cores or prepregs, thickness should never be the only selection criterion.

Engineers should compare:

• Tg
• Z-axis CTE
• Dk
• Df
• Resin system
• Moisture absorption
• Thermal stability
• Copper compatibility
• Lamination behavior

For high-speed applications, replacing a material with a different Dk can alter controlled impedance even if the nominal dielectric thickness remains unchanged.

For high-reliability applications, differences in thermal expansion and resin chemistry can also affect interlayer stress and long-term reliability.

5. Material and Prepreg Selection During Stackup Conversion

A successful thickness conversion requires more than selecting a different core thickness.

The relationship between PCB core, prepreg, copper density, and lamination pressure determines the final dielectric construction.

Prepreg thickness after lamination is influenced by resin content, glass style, copper pattern density, and resin flow. Therefore, engineers should use the manufacturer’s validated pressed thickness rather than simply adding nominal prepreg thickness values.

For high-speed designs, the material substitution process should also consider:

• Dk tolerance
• Df
• Glass-weave effects
• Copper foil roughness
• Resin distribution
• Moisture behavior
• Frequency-dependent electrical performance

This is especially important when the original design has tight impedance tolerances.

6. How to Control Cost Without Sacrificing Performance

The purpose of converting a non-standard thickness should not be simply to reduce the numerical board thickness or force the design into a standard specification.

A better objective is to optimize the entire manufacturing structure.

Potential cost-reduction opportunities include:

Use commonly available materials:
Where technically appropriate, standard core and prepreg constructions can simplify material sourcing and reduce special procurement requirements.

Reduce unnecessary custom structures:
Avoid creating a unique core/PP combination solely to achieve a mechanically convenient thickness when an established stackup can satisfy the electrical and mechanical requirements.

Preserve existing routing where possible:
For controlled-impedance designs, keeping critical dielectric geometry unchanged can reduce PCB layout modifications.

Reduce engineering iterations:
A clearly defined stackup before prototype fabrication can prevent repeated impedance recalculation, layout changes, and manufacturing trials.

Consider total cost rather than material price alone:
The relevant cost includes material, engineering, setup, testing, yield, procurement, logistics, and production volume.

A standard thickness is therefore valuable not simply because it is “cheaper,” but because it may allow the PCB manufacturer to use a more established and repeatable manufacturing structure.

7. Engineering Change Process

A controlled thickness conversion should follow a formal engineering change process.

Step 1: Define the Original Construction

Document:

• Finished thickness
• Layer count
• Core and PP specifications
• Copper thickness
• Dk/Df
• Controlled-impedance requirements
• Via structures
• Mechanical interfaces

Step 2: Identify Critical Parameters

Determine which dimensions must remain unchanged.

For high-speed designs, this usually includes critical signal-to-reference-plane geometry, material electrical properties, and controlled-impedance trace structures.

Step 3: Build the New Stackup

Create a complete revised PCB stackup using available material combinations.

Do not modify only the finished-thickness value in PCB design software.

Step 4: Perform Electrical Verification

For controlled-impedance designs, perform impedance recalculation and SI simulation where appropriate.

Review:

• Single-ended impedance
• Differential impedance
• Return paths
• Via transitions
• BGA breakout structures
• Eye diagrams
• Crosstalk
• Insertion loss, where relevant

Step 5: Perform Mechanical and DFM Review

Check:

• Board dimensions
• Assembly clearance
• Board stiffness
• Warpage
• Via aspect ratio
• Drill capability
• Lamination capability
• Copper balance
• Manufacturing tolerances

Step 6: Prototype and Validate

Where the change affects critical electrical or mechanical characteristics, build prototypes using the revised stackup.

Impedance coupons can be included when required. TDR or other suitable measurement methods can then be used to compare manufactured impedance against the specified target.

Only after the revised construction has passed the required engineering validation should it be released for mass production.

8. Final Checklist for Non-Standard-to-Standard Thickness Conversion

Before approving the change, engineers should confirm:

• The new PCB stackup is fully documented.
• Critical dielectric thicknesses have been identified.
• Core and prepreg materials are compatible.
• Dk/Df requirements have been reviewed.
• Controlled impedance has been recalculated.
• SI simulation has been completed where necessary.
• Copper thickness remains suitable for power and signal requirements.
• Copper distribution is sufficiently balanced.
• Mechanical clearance has been verified.
• Board stiffness and warpage have been evaluated.
• Via aspect ratio and plating capability have been checked.
• BOM and fabrication drawings have been updated.
• DFM requirements have been reviewed.
• Prototype testing has been completed where necessary.

Conclusion

Converting a non-standard PCB thickness to a standard thickness is fundamentally a PCB stackup optimization task rather than a simple change to the finished-thickness specification.

The key is to separate total board thickness from the local dielectric geometry that controls signal behavior. For high-speed designs, critical signal-to-reference-plane structures should be identified and preserved whenever practical. Other portions of the stackup can then be redesigned using compatible, readily available materials.

At the same time, engineers must evaluate mechanical clearance, board stiffness, warpage, via aspect ratio, material properties, impedance, signal integrity, and manufacturing capability.

A properly validated standard-thickness construction can simplify procurement, reduce manufacturing complexity, and improve production consistency without unnecessarily changing the original PCB design.

Kingda can support multilayer PCB stackup design, material selection, impedance analysis, DFM review, prototype validation, and manufacturing engineering to help convert non-standard PCB constructions into production-ready standard designs.