PCB Flatness Tolerance Standards

June 25, 2024

Cadence PCB Solutions

Key Takeaways

Ensuring PCB flatness is crucial for accurate component placement, proper soldering, and minimizing mechanical stress.
IPC-6012 and IPC-A-600 define PCB flatness tolerances for different performance classes and applications, including SMT and non-SMT boards.
Flatness issues are often assessed in terms of bow (cylindrical curvature) and twist (diagonal deformation), with strict tolerances to maintain.

The flatness of a PCB can refer to its twist/ bow, or the flatness of its layers

The flatness of a PCB can refer to its twist/ bow, or the flatness of its layers.

Flatness tolerance refers to the allowable deviation from a perfectly flat surface, ensuring that the PCB can be properly assembled and function correctly in its final application.

A flat PCB ensures accurate component placement, essential for proper soldering and electrical connections. Deviations from flatness can also lead to mechanical stress, misalignment, or interference with other parts. PCB flatness tolerance can refer to multiple aspects of a PCB including bow and twist and board thickness parameters.

PCB Flatness-Related Standards

Standard	Description	Key Flatness Tolerances
IPC-6012	Qualification and Performance Specification for Rigid Printed Boards. Defines performance classes and criteria for various board characteristics including flatness.	Class 1: Cosmetic imperfections allowed. Class 2: High performance, extended life, minor imperfections allowed. Class 3: Critical applications, minimal imperfections allowed.
IPC-A-600	Acceptability of Printed Boards. Details visual and dimensional inspection criteria, including bow and twist tolerances.	Bow and twist tolerances: SMT applications - ≤0.75% of diagonal length, Non-SMT applications - ≤1.5% of diagonal length.
IPC-9641	High-Temperature Printed Board Flatness Guideline. Addresses flatness issues during thermal processing and provides standardized measurement methodologies.	Focuses on warpage control during thermal cycles, especially reflow soldering.

PCB Flatness Tolerance In Terms of Bow and Twist

Flatness in PCBs is assessed by two characteristics: bow and twist.

Bow is characterized by a cylindrical or spherical curvature, with all four corners of the board remaining in the same plane.
Twist refers to deformation along the board's diagonal, causing one corner to be out of plane with the other three.

Bow and twist can be affected by the board design, circuit configurations, and multilayer constructions, which introduce different stress conditions. Material properties and board thickness also play a role in flatness. Bow and twist are measured and calculated as a percentage according to IPC-TM-650 – 2.4.22.

The acceptable limits for bow and twist are:

For printed boards with surface mount components: 0.75% or less.
For all other boards: 1.5% or less.

Thickness and Tolerance

For standard PCBs that are 31 mils or thicker, the thickness tolerance is typically ±10%. This means that a board specified to be 62 mils thick could actually measure between 55.8 mils and 68.2 mils and still be within acceptable limits. For thinner boards (less than 31 mils), the tolerance is tighter, at ±3 mils. Thus, a board specified to be 20 mils thick could measure between 17 mils and 23 mils.

Advanced PCB Thickness Tolerance

For more demanding applications, especially where precise control of impedance and other electrical properties is required, advanced PCBs might have tighter thickness tolerances. These could be as precise as ±5%.The thickness tolerance is crucial because it directly affects the electrical properties of the PCB, such as impedance, capacitance, and inductance. Variations in thickness can lead to variations in these properties, which can affect the performance of high-frequency and high-speed circuits.

Staying Within Tolerance With OrCAD X

Design Consideration	Description	Best Practices	Associated OrCAD X Features
Balanced Copper Distribution	Ensuring even copper distribution across the PCB to prevent differential expansion and contraction.	Use copper pour on unused areas of the PCB to balance the copper distribution. Design the PCB with symmetrical layers to ensure even copper distribution. Avoid areas with high copper density adjacent to low copper density areas.	Easily create and manage copper pours and shapes, including automatic adjustments for components and traces.
Symmetrical Layer Stack-up	Designing the PCB stack-up symmetrically to maintain even stress distribution during thermal cycling.	Use an even number of layers to balance the mechanical stresses. Ensure inner layers are mirror images of each other in terms of copper content. Maintain consistent dielectric thickness between layers to avoid differential stress.	Set up and visualize different stack-ups, ensuring symmetrical layer design and consistent dielectric thickness with the cross section editor.
Controlled Impedance Design	Designing for controlled impedance to maintain electrical performance while ensuring mechanical stability.	Ensure traces are designed for impedance matching to prevent signal integrity issues. Use continuous ground planes to provide stability and reduce warpage. Avoid cross-hatching patterns that can create weak points in the structure.	Set up and enforce impedance control and other electrical constraints to ensure signal integrity with the constraint manager options.
Thermal Management	Incorporating thermal management strategies to minimize warpage due to temperature changes.	Use thermal vias to dissipate heat evenly across the PCB. Integrate heat sinks and spreaders to manage localized heating. Place high-heat-generating components evenly to prevent localized warping.	3D Thermal Simulation: Visualize thermal distribution and manage heat sinks, spreaders, and thermal vias effectively. This is an add-on functionality to OrCAD X.
Mechanical Considerations	Designing for mechanical robustness to maintain flatness during and after manufacturing.	Select appropriate board thickness to provide mechanical stability. Use stiffeners or reinforcements in critical areas. Maintain adequate edge clearance to prevent stress concentrations and warping.	Integrate 3D models to ensure mechanical robustness and proper fit within enclosures. DFM Checks: Perform design for manufacturability checks to ensure mechanical reliability.

To try out these OrCAD X features and ensure your PCB's flatness is within tolerance, start your free trial today!

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