How to Measure Bow and Twist on a PCB: Techniques and Tools (2024)

Published Date

Author Cadence PCB Solutions

Key Takeaways

  • Bow and twist in PCBs are measured using specific IPC standards. Bow is assessed by the maximum distance from a flat surface to the highest point, and twist is assessed by the height difference at one corner while others remain flat.

  • According to IPC-6012 standards, bow and twist should not exceed 0.75% for PCBs with surface-mounted components.

  • To reduce the risk of bow and twist, evenly distribute copper across your PCB. Regions with high and low copper density may lead to internal dresses causing bow or twist.

How to Measure Bow and Twist on a PCB: Techniques and Tools (1)
How to measure PCB bow as a percentage

PCBs ideally need to be completely flat, but achieving this is often challenging. Warping, known as bow and twist, typically happens during and after the PCB production process. A PCB bow describes a curvature where the board bends into a convex or concave shape along its length or width. On the other hand, a PCB twist involves the board warping in a rotational manner around its center, resulting in its corners being uneven and not lying on the same plane. Read on as we explain how to measure bow and twist on PCBs.

Contributing Factors to PCB Bow and Twist

  • The presence of large copper planes or traces concentrated in specific areas
  • Asymmetrical layer arrangements with varying thicknesses
  • Board pressing parameters
  • Material selection
  • Component placement

How to Measure Bow and Twist on a PCB

According to IPC TM 650, bow and twist in PCBs are measured differently. According to IPC6012, the maximum bow and twist for boards with SMD components is less than 0.75% and 1.5% for those without.

  • A bowed PCB has all four corners touching a reference plane, and its deflection is measured from the highest point off the table. The bow calculation considers the deflection relative to the board's length or width.
  • A twisted PCB does not have all four corners touching the reference plane; there is a deflection along the diagonal. One method to measure this involves restraining three corners and allowing the fourth to rise freely, then measuring the deflection at the raised corner.

How to Measure Bow on a PCB

Based on the procedure from IPC-TM-650, bow can be measured with the following equation.

PCB Bow Measurement

Bow% = (BowDist / LBoard)* 100%

Where:

  • Bow% is the percent bow of the board, and should be kept less than 0.75% if with SMD components.

  • LBoard is the length of the board flat. This can be measured through your EDA software.

  • BowDist is the maximum bow distance — when placed on a flat surface, it is the distance from the surface to the highest bowed point on the board, as shown in the first diagram.

How to Measure Bow and Twist on a PCB: Techniques and Tools (2)

How to measure PCB twist as a percentage

How to Measure Twist on a PCB

A PCB has twist if, when laid on a flat reference surface, two or three corners of the board remain touching the flat surface. To measure twist, affix the third corner to the reference plane such that only one corner of the board is raised as shown in the diagram. Then, measure the twist distance.

PCB twist can be measured using the following equation.

PCB Twist Measurement

Twist% = (TwistDist / 2×DBoard)×100%

Where:

  • Twist% is the percent twist of the board and should be kept less than 0.75% if with SMD components

  • DBoard is the diagonal length of the theoretical flat board, specifically the diagonal with two corners that lay flat.. This can be measured through your EDA software.

  • TwistDist is the maximum twist distance — when placed on a flat surface, it is the distance from the surface to the highest twisted point on the board, as shown in the diagram.

To ensure the accuracy of this method, three of the board's four corners must make contact with the reference plane, with only one corner being physically pressed. If this setup is not feasible, the IPC-TM-650 outlines a more intricate 'referee method' that can be employed.

The Consequences of Not Measuring Bow and Twist

Warping and bowing can result in misalignment during assembly, complicating the precise placement and securing of components into the board's holes and pads. Moreover, these deformations can cause problems with signal integrity, induce mechanical stress, degrade performance, and eventually lead to component failure over time.

Measuring Bow and Twist With OrCAD X

To reduce the risk of bow and twist, it is crucial to meticulously plan the layer stack-up design and copper distribution, handle and store PCBs properly during and after manufacturing, and select PCB materials with low thermal expansion coefficients. These measures help maintain the flatness and stability of PCBs throughout their lifespan.

In order to minimize bow and twist on your PCB, it’s important to distribute copper as evenly as possible throughout the board, thus minimizing internal stresses. OrCAD X has a variety of tools that you can use to do this.

OrCAD X Tools to For Managing Copper Distribution

Section

Description

OrCAD X Tools

Cross Section Editor

The cross-section editor allows you to set up and adjust the stack-up. You can add or remove layers, set thickness, material, and surface finish for each layer.

  • Cross Section Editor: Access via Tools > Cross Section. Add/remove layers, adjust thickness and materials, set dielectric constants for impedance.

Managing Copper Shapes

Copper shapes can be managed using tools to create, merge, and assign nets to shapes. Shapes can be created as rectangles or polygons and merged if needed. The Z Copy method can be used to copy shapes to other layers or with an offset.

  • Shape Menu: Access for creating and managing copper shapes.

  • Merge Shapes: Combine multiple shapes into one.

  • Z Copy: Copy shapes to other layers or with an offset. Assign Net: Assign nets to shapes for connectivity.

  • Assign Net: Ensure the planes are connected to the correct nets.

Now that you understand how to measure bow and twist on a PCB, enhance your future designs with OrCAD X. Experience these features firsthand—sign up for a free trial of OrCAD X today!

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How to Measure Bow and Twist on a PCB: Techniques and Tools (2024)

FAQs

How do you measure for a bow and twist? ›

Bow and twist in PCBs are measured using specific IPC standards. Bow is assessed by the maximum distance from a flat surface to the highest point, and twist is assessed by the height difference at one corner while others remain flat.

What is the bow and twist spec for PCB? ›

As per the IPC-A-600 standard, all boards should have bow and twist less than 1.5%. For boards with SMD components, the bow and twist values should be less than 0.75% or less.

How do you measure PCB warpage? ›

Basic Principal Of Warpage Measurement

Within the measurement area, measure the difference between the maximum height and the minimum height by comparing it with the maximum height allowance. In these ways, it is possible to measure warpage although the PCB surface is tilted.

What is the formula for PCB warpage? ›

Circuit board warpage formula will be placed on the desktop, the four corners of the circuit board on the ground, measuring the height of the center of the arch, which is calculated as follows: warpage = arch height / PCB long side length * 100%.

How is bow size measured? ›

Bow size is made up of LIMBS+RISER:

The standard riser size is a 25 inch riser. If you combine this with the length of the limbs you will get the bow length. A long limb with a standard (25 inch) riser will give you a 70 inch bow.

What is the acceptable warpage for PCB? ›

According to IPC standards, the permissible warpage for a PCB board with surface mount components is 0.75%, whereas for a PCB board without surface mount components, it is 1.5%.

What is the bow tolerance for PCB? ›

Bow & Twist Tolerance - with & without SMD

4 of IPC-2221, the printed board shall have a maximum bow and twist of 0.75% for boards that use surface mount components and 1.5% for all other boards.

What is the difference between twist and bow? ›

The bow condition is characterized by a roughly cylindrical or spherical curvature of the board while its four corners are in the same plane. Twist is the board deformation parallel to the diagonal of the board such that one corner is not in the same plane to the other three.

What tools are used to measure warpage? ›

Measuring instruments used to measure warpage of a surface include dial gauges, profile measurement systems, and coordinate measuring machines.

What is the root cause of PCB warpage? ›

PCB warpage is often caused by stress, which occurs during the PCB construction process when the board undergoes expansion and shrinking, leading to internal stress that causes warpage. Temperature changes during fabrication can also cause warpage in the PCB.

What is the allowance for PCB warpage? ›

According to IPC-A-600, the allowance on PCB warpage is limited to 0.75% maximum; this provides a threshold above which a board can be scrapped. The same ideas apply to a PCBA that has made it through reflow/wave soldering and is ready to be shipped.

How do you reduce PCB warpage? ›

Solutions to Defeat PCB Warpage

The primary method to balance copper residue between layers of the board lies in adding copper pouring in blank. To reduce deformation stress of the board, it's an agreeable idea to shrink panel size with a rotation panelization method.

How do you Unwarp a PCB? ›

You probably can't fix warping in your PCB after it's already been fabricated. You can prevent an unwarped board from becoming warped during assembly, but only as long as materials were selected properly and the board is put into reflow correctly.

Do you measure a bow strung or unstrung? ›

Unstring the bow and measure from string groove to string groove, following the curve of the limbs and along the bow's belly, but not going into the grip.

What is the bow and warp measurement? ›

Bow is the distance between the surface and the best-fit plane, whereas warp is the sum of the deviations from the best-fit plane. Accurate measurement and control of wafer bow and warp are crucial for ensuring high-quality semiconductor production.

References

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