Calculating Allowed Bow and Twist for Circuit Card Assemblies
Resources:
Institute of Printed Circuits official website — https://www.ipc.org/
Right the First Time by Lee Ritchie http://www.thehighspeeddesignbook.com/ (Free Download)
The bow and twist allowance in a circuit card assembly (CCA) or a printed circuit board (PCB) extend from the manufacturing processes and result to the installation and mechanical packaging while in operation.
What Specification Standard is Right for My Design?
The Institute of Printed Circuits, or IPC, is the global association trusted by designers and manufacturers to provide industry standards based on testing and peer reviewed research. The associated has developed a series of specification standards labeled “IPC” that are now widely accepted as best practice and standard guidelines.
The structural integrity of the board relies on the proper manufacturing process detailed by specifications called out by the design authority. In the case of circuit card assemblies and printed circuit boards, the critical design specifications fall on either IPC-6013 and IPC-6012.
IPC-6012 - Qualification and Performance Specification for Rigid Printed Boards
IPC-6013 – Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards
Both standards either dictate directly or through referenced specification the acceptability standards, requirements for soldering, cable and wire harness assembly requirements/acceptance, and standards for manufacture, inspection, and test.
What is Bow and Twist?
Manufacturing always includes irregular construction anomalies that creates a deviation of dimension from nominal engineering requirements. These deviations are allowed, to an extent, as described in the printed wiring board (PWB) documentation and the specification standards.
A prominent design and testing requirements on a large majority of CCAs is the bow and twist requirement.
The bow of a CCA is defined as the largest gap or distance measured from a flat surface to the bottom of the PWB when two opposite ends are constrained.
The twist of a CCA is defined as the largest gap or distance measured from the flat surface to the bottom of the PWB when three of the four corners are constrained (assuming a four-sided board).
Both bow and twist gaps are measured with a Coordinate Measurement Machine (CMM), pin gauge (Go/No-Go), or some other equivalent method. The maximum allowed gaps will be calculated below.
How to Calculate Bow and Twist Allowance
The bow and twist requirements will be different for designs with surface mounted components and designs with no surface mounted components.
The maximum allowable bow and twist for a rigid portions of printed boards shall be 0.75% for boards with surface mounted components, and 1.5% for all other applications.
This standard requirement is detailed in the IPC-6013 and IPC-6012 Section 3.4.3. The test method for validating the manufactured part is described under the IPC-TM-650, Method 2.4.22 (linked to the IPC documentation here - PDF).
Calculating Bow
General rule of thumb – Allow for a maximum of 0.0075 inches of bow per inch of length with surface mounted components. Allow for a maximum of 0.015 inches of bow per inch of length with no surface mounted components.
The maximum allowed bow in a CCA or PCB will be measured in both the length and width direction of a four-sided card. More complex boards may require more measurement directions – this will be directed by the design authority, and can be due to types of mounted components, shape of board, critical mechanical packaging requirements, etc.
Where:
Length = The span of CCA or PCB you are measuring for bow across
B = The maximum allowable bow percentage (either 0.75% or 1.5%)
Example:
Let’s take a rectangular shaped CCA with surface mounted components. In this case, the width of the CCA is 3.5 inches and the length is 4.5 inches. The two width edges are constrained on a flat surface acting as a datum, and we need to calculated what the maximum allowable bow gap is.
First, we know this CCA does include surface mounted components, so we will use the 0.75% standard bow percentage.
Second, we know the length edges are being constrained, so we will be calculating the bow allowance across the length of the CCA.
The formula:
So, for this board, a bow of approximately 0.03 inches is acceptable across the length.
Calculating Twist
General rule of thumb – Allow for a maximum of 0.0075 inches of twist per inch of length with surface mounted components. Allow for a maximum of 0.015 inches of twist per inch of length with no surface mounted components.
The maximum allowable twist will be measured between two corners of the CCA or PCB. Again, for more complex boards, this requirement may be further directed by the design authority.
Because the testing procedure assumes the board is measured in a constrained position, a factor of 2x is added to the equation. This accounts for the actual twist acting in two directions.
Where:
D = The diagonal length span of the CCA or PCB you are measuring for twist across
T = The maximum allowable twist percentage (either 0.75% or 1.5%)
Example:
Let’s take the same board example we used above, but this time remove all surface mounted components. The three corners labeled “A” are constrained to a flat surface acting as a datum, and we need to calculate what the maximum allowable twist gap is at corner “B”.
First, we know the board does not include any surface mounted components, and thus is less at risk of damage due to twist. We can use the 1.5% standard allowable twist.
Second, we see the diagonal span we are measuring twist with respect to is 5.7 inches.
The formula:
So, for this board, a twist of approximately 0.17 inches is acceptable across the diagonal span.
What Can Cause Increases in Bow and Twist?
To finish up, here is a list of processes and design decisions that may affect the bow and twist gaps.
Copper distribution
Symmetry of design
Board pressing parameters for multi-layer boards
Material selection and procurement
Orientation and surface used to cool boards
Thermal shock
Avoiding excess bow and twist is as important during the design phase as it is during the manufacturing processes. If a pre-design review with a supplier or the design authority brings up the phrase “dimensions will be altered” or “tolerance will be hard to hold”, further design and reviews should be considered. If this is a critical positioning of components and copper that is causing these concerns, a more detailed manufacturing process discussion should be held.
Finally, keep in mind the mechanical packaging requirements and mounting features on the board. How will the board further flex after installation and during operation?
Jarrett Linowes
Mechanical Engineer
omniamfg@gmail.com
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