According to the IPC-A-600 standard the definition of bow and twist in a PCB is:
“Flatness of printed boards is determined by two characteristics of the product; these are known as bow and twist. 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. Circular or elliptical boards must be evaluated at the highest point of vertical displacement. Bow and twist may be influenced by the board design as different circuit configurations or layer construction of multilayer printed boards can result in different stress or stress relief conditions. Board thickness and material properties are other factors that influence the resulting board flatness.”
Why is the Flatness of a PCB Important?
During the manufacturing process of the PCB the flatness of the production panel is important for handling and for positioning on the production machines.
During the assembly process the flatness of the panel is important for correct solder paste deposition and component mounting.
Flatness is an aspect of the visual quality appearance of the PCB.
What are the Acceptability Criteria for Bow and Twist?
For all PCB’s the bow and twist should be 1.5 % or less
For PCB’s using SMD components ( the majority of the boards) the bow and twist shall be 0.75% or less.
How to Measure the Bow and Twist?
The IPC-TM-650 test methods manual describes the how to calculate the bow and twist percentages.
Warp Determination: The percentage K of the deflection (tk) relative to the length (L) of the curved edge
K= tk/L*100 %
If the warp occurs both lengthwise and crosswise, the largest value counts.
Twist determination: Twist is the percentage V of the deflection (tv) relative to the length (D) of the diagonal:
V=tv/D *100 %
What can the PCB Designer do to avoid Bow and Twist?
Create a symmetrical copper distribution. As far as possible aim for an even copper distribution across each layer. For multilayers as far as possible arrange signal and plane layers symmetrically around the centre of the PCB. If there are areas of very low copper density and areas of high density or full copper its a good idea to add copper to the low density areas to balance out the copper distribution.
Select a symmetrical build-up of cores, pre-pregs and copper thicknesses.
What can a PCB Producer do to avoid Bow and Twist?
Select base materials that are suitable for lead-free soldering. To find out more about the materials we use in Eurocircuits please see the Base Materials section of our Download page.
Use proper pressing parameters for multilayers to reduce stress in the final PCB.
Do not mix materials from different types or vendors, and lay up the material warp and weft correctly.
Use horizontal ovens for the curing processes.
Cool down the panels on a horizontal surface (for instance after hot-air solder-levelling).
What can an Assembly House do to avoid Bow and Twist?
Avoid heavy thermal shock during the soldering process by using a suitable soldering profile.
Organise adequate support during the soldering process.