What is Bow and Twist?
According to the IPC-A-600 standard bow and twist (flatness of the board) 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 printed circuit board important?
- During the production of the board the flatness of the panels is important for handling and for positioning the panels on the machines
- During the assembly process the flatness of the panels is important for correct solder paste deposition and component mounting
- Flatness is an aspect of the visual quality appearance of the boards.
What are the acceptability criteria for bow and twist?
- For all boards the bow and twist should be 1.5 % or less
- For boards 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 method to calculate bow and twist percentages
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 it”s 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. We use for instance IS400 from Isola or NP155 from Nanya
- 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.