Bow and twist are the two characteristics used to determine the flatness of printed circuit boards. The bow condition is a spherical or cylindrical curvature of board, when its four corners lie in the same plane. However, twist is the condition where the deformation lies parallel to the diagonal of the circuit board. This means one corner is not same as the other three corners. 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. At Twisted Traces, we provide printed circuit boards with SMT values ≤0.75%, and non SMT values ≤1%.
Steps Taken to Avoid Bow and Twist
Bow and twist is affected by the board designs. Material properties and board thickness are the two other factors influencing the board design. At Twisted Traces, PCB designing and assembly is handled by qualified and expert PCB designers. They take the following steps to avoid bows and twists:
- A symmetrical build-up of copper thicknesses, pre-pegs, and cores is selected in the PCB manufacturing process. The risk of bows and twists is high in multilayered circuit boards. This risk is minimized by adding pre-peg to the mix.
- All base materials are selected keeping RoHS compliance in mind. We take care to work with materials that are ideal for lead-free soldering.
- During the construction of a multilayer circuit board, we take care to organize a balanced stack-up of layers. Always, we use two laminates that are affected differently by thermal processes. Balanced stack-up always helps oppose thermal expansion values of laminates against each other, and maintains an even bow and tie arrangement.
- Copper distribution is the key factor that we concentrate upon. Unbalanced copper weights have negative impact on bow and twist. If there are areas with low or high density copper distribution, our designers add copper to balance the low density areas.
- In multilayer PCBs, plane and signal layers are arranged symmetrically around the center of PCB. This is because solid layers expand differently than signal layers, when the signal density is light.
- We take care to avoid mixing materials from different vendors.
- Special attention is given to lay up the material warp and weft appropriately.
- We use proper parameters of pressing in multilayers to reduce the stress on the PCB.
- We understand that heavy thermal shocks during soldering may cause bows and twists. This is why we take care to avoid it during the soldering process. Hence, we only use suitable soldering process. Also, the soldering process is supported with adequate support.