Flatness Tolerance
Maximum allowable deviation from flatness describes the structural integrity of a printed board. Standardized bow and twist limits define the permissible curvature across the diagonal or along the length of a panel. Values for rigid boards typically remain at 0.75 percent for surface mount applications or 1.5 percent for through hole designs.
Measurement Procedure
Verification involves placing the board on a precision surface plate with the convex side facing upward. A feeler gauge or a dial indicator quantifies the displacement at the center or the corners. Physical constraints during this test ensure that no external pressure modifies the natural state of the laminate.
Assembly Constraint
Excessive deformation prevents the accurate placement of fine pitch components and leads to solder bridging or open joints. Automated assembly equipment relies on a flat surface for vacuum nozzle stability and stencil contact. When a board exceeds the specified boundary, the pick and place machine may fail to register fiducials or deposit paste uniformly.
Reflow temperatures can further exacerbate internal stresses and increase the final curvature. Thermal profiles must remain within the limits of the material Tg to prevent permanent warping. Corrective baking cycles sometimes reduce these stresses before assembly begins.
