Geometric Distortion
Deviation from a perfectly flat profile occurs when internal stresses cause a printed circuit board to bow or twist away from its intended horizontal plane. This out of plane warpage is typically measured as a percentage of the total board length or as a maximum displacement height. It is a critical metric for ensuring the board can be processed by automated assembly equipment.
SMT Yield
Successful soldering of surface-mount components requires that the board remains flat so that every lead or ball makes contact with its corresponding paste deposit. If the out of plane warpage is too high, the edges or center of the board may lift away from the stencil during printing or away from the solder during reflow. This results in open circuits or bridged joints that require expensive manual rework.
Thermal Cycle
Stresses that cause warping are often dormant until the board is heated above the glass transition temperature of the resin. During the cooling phase of the reflow process, different rates of contraction between the copper and the dielectric pull the board into a distorted shape. Managing the stackup design and the cooling rate is the primary way to minimize this distortion in the final assembly.
