Material Deformation
Differential expansion rates along different physical axes within a laminate material cause mechanical stress during thermal cycling. In a multilayer board, thermal expansion anisotropy occurs because the glass fibers constrain the resin in the horizontal plane while allowing more movement in the vertical direction. This imbalance creates tension at the interfaces between the copper and the dielectric.
Engineers measure these rates using thermomechanical analysis to predict the longevity of the assembly.
Axis Variance
Movement of the material in the Z axis is typically several times greater than the movement in the X or Y axes. This thermal expansion anisotropy puts a significant load on the plated through holes and vias during the soldering process. If the vertical expansion is too high, it can lead to barrel cracking or pad lifting.
Designers choose materials with a low vertical expansion coefficient to minimize the risk of these failures in thick boards.
Joint Integrity
Reliability of the solder connections is directly affected by how much the substrate moves relative to the components. High levels of thermal expansion anisotropy can cause fatigue in the solder balls of grid array packages. Managing this behavior is a requirement for boards that undergo multiple reflow cycles or operate in harsh environments.
Testing involves repeated temperature cycling followed by cross sectional analysis to check for internal cracks. The match between the board and the component expansion is a main factor in long term reliability.
