Stress Boundary
The theoretical plane inside a multi-layered structure where the longitudinal stresses are zero during bending separates the regions of tension and compression. Identifying the neutral mechanical axis is essential for determining the strain experienced by components mounted on the surface or embedded within the layers. This axis sits at the geometric center of the board only when the materials and the copper distribution are perfectly symmetric.
Component Reliability
Placing sensitive components such as ceramic capacitors or fine-pitch ball grid arrays near the neutral mechanical axis minimizes the mechanical stress they experience during board flexure. Because the strain increases with the distance from this axis, components on the outer surfaces of a thick board are more susceptible to solder joint cracking or pad cratering. Designers sometimes shift the axis by adding stiffeners or adjusting the copper balance to protect critical parts, ensuring they remain within the safe operating region even during thermal cycles.
Strain Calculation
Mathematical models of the board cross-section calculate the position of the axis by considering the elastic modulus and thickness of every layer. When the board undergoes thermal expansion or mechanical bending, the distance from this zero-stress plane determines the magnitude of the displacement for each trace and via. This calculation is a fundamental part of predicting the fatigue life of an assembly in harsh environments.
