Material Strain
Latent mechanical forces locked within a manufactured component or laminate sheet exist independently of external loads or thermal gradients. This condition, called internal residual stress, arises during the high-temperature laminate pressing and curing process. Differences in the thermal contraction rates of resin and copper generate tension and compression within the finished board.
If these forces remain unbalanced, the board undergoes dimensional changes when the metal is etched or the board is heated. Controlling these forces ensures the dimensional stability of multi-layer boards.
Thermal Origin
Lamination processes and plating stages contribute directly to the buildup of these internal stresses. When the epoxy resin cures at high temperatures and subsequently cools, it contracts faster than the glass fabric and copper foil. Electroplating steps also deposit metal under tension or compression, depending on the chemical additives used in the copper bath.
This mismatch creates an ongoing mechanical tug-of-war within the dielectric layers that remains hidden until subsequent assembly stages.
Mechanical Consequence
Unbalanced stress fields cause the board to warp during solder reflow, leading to assembly defects like head-in-pillow joints. When internal residual stress is released during the heat of assembly, the board bends and shifts. This distortion prevents proper contact between component balls and the solder paste on the pads.
