Dimensional Change
Three-dimensional enlargement of a cured polymer coating or dry film occurs when it absorbs thermal energy or moisture from the environment. The occurrence of volumetric film expansion is a critical factor in the reliability of printed circuit boards using protective polyimide or acrylic coverlays. This change alters the physical dimensions of the board and creates internal stress.
The expansion continues until the film reaches thermal or moisture equilibrium with its surroundings.
Thermal Effect
Temperature changes drive the expansion rate according to the coefficient of thermal expansion of the polymer material. When the board is heated above the glass transition temperature of the film, the rate of expansion increases dramatically. This rapid change in volume can damage fragile copper traces or micro-vias that are embedded in the laminate layers.
Careful selection of materials with matched thermal expansion rates prevents this damage.
Interfacial Stress
Mismatched expansion rates between the flexible polyimide film and the rigid copper traces generate high shear stress along the adhesive interface. This stress leads to delamination, coverlay lifting, and trace cracking during high-temperature solder reflow. Post-reflow inspection with automated optical systems or cross-sectional analysis verifies the integrity of the adhesive bond.
Ensuring the film has been properly cured minimizes the residual stresses and limits the expansion during assembly. Fabricators use pre-baking steps to ensure that all moisture is removed before reflow, as absorbed moisture would otherwise superimpose a moisture-induced expansion onto the thermal expansion, exceeding the mechanical threshold of the adhesive layer.
