Material Expansion
Polymer laminates exhibit measurable dimensional changes along all three axes when subjected to rising temperatures. The volumetric increase known as dielectric expansion describes the physical swelling of the non-conductive resin matrix within a printed circuit board. When the board is heated above the glass transition temperature of the polymer, the rate of this swelling increases sharply.
The mechanical swelling dominates the mechanical stress profile of the layered structure.
Via Strain
Mismatch in the coefficient of thermal expansion between the polymer and the metal traces puts immense tension on plated through-holes. Because dielectric expansion occurs much more rapidly than the expansion of the copper barrel, the polymer pulls on the plated metal. This tensile force can cause micro-cracking, barrel cracking, or pad lifting during solder reflow cycles.
Using materials with a lower z-axis expansion coefficient mitigates the hazard of via failure. Fabricators control this through the selection of high-glass-transition resins and woven glass reinforcement styles.
Reliability Assessment
Thermal cycling tests evaluate the durability of the copper vias against the cyclic stretching forces. If the dielectric expansion is too severe, early circuit disconnection occurs during thermal stress tests. Specifying stable low-expansion laminate materials prevents electrical discontinuities during high-temperature operations.
