Ductility Property
Tensile stress limits for conductive metal layers define the capacity of a material to withstand mechanical pulling without tearing. Copper foil elongation measures the percentage of permanent deformation that occurs when a sample is subjected to a unidirectional load until it breaks. This property determines how well the circuit tracks and plated holes survive the expansion of the substrate during soldering.
Stress Mechanism
Heat induces volumetric growth in the epoxy resin which pulls on the copper structures within the board. Because copper foil elongation varies by thickness and manufacturing method, high ductility foils are often required for thick panels or high layer counts. The test involves a universal testing machine that pulls a standardized strip of foil at a controlled rate.
Grade 3 foils provide higher values than standard Grade 1 materials to ensure the survival of through-hole barrels during assembly.
Acceptance Limit
Plating parameters and grain structure establish the boundaries of the metal performance. Electrolytic bath chemistry directly affects the crystal size within the metal which in turn governs the copper foil elongation of the finished product. If the metal is too brittle, the rapid heating of a reflow oven will cause the pads to lift or the internal traces to crack.
