Physical Metric
Measurement of the ability of a material to stretch or deform plastically before it reaches the point of fracture. In the context of copper foils and traces, tensile elongation represents the percentage increase in the length of a specimen during a pull test. This property indicates the ductility of the metal and its ability to absorb stress without cracking.
High values are necessary for copper layers that must withstand the z axis expansion of the epoxy glass laminate during soldering operations.
Ductility Requirement
The thermal expansion coefficient of the organic resin in a printed circuit board is much higher than that of the copper plating. When the board is heated, the resin expands and pulls on the copper barrels of the through holes. If the tensile elongation of the copper is too low, the metal will snap rather than stretch.
This results in open circuits that are difficult to detect without specialized testing. Plating chemistry and the concentration of organic additives are the primary factors that determine the final ductility of the copper. Annealing the board after plating can often improve this metric by allowing the grains to reorganize into a more stable and flexible structure.
Mechanical Evaluation
Standardized tests involve pulling a strip of foil until it breaks. The change in the distance between two marked points on the foil is measured to calculate the percentage.
