The modulus of elasticity shows us how much the material will distort for a given level of strain. Copper has a higher Young’s Modulus than silver, and both are substantially higher than the figure for an epoxy resin. The inference of this is that, when stress is applied to a composite assembly, it is the laminate that will deform, rather than the solder joint, accommodating stresses due to flexure or changes in temperature.
As you will see later, it is important that copper is ductile, rather than brittle, else there is potential for fracture and consequent open-circuit. Whilst it is easy to understand how copper may be stressed when a board is handled, in practice the point at which copper ductility is most important is in the through-hole connections. This is because the laminate has a high CTE in the Z direction, tending to force apart the ‘copper rivet’, which is the internal plating connecting to via pads. Over the years, substantial improvements in ductility of plated copper have greatly reduced the incidence of stress cracking in the through-hole plating.