Deflection produced in the target surface for a given amount of force is inversely proportional to Young’s modulus and thickness of the material. A given amount of force produces a larger deflection in a thinner or less stiff (lower Young’s modulus) material than a thicker or stiffer (higher Young’s modulus) one. Figure 1-4 shows how deflection decreases with respect to target thickness for different materials for a fixed button size.

Figure 1-4 Deflection Decreases for More Rigid Materials