In the field of electronic packaging, the manufacturing processes involve electroless plating. The microstructure of these materials differs from bulk materials, so do their mechanical properties. However, bulk material properties are commonly attributed to them. This paper is focused on the characterization of the mechanical properties of a multilayer coating of copper and nickel–phosphorus deposited on a Polymer Core Solder Ball (PCSB) used for BGA (Ball Grid Array) connections. In this coating, the copper is nanocrystalline, while the nickel–phosphorus is amorphous. In order to understand the influence of these microstructural differences on the mechanical properties of the PCSB, a multiscale study is performed. First, at the microscopical scale, the characterization of the mechanical properties (Young modulus (E) and Yielding strength (σ)) of two coatings (Cu, Ni-P) is conducted. For this purpose, insitu micro-mechanical tests (Micro pillar compression and nanoindentation) are carried out. Then, at the mesoscopical scale, the previous properties are validated by applying a compressive loading to a PCSB solder ball. A correlation between the numerical and experimental approaches is performed and revealed that the coating properties are different from those of the bulk material.