This work investigates the effect of adding nickel (Ni)-coated micro-sized precipitated calcium carbonate (PCC) on the hardness performance of near-eutectic tin-zinc (Sn-9Zn) solder and its corresponding microstructure. The surface modification of PCC was acquired by implementing electroless Ni plating, resulting in the deposition of a Ni layer onto the PCC as filler in the Sn-9Zn composite solder. The sample is subsequently subjected to analysis by employing Zeta potential and microstructure analyses to evaluate the effect of the Ni coating to the PCC. The addition of 0.5 wt.% micro-PCC and Ni-coated micro-PCC revealed a significant increase in hardness, particularly with Ni-PCC (SZNMP), reaching 18.5 HV. The enhancement is attributed to the effective dispersion of Ni-PCC within the solder matrix, which mitigates agglomeration issues which associated with uncoated PCC. The Ni coating promotes better filler-matrix interaction, enabling a more uniform distribution of reinforcement particles that helps in producing finer Zn-rich phase. This uniformity is crucial for the Zener pinning effect, where Ni-PCC particles effectively pin grain boundaries, refining the microstructure and increasing hardness.