Abstract: The size effect of solder ball on the liquid-solid interfacial reaction and microstructure evolution in the soldering process of aerospace electronic micro-solder joints was investigated, and then the soldering process was optimized. The size effect of Sn-3.0Ag-0.5Cu and Sn-37Pb solder balls with different diameters(200, 400 μm) after first reflowed with electroplated Ni/Au on Ball Grid Array (BGA) side to form micro joints was studied. The results showed that the Intermetallic Compound(IMC) at interface of Sn-37Pb micro solder joints was needle-like or short rod-like Ni₃Sn₄, while the interfacial IMCs of Sn-3.0Ag-0.5Cu micro solder joints were irregularly bulk (Cu,Ni)₆Sn₅. Both two types of micro solder joints showed an obvious size effect on liquid-solid interfacial reaction in the soldering process, i.e., when the solder ball becomes smaller, the grain diameter and thickness of interfacial IMC will be larger. Based on the Concentration Gradient Controlled(CGC) interfacial reaction theoretical model, the size effect was attributed to the concentration gradient of interfacial solute atoms. The interfacial solute atomic concentration gradient was smaller in micro joints with smaller size. The IMC grains in smaller micro joints were larger. Moreover, based on the CGC interfacial reaction theoretical model, the process parameters of the Sn-3.0Ag-0.5Cu (200 μm)/Au/Ni solder joints were optimized. The average diameter of interfacial IMC grains was reduced by about 60 % under the optimized process parameters, and the shear strength of solder joints aged at 150 ^oC for 1000h was improved 34 %.