Advanced-Internal-Magnesium-Infiltration (AIMI) MgB2 strands are typically processed with the temperature higher than Mg melting point (650 oC) and heating times of 1 ~ 4 h; therefore, the formation of dense MgB2 layer in typical AIMI strand results from liquid - solid state reaction between liquid magnesium and nano-sized boron powder. In this study, the microstructures and critical current densities of AIMI strands heated with 625 oC were explored. We observed that more uniform MgB2 layer were formed in these AIMI strands through solid - solid state reaction between Mg and B. Both optical microscope and SEM were used to observe the uniformity of MgB2 layer and the distribution of voids and B-rich phases in these AIMI wires. Since the rate of the solid - solid reaction is much lower than that of liquid - solid reaction, long heating times (8 ~ 64 h) were utilized to obtain a thick MgB2 layer for these strands which reduced Je variation along the filaments. The further application of this method to increase the engineering critical current density (Je) of the AIMI strands at 4.2 ~ 20 K is discussed.