Laser-based powder bed fusion (L-PBF) of AlSi10Mg is used to fabricate complex, light-weight structures with high thermal conductivity. Much effort has gone into investigating the mechanical behavior of L-PBF components; however few studies investigated their thermal properties. This investigation characterizes the effect of process parameters on the relative density of AlSi10Mg components fabricated by L-PBF to understand how these parameters contribute to thermal conductivity. Exposure time, laser power, pointwise distance, and build orientation were examined. Results show that these parameters affect the effective thermal conductivity of printed parts by up to 22%. Pointwise distance had the most impact on melt pool size and on effective thermal conductivity compared with other parameters. As the pointwise distance increased, both the conductivity and the melt pool width decreased, whereas the laser power had a negligible effect on both. The effect of exposure time was mainly dependent on the pointwise distance. We show that thermal conductivity is not only related to the relative density of the samples, but the number of the melt pool boundaries in the microstructure also plays a significant role in interrupting the heat flow. A new factor which accounts for the number of melt pool boundaries per unit length in the direction of heat flow is introduced to quantify the phonon scattering associated with the microstructure evolution induced by the process parameter changes. This helps explain the variation in thermal conductivity for samples manufactured with high energy densities which had negligible difference in relative density.