What are primary and secondary sides of PCB?

I am designing a 2 layer PCB with components on both side of it. SMT components are one one side of PCB, but TH (through hole) components are on both sides. I need to understanding something mentioned by the supplier called primary and secondary sides.

PCB Assembly
November 23, 2023

The primary side is the side that has most of the components, especially the ICs. The most common soldering technique these days is reflow soldering. It is simply easier to get precision temperature profiles on one side of the board only.

The secondary side can also support components, but

it’s harder to control the temperature precisely, and
gravity works against you as the solder paste begins to melt and the components on the underside of the board want to fall off.

Oliver Smith

Oliver is an experienced electronics engineer skilled in PCB design, analog circuits, embedded systems, and prototyping. His deep knowledge spans schematic capture, firmware coding, simulation, layout, testing, and troubleshooting. Oliver excels at taking projects from concept to mass production using his electrical design talents and mechanical aptitude.

Oliver Smith

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