An SMD assembly line is essentially made up of three phases
During the first phasem solder paste or adhesive is placed on the circuit where the SMD componentes are to be placed.
In the next phase, known as pick and place, the components are placed in their correct positions before moving on to the finla phase of the the assembly line.
During the final phase of the assembly process the circuits are placed in reflow ovens which produce enough heat for the the solder to pass into the liquid state and join the substrates in a uniform fashion. After a brief cooling period, the circuits are ready to be used.