PCB assembly is a process of fixing components on PCB. This is generally done using soldering process. Soldering involves filler metal and it has usually lower melting points. When subjected to heat, solder melts and joins the adjoining metal over conductive bridge. In case of PCB soldering, there are two types: SMT soldering and through hole.

smt soldering
SMT – Surface mount technology
THT – Through hole technology
In general approach, Bill of materials (BOM) is verified as per schematic and layout. This is done to assure that components are aligned with PCB and can be soldered directly.

Surface Mount Technology:
Solder paste printing – Solder paste is applied on PCB using Stencil. Stencils have slots where pads are there.PCB Component Placement – Placement of the small components is performed by a pick and place machine. Placement by hand is critical and not recommended for SMT technology. For a double sided surface mount assembly Glue dispensing is necessary to hold the components in place on the opposite side.

smt stencil


Solder Reflow – Solder reflow is a in which temperature above which a solid mass of solder alloy is certain to melt (as opposed to merely soften). If cooled below this temperature, the solder will not flow. Warmed above it once more, the solder will flow again—hence “re- flow”. When components are placed and aligned on solder paste, it is taken on reflow oven conveyer belt, which have temperature controlled environment. The solder paste melts over a certain temperature and then solidified when cooled to adjoin two PCB pads to components pins. Then boards are finally inspected for integrity.

Through hole technology:
As components are through hole, they are manually inserted in PCB slots fixed for components. Legd of components are properly cropped and slipped before manual insertion and bent in the direction so that it easily inserts and fits into PCB. After this process, a wave of solder is passed through PCB. In this process, wherever there is scope of electrical soldering, solder gets fixed and electrical connections are made. In an alternate approach, pot solder process is used. In pot soldering, holes and component pin are held close and hot soldering iron is used for point to point connection. This is most basic type of soldering and not used for bulk assembly.

After all these processes, final inspection is done. Printed circuit assembly quality is verified in this stage with the available inspection machines, such as x-ray and conventional AOI machine. There should be no shorts, no solder bridges between solder balls, and no loose solder balls under or around the component or component leads. Also manual inspection is important so that component with recommended rating is put on. Also components are checked for polarity alignment.
PCB is manufactured for prototyping as well as for bulk production. In case of prototyping, small scale production is required as board has to be tested for operation and integrity. Manual process is used in case of prototyping stage so as to avoid high value of machine set-up cost. In case of bulk manufacturing, machine and automated process is best approach. Best practices are opted to get error free results in one go.