What is PCB Assembly

Electronic PCB assembly is a term used in electronic product and device manufacturing. This involves the assembly of various electronic components and computer parts, onto a printed circuit board (PCB).

The printed circuit board manufacturing, production and assembly process is complex and combines several stages. Furthermore, the process involves identifying, correctly sourcing  and handling components that are then fitted to a PCB either manually or by machine. 

An Overview of The Printed Circuit Board (PCB) Assembly Process

The individual stages necessary in the PCB assembly process are split in 2 sections SMT (Surface Mount Technology) or PTH (Plated through Hole Technology).

In the case of SMT – the manufacturing sequence involved is rudimentarily described as:

  1. Application Solder paste to the board  (High precision printing machine)
  2. Pick and place the components (High accuracy Pick and Place Machines)
  3. Soldering  (Controlled thermal reflow using Convection Reflow  Ovens)
  4. Inspection and test. (Automated Optical Inspection)

Constant monitoring of each process is vital and helps ensure that the finished product is of the highest quality possible.

Below, we examine each stage in more detail. Please note that each description is configured on the assumption that surface mount is being used, as pretty much most  modern  PCB assembly uses surface mount components today.

1. Solder paste 

Before the components are added to a board, solder paste needs to be applied to the areas of the board like component pads where required using a solder screen. Small grains of solder combine with flux in a gel matrix  to make the solder paste. Using the solder screen directly on the board and fixed in the correct position, a metal squeegee blade moves across the screen squeezing a tiny controlled amount of solder paste through the apertures in the screen and onto the board.

The solder screen has been formed from the printed circuit board files, it has holes perfectly positioned on the solder pads, ensuring solder is inputted on the solder pads alone. The level of solder applied needs to be controlled to guarantee the right amount in each joint – this is a critical part of the assembly process.

2. Pick and Place

The next part of the assembly process sees the solder-loaded board moved into the pick and place process. It is here where a machine loaded with reels, tubes or trays of components will pluck the components from the reels and other dispensers and offload them into the correct position on the board, in the correct orientation. The components dispensed onto the board are held together by the tack strength of the solder paste.

As long as the board is not jolted, this is sufficient to keep them in place. In certain assembly processes, the pick and place machines add small droplets of glue to fix the components to the board as a supplemental process. However, this is usually only completed if the board is to be wave soldered.

The downside of this is that any repair is made more challenging by the presence of the glue, despite some glues being designed to degrade throughout the soldering process. The pick and place programming is considerably simplified by the PCB design information which determines the position and component information required to programme the pick and place machine. 

Modern machines have laser and on the fly vision which allow components to be inspected for damage before placement during the assembly process.

3. Soldering

Now the components have been placed on to the board, the next stage of the assembly production process involves a soldering machine. This effectively fuses the component joints with the corresponding pads on the PCBs. Reflow soldering techniques are used more often for surface mount components than using wave soldering (used for PTH components), although some boards may be passed through a wave soldering machine.

If wave soldering is used, as the solder is provided by the wave soldering machine then solder paste is not added to the board.

The boards are inspected after the completion of the soldering process.  Automated optical Inspection machines are available to undertake board inspection because there are usually hundreds of components. They rapidly and consistently inspect boards and detect poor joints, misplaced components, and under some instances the wrong component which could be missed by the naked eye during manual inspection.

4. Test And Feedback

Testing can be completed in various ways and is an essential part of the process. All boards are tested vigorously before leaving the factory. Monitoring the outputs and investigating any detected failures are the most efficient methods of ensuring that the manufacturing process is running satisfactorily.

The optical inspection stage is the optimum opportunity to intercept a problem. By inspecting and intervening if necessary, at this stage means that process errors can be identified quickly and rectified before too many boards are built with the same defects. Functional testing at the final stage ensures the client receives a fully working board as per their expectations.