A guide to PCB assembly processes

The PCB assembly process consists of many steps and stages, some manual, some automated, with important choices to be made along the way that will influence the design of the finished board, its manufacture and the costs and timescales involved.

One of the key choices for manufacturers of PCB assemblies is which mounting or assembly technology to use: Through-Hole Technology or Surface Mount Technology, or possibly a mixture of the two – Mixed Technologies.

Through-Hole Technology

Through-Hole Technology (THT) is particularly suited to PCBAs featuring components with leads or wires that need to be passed through holes on the board and then soldered on the other side of the board. For example, Thru-Hole Technology would be the choice for PCB assemblies featuring large components, such as capacitors.

Through-Hole Technology Assembly is a traditional PCB assembly method, with the thru-hole mounting process involving both manual and automatic procedures. The THT process follows the following method:

  • A technician manually places components on corresponding positions according to the PCB design files.
  • The board is then inspected to determine whether components have been accurately placed. Any issues concerning component placement can be remedied prior to soldering.
  • The THT components are soldered onto circuit board using the wave soldering system, where the board moves slowly over a wave of liquid solder at high temperature, attaching the thru-hole components to the board.

Surface Mount Technology

Surface Mount Technology is suited to PCB assemblies featuring sensitive and/or extremely small components, such as resistors or diodes, and integrated circuits. These are placed automatically on to the surface of board (known as Surface Mount Device (SMD) assembly).

The Surface Mount Technology (SMT) assembly is considered a more efficient PCB assembly process than Through-Hole Technology assembly, as it consists of a totally automatic mounting process. The SMT assembly process follows the following method:

  • Solder paste is applied on the board through a solder paste printer, using a template (also known as a stencil or solder screen) to ensure it is accurately applied to those places where components will be mounted. An inspection will pick up any defects and the printing reworked or the solder paste washed off prior to second printing.
  • The PCB is then sent to the pick-and-place machine where components or ICs are mounted on to their corresponding pads via component reels in the machine.
  • Once all the components are in place the board goes through reflow soldering, passing through a furnace, causing the solder paste to liquefy and bind the SMD components to the board.

Mixed Technology

In a world where there are now daily advancements in science and technology, electronic products are increasingly complex, requiring evermore complicated, integrated and smaller Printed Circuit Boards to operate them. Today’s PCBAs are therefore more than likely to feature more than one type of component technology – Mixed Technology.

Mixed Technology assembly includes:

  • Single side mixed assembly – whereby a single-side PCB undergoes both SMT and THT assembly processes.
  • One side SMT and one side THT – whereby one side of the PCB is an SMT assembly and the other is a THT assembly.
  • Double side mixed assembly – whereby both sides undergo both SMT and THT assembly processes.

How to choose an assembly technology

PCB assembly is a complex, technical process, with numerous factors needing to be taken into consideration and many key choices and decisions to made, with even the smallest change potentially having far-reaching consequences for production schedules, costs and quality.

Determining which manufacturing technology is right for a customer’s requirements is something that an expert contract electronics manufacturer will consider from the moment they receive the initial enquiry and keep in mind throughout the design process.

Find out how MPE Electronics’ high quality PCB assemblies and reliable service can help your business – call us now on 01825 764822 or email enquiries@mpe-electronics.co.uk.