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PCB assembly processes: a brief guide

15 March, 2021

In short, circuit board prototyping is rather complex, with many different components needed for each different process. PCB assembly is often cited as being one of the most important; there are many steps and stages to this particular process - some are automated, some are manual and some use both procedures.

Making some key choices will have a significant bearing on the design of the finished board and its manufacture, as well as what type of costs and timescales you are dealing with.

One of the most important questions which manufacturers of PCB assemblies need to ask themselves before they start is which mounting or assembly technology should they use? Could it be Through-Hole Technology or Surface Mount Technology? Or possibly a combination of both, known as Mixed Technologies? We will take a closer look at each type to help PCB assembly manufacturers make an informed decision.

Through-Hole Technology

Regarded as the simplest PCB assembly method, Through-Hole Technology (THT) utilises a combination of manual and automatic procedures. It works well on components with leads or wires that need to be mounted on the board by passing them through holes on the board and then soldering them to the underside of the PCB.

So, for example, in the case of PCB assemblies featuring large components, such as capacitors, or for products where overall board size is not a significant concern, Through-Hole Technology would be the ideal choice.

Classed as a traditional PCB assembly method, the THT process is as follows:

  1. Components are manually placed by a highly experienced technician on corresponding positions in accordance with the PCB design files. The manual pick and place component mounting process requires the utmost precision and speed to ensure the component placement.
  2. The board undergoes an inspection to verify accurate placement of components. If there are any issues found, placement can be rectified at this point only, before soldering takes place.
  3. The THT components are soldered on the underside of the circuit board via the wave soldering system, which solidifies the solder paste and keeps the components intact at their specific positions. The component mounted PCB board moves slowly over liquid solder, heated at a temperature of over 500° F, in a wave motion.

Surface Mount Technology

Known as being more efficient for complex PCB assembly processes, Surface Mount Technology (SMT) consists of a totally automatic mounting process. It is suited to PCB assemblies featuring sensitive and/or extremely small components, such as integrated circuits, and resistors or diodes. Consumer demand covering criteria such as smaller product size and higher function and reliability have forced Surface Mount Technology to replace Through-Hole Technology.

The SMT process involves components being placed automatically on to the surface of the board, known as Surface Mount Device (SMD) assembly.

The SMT assembly process follows the below method:

  1. Using a template as reference (also known as a design stencil or solder screen), the solder paste is applied on the board through a solder paste printer. This ensures that the solder paste is accurately applied at specific locations where components will be mounted, in satisfactory quantity. The component placement is automatic in the SMT assembly.
  2. During inspection, any defects or missing parts will be detected and the printing reworked or the solder paste washed off prior to second printing.
  3. Next is the placement operation. The PCB is then sent to component mounting which is done via a high-speed placement machine. This is quicker than the manual process and so is much more time efficient; it also ensures the precision of specific component locations.
  4. Once all the components have been mounted, the PCB passes through a furnace and goes through reflow soldering, where the solder paste liquifies and settles around the SMD components. Coolers then secure them to the board.
  5. Finally, the PCB is cleaned to remove the contaminants exposed during fabrication and PCB assembly.

Mixed Technology

Mixed technology PCB assembly is designed to provide solutions for applications where a combination of through hole and surface mount assemblies are required. This technology has come to the fore due to the rapid evolution of computerised technology, which has resulted in an increasing number of PCBs being made with ever smaller parts.

Mixed Technology assembly will encompass one of the following approaches:

  1. A single-side mixed assembly - where a single-side PCB undergoes both SMT and THT assembly processes, both on the same side of the board.
  2. One-sided SMT and one-sided THT (also known as a split assembly) where one side of the PCB is assembled with Surface Mount Technology and the other side of the board is assembled with Through-Hole Technology. PCBs like these have normal-size components on one side and smaller components on the other side.
  3. A double-side mixed assembly – whereby both sides of the PCB are assembled with a combination of Through-Hole Technology and Surface Mount Technology. A board of this type features regular components as well as tiny and delicate ones on both sides.

In today’s high-tech world, PCB assembly is becoming an increasingly complex, technical process. As computer devices and electronics shrink in size, PCBs will also need to follow suit in order to efficiently power these products. This means that advanced levels of engineering will be called upon in the manufacture and production of printed circuit boards.

We have shown in this article that when it comes to choosing the correct PCB assembly process, there is a wealth of factors which must be taken into account requiring many key choices and decisions to be made. Even the smallest change or oversight can potentially have a far-reaching impact on production schedules, costs and quality.

Matching the right manufacturing technology to a customer’s specific requirements calls for a fully experienced, expert contract electronics manufacturer, one with a track record of providing high-quality PCB solutions. From Surface Mount Technology to Through-Hole Technology and Mixed assembly, we do it all, ensuring we keep our customers’ exact requirements in mind right from the moment we receive the initial enquiry and throughout the design process.

Find out why UK Circuits is the reliable go-to source for all your PCB needs and the only PCB assembly solution to help your business, by calling us now on +44 (0)161 654 5969 or contacting us via email: