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What is PCB assembly?

The PCB assembly process populates a circuit board with all the desired components. The assembly of the components is based on the floor plan and schematic conceived during the design stage. This process can be automated or manual, although, for high-volume production of circuit boards, manual placement is not practical. For the intended efficiency and output, proper planning should be done before the assembly process takes place. This is one of the best ways to reduce the cost and ensure that the board is assembled in the fastest possible way.

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Stages of board assembly

The stages involved in board assembly can be depicted using the flowchart below.

pcb-assembly-flowchart

    1. DFA: Design for assembly is the first stage in which the design is verified with respect to a number of documents such as the BOM, Gerber, Centroid, etc.
    2. PCB material knitting: Once the DFA check is carried out, the components and other items required for the assembly are collected.
    3. SMT assembly: Before the SMT components are mounted, they are checked for any non-washable components. If they are present, their assembly is kept for a later stage. SMT assembly consists of the following steps:
      1. Solder-paste screening/printing: The process of applying solder paste uniformly using stencils is called SMT solder paste screening/printing.
      2. Component placement: After the solder paste is applied on the board, the components are mounted using an automated pick and place machine. Before the mounting, the non-washable surface-mount components are taken away and are soldered only at a later stage after washing the board.
      3. Reflow soldering: The circuit board along with the mounted components are passed through the reflow oven. The applied solder paste is melted and upon cooling the components are fixed to the board.
      4. Automated optical inspection (AOI): AOI inspection detects missing components, quality defects, etc.
      5. X-ray inspection: This method is used for multilayer and complex circuit boards. It reveals any defects in the solder joints of the board that are otherwise difficult to find out.

Mounting of surface-mount components is completed after this stage. Further, the through-hole components are gathered and checked for any non-washable components. If these are present, then just like surface-mount non-washable components, these are also kept away and soldered only after the cleaning stage.

    1. Through-hole assembly: The placement of through-hole components can be manual or automated. Once the components are placed, the soldering process is carried out. There are three types of soldering techniques used in the case of through-hole components.
      • Wave soldering/selective soldering/hand soldering: These three types of soldering can be performed on THT components. The feasible type out of the three is chosen as per requirement.
    2. Cleaning of the assembled boards: After both types of components are mounted on the board, the board is cleaned using deionized water.
    3. Soldering of the non-washable components: If there are any non-washable components in the design, which have not been added, they are soldered to the board at this stage.
    4. Final inspection and testing: Once the assembly is completed, the board is sent for final inspection and later all the required tests are carried out.
    5. Conformal coating: Conformal coating is done to protect the board from moisture and other environmental conditions. Afterward, the assembled boards are packed for delivery.
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Design considerations for PCB assembly

The important decisions to be made regarding the board assembly are choosing the type of components, the method of component placement, and the type of soldering. The type of soldering chosen depends on the components.

Based on the choice of these aspects, the assembly process will yield maximum efficiency and take less effort and time.

Component selection

component-selection-for-pcb-assembly

As per the schematic of a circuit, components with certain values are selected. Later, the type of mounting technology is chosen. PCB designers should stick to the following guidelines during component selection:

  • Reduce the total part count
  • Replace arrays of components with single packages if available
  • Use similar types of packages if the design allows
  • Select SMT components as much as possible and choose THT components only if their SMT versions are not available

Advantages of SMT components:

  • Lower cost
  • Less maintenance
  • Make boards smaller and lighter
  • Easier to automate compared to THT components

Disadvantages of SMT components:

  • Easily damaged with temperature fluctuations
  • Placement requires more accuracy

Only in special cases where the component required is not available in the surface-mount form and is particularly sensitive to temperature, THT components are chosen.

Component placement methods

pcb-component-placement

Placement of components can be done automatically or manually. During this process, we have to consider that the following aspects are always in accordance with the design:

  • Part orientation
  • Proper spacing
  • Pad and hole spacing
  • Pitch of components used
  • The polarity of the components
  • Solder mask bridge size
  • Tented vias (vias covered with solder mask)

Manual placing is feasible only in the case of low-volume assemblies.

Selection of the type of soldering

The type of soldering technique which goes well with the type of components being soldered is critical for the success of soldering. A few points to be noted are:

  • Reflow soldering is not used in the case of designs with a high number of THT components.
  • Wave soldering is used specifically for THT components in the assembly. In this technique, there is a shorter period of exposure to high temperatures as compared to reflow soldering.
  • Selective soldering allows soldering the through-hole parts without harming the delicate SMT packages by targeting specific areas on the board. Different methods are available to execute selective soldering, but the most common is a programmable machine.
  • Hand soldering is rarely used in the assembly process. This is done when the soldering of a particular THT component cannot be performed using wave/selective soldering methods.

Before the beginning of the assembly process, the type of components to be used and the placement requirements of those components are thoroughly verified. The type of soldering to be used should also be decided upon. These are the basic aspects that determine the smooth completion of this process. Once these parameters are chosen properly, the PCB assembly process will yield maximum efficiency.

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