How Does A PCB Assembly Process Work? – Here Is All You Need To Know

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Have you heard of the PCB assembly processes? Are you wondering what they are and how they help our PCB Assembly services? This article will give you a clear understanding of how the PCB assembly process work. These processes are essentially what enable us to be experts in our field.

A PCB assembly process is the final stage of a PCB design before the product can be shipped. During this process, it is impossible to make any more changes to the product design.

Therefore your PCB assembly must be done right, so you will have to choose a board assembly company that provides high-quality services.

The PCB assembly service is a kind of electronic circuit board manufacturing service. It is the process of soldering or assembly of electronic components to a PCB or printed circuit board. It is done manually, by machines, and through reflow ovens.

 PCB assembly

Before the Assembly Process

Before the assembly process starts, many details need to be taken care of to ensure the quality and functionality of the final product. This is particularly important for printed circuit boards because they include many different components.

The first thing that happens is that some engineers will look at your idea and figure out how it can be fabricated. They will also use specialized software to create a design.

Once the design is completed, they will create prototypes of the circuit board to test it out and make sure that it is functional before any large-scale production takes place.

If you think about it, this step alone can save you and your company time, money, and effort. It means that you do not have to build thousands of units only to find out later that the design has a problem.

Another essential step before assembly occurs is to order any required parts in advance. Many different components may be used on one circuit board, so proper planning is necessary to ensure that everything goes smoothly during the manufacturing process.

Before the assembly process begins, the printed circuit board (PCB) design is sent to a PCB manufacturer. This manufacturer creates the bare PCB from copper foil, fiberglass, and epoxy resin layers.

The PCB manufacturing process can be split into two categories:

  1. The PCB Fabrication Process

The manufacturer takes the Gerber files from the design and uses them to create layers of copper foil patterns on an epoxy board.

  1. The PCB Assembly Process

The populated board is created by attaching components to the fabricated panel. These components include integrated circuits, discrete components, switches, connectors, etc.

After both processes are completed, you will have a fully functioning printed circuit board assembly!

PCB Assembly Process Overview

PCB assembly, also known as PCBA, is the soldering or assembly of electronic components to a PCB (Printed Circuit Board) to form a functional printed circuit assembly (PCA). A PCB is a thin board of fiberglass, composite epoxy, or other laminate materials.

It has copper tracks to connect different components on the board. The electronics parts are generally mounted on the surface of the PCB. However, some specialized boards, like multilayer boards, have features inside the layers of the PCB.

The process of PCB assembly includes mounting various electronic components on it. The details can be passive or active depending on their functionality and type. Passive features include resistors, capacitors, and inductors, while transistors and microchips are functional components. Depending on their size and other factors, these parts can be surface mounted or through-hole mounted

The PCB assembly process can be challenging to visualize, especially for those new to electronics manufacturing. To help visualize the process, we've created a simple overview of the steps involved below.

  1. Component Preparation:

Before the assembly begins, components are prepared by forming leads that will be inserted into plated through holes (PTH) in the board.

  1. Board Handling:

The bare board is inspected and cleaned to remove contaminants before the surface mounting technology (SMT) phase begins.

  1. Surface Mounting Technology (SMT):

Components are placed on the surface of the bare board using a pick and place machine. Solder paste is applied to each component pad before placement. The boards then enter a reflow oven where the solder paste melts, joining components to the board.

  1. Automated Optical Inspection (AOI):

After reflow soldering is complete, the boards are inspected using an AOI machine that takes photographs of each side of every board and compares them to specifications for missing or incorrectly placed components. Any panels with incorrect placements or missing parts are flagged for repair/re-work.

Actual PCBA Process Steps

    1. Solder Paste:

The bare etched PCB is first placed on a "sticky" jig. This is a surface with adhesive which will hold the board firmly in place. The jig has holes that correspond precisely to the location of the pads on the board.

A solder paste stencil is then laid over the top of it all, and a squeegee is used to push solder paste through the holes in the stencil onto all of the pads, leaving a small amount of paste on each place.

Several types of solder pastes are available, but all are made from two main ingredients: alloy solder powder and an organic vehicle (sometimes called flux). Once dispensed onto the PCB, this paste will be used to join together SMT components with their respective pads.

    1. Pick and Place:

The SMD components are picked up by a vacuum nozzle and placed over the corresponding pads with high precision. This process requires exact machines which can detect slight variations in component height and position them accordingly over the pads with an accuracy of ±0.3mm or less.

Next, a pick and place machine is used to pick up each component one at a time, placing them in their designated locations on the PCB. This process is highly automated and generally goes very quickly as tens or hundreds of parts per minute can be placed on the PCB.

The exact location of each component is controlled by a CNC (computer numeric control) system that reads in CAD files containing data about which part is supposed to go.

    1. Soldering:

In this process, soldering iron is used for soldering one component at a time. The process takes place in two steps:

  • Dipping the soldering iron into the solder pot.
  • Use a soldering iron to melt the solder on the PCB and then put the component in the molten solder.

Once placed onto the PCBs, components need to be connected with solder. This is done by melting all solder joints with an oven (reflow oven).

    1. Reflow Oven:  

Reflow soldering uses an oven or hot air convection system to heat the board and melt the solder paste.

The exact temperature profile depends on the solder alloy being used; lead-free alloys require higher temperatures than tin/lead alloys, but they must not be allowed to become too hot, or they will not flow correctly (the term for this problem is "tombstoning").

  1. Wave Soldering:  

This process is used for assembling double-sided PCBs. The machine automatically carries out all operations on a conveyor belt. The board is passed over a wave of molten solder, which results in adhering of components to both sides of the board.

  1. Cleaning:

Once this process is complete, any excess solder paste will have leaked out of the stencil onto the surface of your PCB assembly in a process known as solder wicking or tombstoning.

This excess solder paste needs to be removed from your circuit board so that it doesn't cause short circuits and reliability issues down the line. Moreover, it's sent through a cleaning process that removes any unwanted residue from your board's surface – leaving only those electrical joints you want to remain intact, untouched.

  1. Inspection:   

The quality of solder paste printing, component placement, and reflow soldering is inspected. The accuracy of the component number is checked for any missing components or miss-inserted components.   

The PCB assembly is inspected at several stages of the manufacturing process and at various points along the production line. This helps to ensure that only high-quality PCBs are assembled and improves the reliability of the boards.

  1. Manual Checks:

The inspection can be done manually or automatically. The manual checks, which are more time-consuming and labor-intensive, are carried out during array testing, component placement, soldering (both before and after reflow), cleaning, conformal coating, and final inspection.

  1. Automatic Optical Inspection:

Automated optical inspection (AOI) systems use a series of cameras to inspect the PCB before and after component placement visually. These check for faults such as solder bridges between pads, missing components, or components in incorrect positions. AOI systems can also check for missing or broken traces on the PCB substrate.

  1. X-ray Inspection

X-ray inspection is used to detect defects in BGA packages that optical inspection methods cannot see.

These include tombstoning (components standing at an angle), lifted pads, cracks in solder joints, and voids within solder balls (which can reduce connection reliability).

  1. Test:   

This step is not required for all PCB assembly jobs but is generally required for PCBAs with more complex electronic functions. Various test devices can be used, including a Flying probe, bed-of-nails, and fixtureless test (In-circuit testing).   

  1. Feedback:   

Feedback helps to improve the quality of the PCB assembly process in the future by identifying factors that lead to defects, including design issues, manufacturing issues, and material issues.

PCB assembly

After PCBA

The After PCBA process is the final step of the PCB assembly process. The after PCBA process consists of tests to ensure that the PCB assembly is working properly. The most common testing methods are the In-circuit test and Functional Test.

       1. In-Circuit Test

The first step in the after PCBA process is to test every component on the board with a tester machine. If any pieces are faulty, they get replaced before trying the rest of the board. This can be done at two levels:

      2. With Flying Probe:

A flying probe tester has two probes that move across the entire length and width of the PCB. The probes move to all points on the PCB, detect short circuits and open circuits, and check for incorrect component placement and polarity.

Flying probe testers are generally suitable for low-volume production runs and prototypes because they require less setup time, but they are more expensive than other testing methods.

    3. With Fixtured Test:

A fixture bed has holes into which components can be plugged or mounted on pegs. The bed can be designed specifically for your PCB assembly to fit perfectly onto it. A fixture bed is used with a tester machine to test accurately.

    4. Functional Test

This test checks that all the components work together correctly to achieve the desired function. An example of functional testing would be checking whether the board turns on or sends and receives data over a USB port.

Bottom Line:

PCB assembly is complex, requiring a great deal of attention to detail. In conjunction with the number of steps, that can lead to mistakes being made along the way that are hard to correct once something has been assembled.

To help avoid these errors, you should establish a process to handle board assembly that works for your company. Hopefully, this guide has given you some insight into how PCB assembly works and how you might be able to improve your PCB assembly process.

If you are trying to decide between outsourcing your next PCB assembly project and setting up an in-house assembly house, we hope that this gives you some good food for thought.


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