Printed Circuit Board Assembly Process

04 September 2024
New

From design to testing, a printed circuit board goes through multiple steps of creation and development, including the assembly process. The step comes after the printed circuit board is fabricated, i.e., the holes are drilled, traces made, the layers stacked over one another, the surface finish is applied, and the holes are plated. The assembly process is where the components are installed and soldered on the printed circuit board. This process is usually followed by testing, where the finished PCB (with components) is tested, particularly for its output/performance and electrical characteristics.

Printed Circuit Board Assembly Checklist

One of the best ways to ensure that the assembling process is smooth and maximally productive is to create a printed circuit board assembly checklist and adhere to it. This checklist can be created using the best assembly practices and following the core steps (in the appropriate order).

A typical printed circuit board assembly checklist would include:

Printed circuit board testing. Before you start assembling components on it, ensuring that the circuit board itself meets the performance requirement of the circuit and is electrically, mechanically, and thermally sound can be critical.
Assess the design for components, their quantities, and compare it to the actual PCB to ensure that all the components can be assembled on the PCB. If the housing of a single component is larger than intended, it may disrupt the placement of all the neighboring components.
Make sure all the relevant documents are available and accessible. This includes the design files in the right formats, the Bill of Materials (BoM), and any additional design instructions that might have been communicated for the assembly process.
Component procurement/availability. This step is critical for the printed circuit board assembly checklist for both designers and manufacturers. Make sure the components that are to be assembled on the PCB are available or you have ways to procure them before the assembly process begins. Even though partial assembly is possible, it comes with several challenges, including design which should accommodate an assembly where you solder some components in one step and remaining components in the next one. Ideally, all the components should be available at the time of assembly.
Inspection of the components and the PCB boards for any defects.
Proper solder paste stenciling and application should ensure that the solder paste is applied on all the relevant places and is not flowing out of the designated locations.
The process of adding components to the PCBs (once solder paste is already applied) can be both manual and automated (pick and place), though the latter is the norm nowadays. The pick and place machine should ensure not just proper placement but the right orientation of the components as well.
With the solder paste approach to assembly, the next step should be the reflow oven. Make sure the reflow process, temperatures, and timing are set based on the solder material and the PCB substrate.
Once these components are in place, through-hole components can be installed and soldered.
Cleaning of the PCB to ensure nothing unwanted remains after soldering.
Final inspection and testing include visual or machine inspection of the solder joints to ensure that they are mechanically and electrically sound. It's also time for functional testing of the PCB since it should offer the desired functionality now that all the components have been installed. It can also be tested for specific electrical, mechanical, thermal, environmental, and signal integrity characteristics.
Once the PCB passes the inspections and testing, it can be packaged and shipped to the customers.

Note that this printed circuit board assembly checklist covers the most common steps/parts of the assembly process. However, some assemblies may require additional steps and considerations, due to their unique requirements.

For more sophisticated designs that may require multiple assembly steps, human interventions, use of high-end machines like automated visual inspectors, etc., the assembly process may also be longer and costlier than usual. It may also come with more comprehensive testing services to ensure that the final/assembled PCB is working as intended.

Prototype PCB Assembly Services

Prototype PCB assembly services differ from typical PCB assembly services in a number of ways. The first main difference is the volume. PCB makers that offer prototype PCB assembly services may prioritize them or have a separate assembly line for the prototypes to ensure a rapid turn around, since that’s something most customers expect from prototype PCB services. The other difference is volume. When providing prototype PCB assembly services, the PCB makers/assemblers usually work with a few units at the most at any given time.

The third difference is testing and the insights/information communicated to the customers after the prototypes are assembled. Prototype PCB assembly services may include comprehensive testing of the final/assembled PCB. The PCB maker/assembler may also offer a more detailed report to the customer, detailing any challenges they faced during assembly, like crowded component placement or inadequate clearance for certain components. The customer might be able to use this information to improve upon the design and ensure that the final design doesn’t have these flaws.

Like other comprehensive PCB production services, Prototyping can also be offered as a package deal that includes everything from fabrication to testing, including the prototype PCB assembly services.

SMT Assembly

Surface Mount Technology (or SMT) as a whole and Surface Mount Devices (SMD components) have become such a ubiquitous part of PCBs that it's difficult to imagine a PCB without any SMDs. SMT assembly is the process of assembling SMDs, i.e., surface mount components on the PCB surfaces (both top and bottom if needed). It's an improvement over the more conventional Through Hole Technology (THT) assembling process, where most or all components that needed to be installed on a PCB relied upon through holes and were soldered on the back end of the PCB.

There are other assembly processes as well, like Ball Grid Array (BGA) and Chip-on-Board (COB), but SMT assembly is by far the most common one. Many PCBs use a combination of these assembling processes, especially SMT assembly and THT assembly, which are often seen together.

The SMT assembly process relies on the automated pick and place machines. As the name suggests, these machines “pick” the SMD that has to be installed on the PCB and “place” it at the adequate spot identified beforehand. They ensure that the component is placed on the location precisely and is oriented the right way, since misorientation might lead to electrical and functional flaws.

How the components "stick" to the board in an SMT assembly may differ based on the overall SMT assembly process you are following. The most common one relies upon solder paste and reflow oven. In this SMT assembly process, a solder paste is applied before component placement, and after the placement, the PCB goes through the reflow oven, which binds the component to the PCB through heat, establishing a solid electrical and mechanical connection. However, an SMT assembly may also rely upon a conductive adhesive, where the component bonds to the PCB in the placement step.

Laser soldering is also becoming a desirable method in SMT assembly, especially for high-density circuits, since it offers precise control. The laser is used to melt the solder paste, binding the component to the board.

PCB Assembly Inspection Checklist

Inspection of the assembled PCB is crucial to ensure that nothing has gone wrong in the assembling process and the final product doesn’t just function as expected but is mechanically, electrically, and thermally sound. A PCB assembly inspection checklist can help PCB makers/assemblers conduct a comprehensive inspection of the assembled PCB.

Some of the core components present in most typical PCB assembly inspection checklists are:

Record any deviation from the original assembly plan or any changes made to ensure that the PCB is assembled to the specs. This may include the replacement of suggested/requested components with alternatives because of different housing sizes.
Comparison of the final/assembled PCB with the BoM and original assembly drawings and ensuring all deviations are identified and recorded.
Visual inspection of the components, ensuring that all of them are present and installed as expected. In cramped designs, it might be possible for a pick-and-place machine to place a component without ensuring that it's pushed down all the way through. This may lead to improper adhesion to the solder paste. So, inspection should ensure that all the components are fixed correctly.
Visual inspection for component orientation. Deviations of 180 degrees are difficult to spot and can have adverse impact on the PCB’s functionality and even its electrical integrity. Any discoloration or damage in the component should also be noted in this stage.
Solder joint inspection is next in the PCB assembly inspection checklist. The inspection process may differ based on how the components are soldered. Even though visual inspection (often aided by microscopes) is the most common, Automated Optical Inspection (AOI) and X-ray inspection may also be performed. For through-hole components, soldering at the back of the PCB can be inspected for physical integrity and the absence of any cracks. For SMD components, it's important to ensure that there are no solder bridges (unwanted electrical connections between components), cracks in the solder, open solder joints (gap between component and solder/PCB), etc. Discoloration of a solder joint is also indicative of problems in the connection.
The final/assembled PCB can also be inspected for its overall finish and any issues with its overall integrity and aesthetics.

A more comprehensive PCB assembly inspection checklist might be needed by PCB makers working on highly sophisticated PCB designs or working with unique soldering requirements or special soldering techniques (like laser soldering). It's worth noting that these are elements of an inspection checklist, which is completely different from final PCB testing. The goal of this inspection is to ensure that the assembly process has been completed as intended or as it should be.

Application of Printed Circuit Board Assembly Services

A completed and often tested PCB that is ready to be put to real-world use is the application of printed circuit board assembly services. Printed circuit board assembly services are often offered by the same vendors that offer fabrication services, and in most cases, it's a good idea to opt for these collected services. When the two services are provided under the same roof and by the same vendor, there are usually no communication gaps, and the services are seamless.

The application of printed circuit board assembly services is also influenced by how comprehensive the services actually are. If the services also include proper testing of the final PCB, the application extends beyond just receiving the finalized PCB. You will also have information about the functional, electrical, thermal, and other characteristics of the PCB. This can be critically important information when you actually install the PCB in the device it was designed for.

Final Words

PCB assembly may not seem as complicated a process as PCB fabrication is, but it’s just as crucial for the production of a final product that’s electrically, mechanically, and thermally sound and works as intended (functionality). So both designers and assemblers should adhere to the best practices and industry standards for the assembly process.

The designers should comply with all the assembly requirements the assembler/PCB maker has conveyed. This may include specific design practices, instruction for assembly, flexible BoM, alternative components, etc. The assemblers/PCB makers should ensure that they inspect/analyze the design thoroughly before starting the assembling process. It's also important to ensure that the fabricated PCB is as per design, and if any deviations have been made in the PCB during the fabrication process, they are conveyed to both the assembler and designer so appropriate adjustments can be made.