How is a PCB Assembled?

Projected to touch a figure of $80 billion by 2026, the global PCB industry is still a segment of the larger electronics ecosystem. That said, understanding the underpinnings of this industry can provide actionable insights into how electronics products must be designed, engineered, and even manufactured.

While most professionals have a considerable understanding of the PCB manufacturing process, the assembly process is often ignored. How a PCB is put together can significantly impact the further quality control process, functionalities of the product, and even the entire project's budget. Thus, it is critical for everyone in the PCB ecosystem – project owners, engineers, designers, and even students to understand the PCB assembly process.

What is PCB Assembly, and How is It Different from PCB Manufacturing?

Several industry incumbents confuse PCB assembly with the PCB fabrication process. The latter deals with the manufacturing of the printed circuit board without any components on it. The former, PCB assembly, is a more intricate process where the board is populated with components that have to be carefully soldered onto the board. This difference highlights why PCB assembly can be a complex process.

Unlike the prototyping phase, the PCB assembly phase is generally a step towards producing your printed circuit board on a large scale. Hence, despite successful prototyping, you must optimize your PCB design for manufacturability. The PCB assembly process can include technical engineers' fees to hand-assemble the components or fees for the automated assembly line. Both the methods can turn expensive and result in massive reruns if the PCB is not ready for manufacturability.

Understanding the PCB assembly process in detail can help you gain an informed perspective into how your design must be prepared for the assembly process to avoid reruns and cost overruns.

The PCB Assembly Process

Depending on the PCB assembly partner you have and the automated or hand-assembled process you are going for, the sequence or the number of touchpoints in the process may change. However, the following sequence is a reliable structure of the PCB assembly process that gives you insights into how you can expect the entire process to unfold:

1. Stencilling the Solder Paste for Accuracy

A solder paste is prepared using tin, silver, and copper. It is mixed with a chemical named 'flux' that ensures the paste will melt and get linked with the board. By the end of this process, the solder paste is ready as a grey-coloured substance prepared to be applied.

An automated assembly process will have a holder that keeps the printed circuit board in one place and another component that uniformly applies the solder paste over it. It is critical to apply the solder paste in the same spaces, or the PCB's performance will become unpredictable. Stencils mitigate this risk to a large extent.

Once you remove the stencil, only the solder paste rests in the places where it is required.

2. Pick and Place

As the name dictates, the process involves picking a printed circuit board and placing the SMD components on it. SMDs form most of the non-connecting components on the printed circuit board, and it is essential to accurately place them in their designated spots.

In an automated PCB assembly process, the printed circuit board with the solder paste applied is picked up from the earlier-mentioned process and then brought to the pick & place step. SMDs are used at the programmed spots, and the process is over within minutes. This process' culmination is a significant step forward from the manual installation of SMDs, which was done for higher accuracy. However, it also resulted in sub-standard results. With programmed devices, it can become easier to deploy the SMDs with the same accuracy but for a larger PCB batch with greater consistency.

3. Solidifying the Solder Paste

If you remember the initial step, you would be able to recall the fact that the soldering paste gets heated and can be cooled to create solder joints. This property is exploited in this very step.

The printed circuit board with all the SMDs and the soldering paste is shifted to a conveyor belt which takes it to a reflow oven. The solder paste gets heated to temperatures north of 250o C and results in a molten form of the soldering paste. As the SMDs and the soldering paste form a soldered joint, the same conveyor belt takes the PCB to a cooling station. As the PCBs get cooled, the SMD and the soldering paste get unified with the solder joint.

The process becomes quite complex with each added layer in the printed circuit board. For instance – two layers would require separate stencils and reflow processes, as the configuration of components on both sides will be distinct.

4. Optical or X-Ray Inspection for Quality Control

It might seem that once the SMDs have been installed onto the soldering paste, the PCB assembly process must reach its conclusion. In reality, this process is yet to undergo a rigorous quality control step.

Even though conveyor belts are engineered with sophistication, even a minute disturbance in the second stage, where the SMDs are placed on top of the soldering paste, can result in malfunctioning connections. Such connections can lead to sub-optimal power transmission, short circuits, or even an absolute absence of a connection.

An Automatic Optical Inspection machine can be used for quality control here. It takes camera data from different angles and detects the PCBs that do not meet the connections' standard. For more complex components like BGAs and layered PCBs, an X-Ray machine is used to perform a similar analysis.

5. Component Installation Using a Through-Hole Process

While SMDs are generally the most common components, other components often fall under the category of through-hole components. These components are installed using through-holes and transmit power from one side of the board to another. Using the conventional soldering paste will not work as the paste will flow across the through-holes.

Hence, these components are either run through a manual soldering process or a wave-soldering process. Such approaches generally do not work for PCBs with more than one layer.

6. Functional Testing

A functional test puts the PCB through circumstances designed to replicate its use-case. Through several such tests, its voltage, direction of flow, and other performance parameters are measured using testers, power connections, and sensors. PCBs that do not meet the performance parameters are generally discarded or sent into rework.

In Conclusion

PCB assembly can be a complex process, even for simple PCB designs. This is the crucial reason why a manufacturer producing prototypes or working just on manufacturing circuit boards cannot automatically get equipped to execute the PCB assembly process. It usually requires a dedicated assembly line, access to specialized knowledge, and experience of having produced PCBs that do not require rework.

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