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Updated by Gopal Radadia on Jun 07, 2018
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Steps in PCB Assembly Process

PCB assembly is a lengthy process involving several automated and manual steps. Each of these steps must be executed properly by paying utmost attention to detail. Minor error in any of the step in the assembly process will lead to the failure of the final assembly. This post aims to familiarize you with the various processes involved in PCB assembly. The post will be extremely useful for those who are new to the PCB fabrication industry.



Solder Paste Application

Firstly, solder paste, which is a paste of small grains of solder mixed with flux, is applied to the board. For this application, most PCB manufacturers use stencils (available in several sizes, and shapes ad in par with the specifications), which allow the right applications of solder paste only to certain parts of the board in the right amounts.


Placing of Components

Unlike in the past, this stage of PCB assembly process is now completely automated. The pick and place of components such as surface mount components, which were once done manually, are now executed by robotic pick and place machines. These machines accurately place the components to the pre-planned areas of the board.



Now, solder paste and all the surface mount components are in place. What is the next step? Solidification of the solder paste to the right specification is critical to properly adhere PCB components onto it. Here comes this relevant part of the PCB assembly process — the reflow soldering. To accomplish this, the assembly with the solder paste, and the components on it is passed through a conveyor belt, which moves through an industrial grade reflow oven. The heaters in the oven melt the solder in the solder paste. Once this melting is done, the assembly is again moved in the conveyor belt, and exposed to a series of cooler heaters. The purpose of these coolers is to cool down the melted solder, and to attain a solidified state.



After the reflow process, the PCB is subjected to inspection to check for its functionality. This stage helps identify poor quality connections, misplaced components, and shorts due to the consecutive movement of the board during the reflow process. The PCB manufacturers employ several inspection steps such as manual checks, automatic optical inspection, and X-ray inspection to examine the functionality of the board, recognize lower-quality solder, and identify any potentially hidden problems. Once the inspection is done, the assembling team makes the crucial decision. The boards which are identified with several functionality errors are usually scrapped. On the other hand, if there are minor errors, the board is again sent for reworking.


Through-hole Component Insertion

Some types of PCBs require the insertion of through-hole components in conjunction with the usual SMD components. This stage is devoted for such component insertion. For this, plated-through hole is created with the help of which PCB components pass signals from one side of the board to the other. The PCB through-hole insertion usually utilizes manual or wave soldering to achieve the results.


Final Inspection

It is now the time for a second level of inspection. Here, the assembled board is tested for its functionality or the PCB is thoroughly inspected to monitor its electrical characteristics including voltage, current or signal output. Today’s manufacturers utilize several advanced testing equipment that help determining the success or failure of the finished boards.



As the soldering process leaves some amount of flux residues in PCBs, it is crucial to clean the assembly thoroughly before shipping the final board to a customer. For this, the PCBs are washed in deionized water. After the cleaning process, the board is dried thoroughly utilizing compressed air. The PCB assembly is now ready for the customers’ check and inspection.