Steps in PCB Assembly Process

Steps in PCB Assembly Process

May 28,2018

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.

Some Basics You Must Know

To better understand the PCB assembly process, you must have a clear understanding of the most basic unit of the PCB- the base. The base of the PCB consists of several layers such as these.

  • Substrates: These are the specialized materials that conduct electricity at a minimal level. The commonly used substrates that serve as an insulating layer between two conducting copper layers are fluorine-series resin, PPO or PPE resin and modified epoxy resin, Fluorine-series dielectric substrate, PTFE, among others.
  • Copper: A thin layer of copper foil is added to increase the thermal resistance and current-carrying capacity of the PCBs.
  • Solder Mask: Solder paste, which is usually green in color, is applied to insulate copper traces from other conductive materials.
  • Silkscreen: The final layer of PCB is silkscreen, which helps provide text designators for the components. Silkscreen layer helps identify test points, part numbers, warning symbols, logos, and manufacturer marks.

The basic layers aforementioned remain almost the same for all the types of PCBs. The only thing that varies across rigid, flexible, metal core, surface mount or thru-hole PCBs is the usage of substrates. The manufacturers choose the substrates after considering the applications.

Design for Manufacturability (DFM) Check

Before the actual assembly process, manufacturers thoroughly check the PCB design file to check the functionality, and manufacturability. This stage, which is termed as DFM, checks design specifications of a PCB, whilst analysing any missing, redundant or potentially problematic features. The stage helps detect design errors, and allows the designers to instantly clear all the flaws, which in turn leads to successful production.

Starting of Actual PCB Assembly Process

Now begins the actual PCB assembly process as described below.
  1. 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.
  2. 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.
  3. Reflow: 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.
  4. Inspection: 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.
  5. 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.
  6. 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.
  7. Cleaning: 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.

Although the PCB assembly process steps remain almost the same for all types of boards, there are yet some changes when it comes to THT Assembly, SMT Assembly and Mixed Technology. Let us wind up the discussion by listing the assembly steps of THT, SMT and mixed boards.

Steps in Thru-Hole Technology (THT) Assembly

  1. Components Placement
  2. Inspection and Correction
  3. Wave Soldering

Steps in Surface Mount (SMT) Assembly

  1. Solder Paste Printing
  2. Components Mounting
  3. Reflow Soldering

Steps in Mixed Assembly

  • Single Sided Mixed Assembly
    1. Solder Paste printing
    2. SMD parts Placement
    3. Reflow Soldering
    4. THT component Placement
    5. Wave Soldering
  • One Side SMT & One Side THT
    1. Surface mount adhesives
    2. SMD parts placements
    3. Solidification
    4. Flipping
    5. THT Component Placement
    6. Wave Soldering
  • Double Side Mixed Assembly
    1. Solder paste printing
    2. SMD parts placement
    3. Reflow
    4. Flipping
    5. SMT adhesive
    6. SMD parts placement
    7. Solidification
    8. Flipping
    9. THT Component Placement
    10. Wave Soldering

Now, you must have gained a good understanding of the basic steps involved in the PCB assembly process. However, as per the complexities of the project, these steps are also subjected to changes. The modifications are more evident when it comes to customized PCBs. Hence, it is always advisable to partner with only those leading PCB fabricators to get your PCB assembly projects done rightly as per the specifications. When you search for such a partner, the name of Sierra Assembly always comes top in the list. The company boasts several years of experience in assembling high quality boards as per the needs and expectations of the clients.