Complete PCB SMT Assembly Process Flow

Modern technology has revolutionized the way printed surface boards are built. From the use of the traditional through-hole technology components, the area of electronics has shifted towards the use of surface mount technology, commonly referred to as SMT. The surface mount technology has been made successful by the introduction of surface mount devices which is a technology of building components that do not have lead terminals and in place utilize pads that are mounted on the surface of the printed circuit board before soldering occurs. It is good to remember that in THT, the components have leads that are fitted on the holes drilled through the PCB before soldering can occur. Even though DIP assembly has the formation of a strong solder joint, SMT technology is becoming the most popular because it allows the formation of miniature, compact, dense, and complex printed circuit boards. The introduction of the SMT assembly was based on the reduction of the cost of manufacturing and also ensuring that there is a better utilization of the space allocated on the printed circuit board surface. In this article, we are going to focus on the SMT assembly process flow, looking into details of the steps that are involved in the assembly line, what those steps are and why they are significant for the process. Let us get started!

Realize SMT manufacturing step by step

SMT assembly is a process flow that is structured to work automatically during the manufacturing of the PCBA boards and it makes use of properly designed diagrams, well-laid-down machine structures, and automatic placement equipment. In order to quickly achieve mass PCBA production, numerous automated equipment such as connection tables, placement machines, printing machines, reflow soldering, AOI, etc. is used in this process. Therefore, it is good to note that the PCB SMT assembly process does not only involve picking, placing, heating, and soldering of the circuit board components, but involves so many arrays of productions. Below are the step-by-step processes involved in the SMT assembly of the printed circuit boards.

Preparations before the start of the SMT assembly process

Circuit Design

This is the starting point before the start of all electronic projects. The design work can be done by yourself or handed over to a turnkey PCBA factory with design capabilities. In this significant stage, the initial layouts of the printed circuit boards are worked on using designated EDA software which includes KiCAD, EagleCAD, EasyEDA, etc.

Prepare Design Documents

After completing the schematic and PCB circuit design through the PCB design tools, the manufacturing files need to be generated for review by the PCBA fabricator. The files are very vital in ensuring that the SMT process flow runs smoothly and that some machines can be set to function automatically. Below are some of the very important files that every printed circuit board designer should generate to have the PCB or PCBA successfully manufactured.

Gerber files: These are files that carry out relevant information about your printed circuit boards such as their layout, spacing, tracks, and layer information.
PNP/CPL Files: PNP stands for pick & place while CPL stands for component placement list. The files are very important in the SMT assembly because they will tell the machine where each part should be placed on the board.
BOM Files: BOM stands for Bill of Materials. This file contains the list of all the electronic components that are contained in the printed circuit board that is being designed.

Every step of SMT in PCBA processing factory

Step 1: Material inspection and stencil preparation

This is a critical process that determines the quality of the entire project. The step will reveal if there are flows that exist in the designed board. In the process of material evaluation, the stencil is not left behind. The stencil is what provides a position that is fixed for the solder pad in the surface mount technology printed circuit board design.

Step 2: Printing of the Solder Paste

Solder paste is a mixture of tin and flax. Solder paste printing involves the connection of the solder pad and SMC to the printed circuit board using squeegees and the stencil. The described is the most used method of printing the solder paste. FS Technologies uses a jet printing process, which is the most popular method for high volume PCB assembly.

Step 3: Parts Placement

After carrying out printing plus SPI inspection on the PCBs, they move to the SMD components placement step. Many manufacturers make use of the Pick and Place technology which is a technology that makes use of the nozzle and gripper to place components in exact designated locations as per the design at a very high speed. After all the components have been placed, the board is taken through a pre-inspection process to confirm that there are no mistakes made during the process.

Step 4: Reflow Soldering

From mounting and inspection of the components, the next step is soldering of these components through a process known as reflow soldering. This involves the use of machines to form important electrical solder joints between the SMD parts and the printed circuit board by heating the whole assembly to a specified temperature. Below are the stages involved in a reflow soldering machine.

Soldering Oven: This is the first stage of the circuit board passing through the soldering furnace, and the PCB will be sent here through the docking station after placement.

Pre-heat Zone: This is the first part of the reflow soldering machine oven. Here the temperature of the PCB and the parts goes up at the same time and in a gradual manner. The standard temperature variance is in the range of 1.0 to 2.0 degrees Celcius until it gets to a maximum of 140 to 160 degrees Celcius in less than 90 seconds.

Soaking Phase: Here is the phase where the board is soaked in the reflow oven.

Reflow Zone: This is the cone where the temperature of the printed circuit board is increased to a pick range of 210 to 2300C. The purpose of the process is for melting the tin that is in the PCB’s solder paste to bind the PCB components to the board.

Cooling Zone: Here the PCB is cooled while exiting the heating zone to ensure that joint defects are prevented from occurring.

Step 5: Post-Reflow Inspection

From the process of reflow soldering, the board is taken through a specialized post-inspection to ensure that there was no single mistake done in the reflow soldering step. The process also ensures that the solder paste used is adequate. The commonly sued method is the AOI inspection.

Step 6: Finished Board Cleaning

Here the finished board is taken through a PCB cleaning process to remove dirt, debris, residual flux, and any other foreign materials from the printed circuit boards. The best chemical used for the process is isopropyl alcohol.

Summary of PCB SMT process

SMT stands for Surface Mount Technology which is a technology that makes use of Surface Mount Devices in the building of miniature printed circuit boards.
SMT Assembly is where the technology of automation utilizes the SMD components in the building of the printed surface boards in a systematic manner.
Six steps are involved in the process of SMT assembly, and electronics manufacturers need to ensure that the design files are correct.
Gerber, BOM, and pick & place files are very significant files that ensure that the process is achieved successfully.
Examination of materials, the printing of the solder paste, placement of parts, reflow soldering, final inspection, and cleaning of the finished PCB are the other steps that complete the process.