PCB assembly technology. Surface-mount technology (SMT)

Surface-mount technology (SMT) is the most widely used method of assembling electronic modules, which reduces cost, and ensures the high quality of the final product. In this video, we will show you, step by step, the automatic SMT process, as well as the additional control steps and automated selective soldering of through-hole components, using an example of contract manufacturing, as we do at Rezonit EMS production. A PCBA order starts with a range of preparation procedures. Processing and handling teams analyze the customer’s data, and prepare internal design data for PCBA production, to resolve all questions and details with the customer’s designers. Electronic components pass through incoming control at the warehouse for quality and quantity.

The components are kept in the ESD area, within a controlled environment. Components that require special conditions are stored in dry storage cabinets. In accordance with the production schedule, components are transferred to the PCBA sites, where they are installed in feeders prior to starting the order. PCB’s for automatic SMT assembly can be arranged in panels, for assembly convenience and optimization. These panels have fiducial marks and process keep-out zones or “fields” for smooth transportation along the entire automatic assembly line.

Each board is 02:09 - labelled with an individual code to track the product through all stages of production and quality control. All product data is integrated into a cohesive manufacturing management system in accordance with the “smart manufacturing” concept. Screen-printing is the main method of applying solder paste to printed circuit boards in mass and medium-scale production. Stencils are made from high-quality stainless steel by the method of laser cutting. The stencil apertures are designed to follow the pattern and location of the pads on the PCB.

It is important to obtain the right apertures and stencil thickness to ensure the 03:01 - correct volume of solder paste is applied to achieve the desired quality of soldered joint. Solder paste is a powdered solder alloy combined with a flux, plus some minor additives. The flux formula helps to protect the contact surfaces from oxidation, and has tacky properties that ensure components stay in position until soldering. Solder paste is applied onto the surface of the pads by automatic screen printers. The printers are equipped with a machine vision system, which ensures the precise alignment of the stencil with the board.

The paste is pressed with a special squeegee 04:01 - through the apertures of the stencil onto the contact pads of the PCB. The quality of the application of soldering paste is controlled by a 3D optical inspection machine, which evaluates the volume, and accuracy of the solder paste application deposits on the PCB contact pads, as well as other possible defects. SMD placement machines arrange components on the PCB in accordance with the design documentation, using a pre-written program. Before the assembly process starts, the machine’s camera determines the coordinates of the reference marks on the panel for correct positioning on the PCB’s contact pads. State-of-the-art equipment provides for PCB assembly using the widest range of components – from the smallest chips, to large IC’s, and other complex components with a small lead pitch, connectors, et cetera.

05:34 - The printed circuit board with installed components moves along the conveyor to the convection reflow solder oven. These state-of-the-art reflow ovens have from 6 to 14 zones, each with a different temperature control for uniform and smooth heating and cooling of the PCB. All ovens are able to provide soldering both with leaded or lead-free technologies. The automatic optical inspection system controls product quality after soldering. These machines are able to control many parameters: the absence of components, deviations in component positioning, the polarity, the bridging between leads, the coplanarity, and other parameters.

06:36 - AOI also evaluates the quality of soldered connections, the height and width of the solder fillet, as well as compliance with local and IPC standards. The boards on which defects were detected are sorted into a separate magazine of the automatic loader, and passed on to the operator. When scanning the PCB’s barcode, the inspection results are displayed on the screen. The controller evaluates the quality of the soldered connections and, if defects are detected, the board is passed on to a reworking station. SPC– Statistical process control – is a tool for managing the products quality immediately in the production process.

Quality data is collected and analysed by a special 07:52 - consolidated software that helps process engineers to improve the current process. The system stores and analyzes the most detailed product characteristics, including images, 3D models, and measurement results. The LINK function allows for comparing data from the SPI and AOI systems to conduct parallel analysis. This makes it possible to identify and eliminate the causes of defects. The quality of soldered components like BGA, LGA, CSP, or any other Bottom Terminated Components (BTC), cannot be determined with optical systems.

Boards with these components 08:51 - are checked at the x-ray inspection machine. An X-ray allows us to control the standard parameters of soldering quality, evaluate the number of voids, check the integrity of the internal PCB conductors and components, and, if necessary, get high- quality images. After the SMD component’s assembly, the THT components are installed, and selectively soldered automatically. Compared with classic wave soldering, in this technology, not all of the underside of the board with components comes into contact with the solder, but certain parts of the PCB individually, one at a time, those that are directly subject to soldering. This method allows for the high-quality assembly of complex double-sided boards with high component density.

09:57 - A drop-jet fluxing device moves under the lower surface of the PCB and applies the flux only where necessary. Soldering takes place in an inert nitrogen environment, which significantly improves the quality of the process. Finished electronic modules pass through the final quality control stage. The operator performs visual quality control of each module using state-of-the-art inspection equipment, including stereo microscopes. The finished modules are packed in a protective antistatic film, and sent to the customer. .