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What is SMT technology? Why should OEMs care about it?
After reflow soldering, a 3D automated optical inspection machine (AOI) is typically used for final inspection. This inspection checks the quality of the solder joints and ensures that no errors occurred during the SMT process. Machines are preferred for this inspection because they are faster and more accurate than manual inspection.
What is SMT?
Surface Mount Technology (SMT) is the process of mounting components onto the surface of a printed circuit board. For the vast majority of electronic products, the components are specifically designed to be mounted directly onto the circuit board rather than hard-wired.
Surface Mount Technology (SMT) has been widely adopted since the 1980s because it allows for automation, saving manufacturers and customers both time and cost. In the past, components were connected via through-hole soldering between circuit boards, a process that required manual labor and was prone to human error. In contrast, compared to the traditional through-hole method, the SMT process not only allows for automation but also better adapts to future technological advancements.
SMT also offers several other advantages, including higher component density, the ability to use smaller components, and better performance under stress. Furthermore, SMT allows for components to be mounted on either side of the board, further optimizing space and design flexibility.
What is a typical SMT process?
The Surface Mount Technology (SMT) process involves three main steps: solder paste printing, component placement, and reflow soldering.
Solder Paste Printing
Solder paste printing is a step performed by a machine to ensure accuracy and speed. In this stage, a printer uses a stencil and squeegee to apply solder paste to the pre-made template of the printed circuit board (PCB). The solder paste is typically a mixture of flux and tin, used to connect the surface mount components (SMCs) to the pads on the PCB. It is crucial that each pad receives the correct amount of solder paste to establish a strong connection when the solder melts in the reflow oven.
Quality control is critical at this stage, as any defects in the solder paste printing can lead to problems in subsequent processes. Therefore, the design of the stencil is crucial, and the assembly team must ensure that the process is repeatable and stable. To achieve this, most solder paste printers can include automated inspections, and sometimes an external machine with 3D technology is used for a more thorough inspection, checking the amount of solder paste on each pad, not just the printed area.
Component Placement
After inspection is passed, the PCB moves to the component placement stage. In this stage, each component is picked from its packaging using vacuum nozzles or grippers and placed at its programmed location on the PCB. The machines used in this process are highly accurate, capable of placing 80,000 components per hour.
Once all components are placed, an inspection is performed to ensure correct placement. This step is crucial, as any undetected placement errors can lead to costly and time-consuming rework if the components are soldered incorrectly.
Reflow Soldering
Reflow soldering. In this stage, the PCB is placed into a reflow soldering machine (commonly known as a reflow oven). Heat transforms the solder paste into solder, creating an electrical connection between the components and the PCB. Precise temperature control is crucial; too high a temperature may damage the components or board, while too low a temperature may hinder proper connection.
For good results, the PCB is placed on a conveyor belt, gradually heated through several zones, and then passed through a cooling zone. The PCB must dwell in each zone for the appropriate time and be fully cooled before handling to prevent warping.
After reflow soldering, a final inspection is typically performed using a 3D Automated Optical Inspection machine (AOI). This inspection checks the quality of the solder joints and ensures that no errors occurred during the SMT process. Machines are preferred for this inspection because they are faster and more accurate than manual inspection.
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