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What Is SMT Assembly & How Does It Work?
- Authors
- Name
- Geeks Kai
- @KaiGeeks
Surface Mount Technology or SMT is a modern methodology of electronic component assembly, where components are mounted onto the surface of a printed circuit board, not by means of holes but rather directly on the board surface, in contrast with traditional through-hole technology. It revolutionised the manufacturing of electronics to include smaller and more compact gadgets, greatly improved production efficiency, and overall lower production costs.
SMT Assembly Explained:
SMT assembly depends a lot on good design for manufacture or DFM. This will include things like adequate pad sizes, proper spacing between components, thermal relief patterns for large pads, and fiducial marks to align machines. The PCB design must also consider thermal management during reflow when different-sized components heat up and cool down at different rates. The process of SMT assembly includes environmental management in a very important way. In fact, the whole process is done within a controlled environment where temperature and humidity, with dust, are controlled. Operators usually wear anti-static equipment to avoid damage to sensitive components, and materials are stored and handled carefully to avoid moisture absorption that can cause problems during reflow.
SMT Assembly - The Process:
SMT assembly begins with the bare PCB, which has specially designed pads and traces on its surface. These pads are meant for the various components to stand on and thus get soldered onto the board. The board itself must undergo several preparatory steps before mounting. Application of solder paste to these pads via the stencil printing process is first.
Solder paste is the most important part of SMT assembly, which is basically tiny spheres of solder metal suspended in a flux medium. The paste is applied using a metal stencil-often called a solder screen-that has openings corresponding to the pad locations on the PCB. The stencil is then placed over the board, and a squeegee moves across it, forcing solder paste through the openings onto the pads below. It requires special care in the quantity and positioning of solder paste since too much or too little may result in defectively finished products.
SMT Assembly - Component Placement:
Following solder paste application, the next process is component placement. This is generally done with automated pick-and-place machines, which are just amazing pieces of equipment that can place thousands of components per hour with incredible accuracy. These machines use vacuum nozzles to pick components from reels, trays, or tubes and place them onto the corresponding pads on the PCB. Modern pick-and-place machines use advanced vision systems for placing with high accuracy and can place components as small as 0.4 x 0.2 mm. The components used in SMT assembly are available for use in different package types: a few common ones are resistors and capacitors, which come in chip packages but referred to based on their dimensions (i.e., 0402, 0603, 0805); integrated circuits in quad flat packages, normally abbreviated as QFP; ball grid arrays, normally referred to as BGA; and small outline integrated circuits also known as SOIC. Each requires special handling and placing, along with programming accordingly of the pick-and-place machines.
SMT Assembly - Creating Connections:
After the parts are placed, the full assembly goes under a reflow oven where the magic happens in permanent attachment. The reflow oven takes the board through a very precisely controlled temperature profile, frequently comprising four zones: preheat, soak, reflow, and cooling. During the preheat stage of the profile, the temperature of the board is increased at a gradual rate to avoid thermal shock. The soak phase allows temperatures to equalize across all components and activates the flux in the solder paste. The reflow zone brings the temperature above the melting point of the solder-usually about 220°C for lead-free solder-causing the solder paste to become liquid and form proper solder joints. Surface tension in the molten solder helps to align the components precisely on their pads, a phenomenon known as self-alignment. The cooling zone finally allows the solder to solidify in a controlled manner, thus forming strong and reliable connections.
SMT Assembly - Quality Control:
Quality control is a vital part of SMT assembly. Boards are usually AOI or X-ray inspected for BGAs and other hidden connections post-reflow. All these systems can detect defects related to missing components, misplaced components, insufficient solder, excessive solder (bridging), cold joints, among others. Common defects in SMT assembly are tombstoning, where one end of a component lifts off the board, solder bridges between adjacent pads, and voids within the solder joints. The process is more complicated for boards with components on both sides. Normally, the components are mounted on the bottom side first, using special adhesives to hold them in place when the board is inverted for top-side assembly. The reflow profile has to be carefully controlled to avoid having already-soldered components fall off during the second reflow cycle.
SMT Assembly - SMT plus Through-Hole Components:
Some assemblies require both SMT and through-hole components. The usual procedure in these assemblies is that the SMT components go first into the PCB, then wave soldering or selective soldering for through-hole components follows. Such a process requires a good layout of the PCB in order not to expose SMT components to the wave soldering process.
In recent years, environmental considerations have had a significant impact on SMT assembly. The move to lead-free solder, mandated by regulations such as the EU's RoHS directive, has forced the use of higher reflow temperatures and more precise process control. Lead-free solders are commonly SAC alloys (tin-silver-copper) and have peak temperatures around 20-30°C higher than the traditional tin-lead solder.