SMT stands for Surface Mount Technology, which is a method of placing electronic components onto a printed circuit board (PCB). In contrast to through hole technology, which involves inserting components into holes drilled into the PCB, SMT components are placed directly onto the surface of the board. In this blog, you'll know everything about Surface Mount Technology.
Surface mount technology (SMT) is a part of the electronic assembly that deals with the mounting of electronic components to the surface of a PCB. In this method, the components are mounted directly on the surface of the PCB. However, it increases the risk of errors because of the denser packing of the PCB board. Electronic components mounted this way are called surface-mounted devices (SMD).
Unlike conventional assembly, SMT does not require components to be inserted through holes, rather components are soldered onto the board directly through reflow soldering.
SMT was first developed and applied by IBM to build small-scale computers in the 1960s, thereby becoming a replacement for its predecessor, Through-Hole Technology and was broadly used in the 1980s. By the 1990s, they were used in most of all high-tech printed circuit assemblies. Conventional electronic components were redesigned to include metal tabs or end caps that could be attached directly to the board surface. This replaced typical wire leads which needed to pass through drilled holes. SMT led to much smaller components and enabled component placement on both sides of the board. Surface mounting enables a higher degree of automation minimizing labor costs and expanding production rates that results in the development advanced of boards.
SMT has proven beneficial for PCB assembly (PCBA), PCB manufacturing, and electronics production in many ways including these:
Allows for smaller components
SMT process encourages increased automation
Maximum flexibility in building PCBs
Improved reliability and performance
Reduced manual intervention for component placement
Smaller, lighter boards
Ease of PCB assembly, using both sides of the board without the hole limitations that exist in the conventional method
Can co-exist with through-hole components, even on the same board
Increased density i.e more SMD components in the same space, or the same number of components in a much smaller frame
Low Cost of materials
Simplifies the production process and reduces the production cost.
Conversely, disadvantages of SMT to electronic manufacturing include:
Making SMT PCB Prototype or Small Scale Production is Expensive.
SMT PCB Assembly Line Requires Huge Investment, since most of the SMT Equipment such as Reflow Oven, Pick and Place Machine, Solder Paste Screen Printer and even Hot Air SMD Rework Station are Expensive.
Components can be easily dropped or damaged when installed.
Most SMD Components are Small and have Numerous Solder Joints, it get very difficult during inspection.
Technical complexity requires high training and learning costs.
The difference between SMD and SMT is that SMD (surface mount device) refers to an electronic component mounted on a PCB. In contrast, SMT (surface mount technology) relates to the method used to place electronic components on a printed circuit board.
SMT is the process in the technology, while SMD is the device involved in the technology. SMT is the technology that uses the method of directly placing and soldering electronic components on a PCB. These components are also sometimes called surface mount devices or SMDs. They are designed to be mounted on a printed circuit board (PCB).
SMDs make for devices produced faster, with more flexibility and less cost, without sacrificing functionality. They promise more functionality because smaller components allow for more circuits on small board space. This miniaturization is the major feature of SMD. Note: Available from: https://history-computer.com/smt-surface-mount-technology/
In summary, SMT is a method of electronic component assembly that involves mounting components directly onto the surface of a PCB, while SMD refers to the actual electronic components that are used in SMT assembly.
|Non-planar HASL finish most common.||Planar finish (ENIG, Immersion Silver, OSP.)|
|Hole required for component lead insertion.||Components mount to surface pads, no hole.|
|2-sided assembly rare.||2-sided assembly common.|
|Component lead spacing typically 0.100” or greater.||Component lead spacing 0.0157” (0.0197” common.)|
|Manual assembly.||Automated assembly.|
|Soldering manual or automated.||Soldering typically automated.|
|Stencil not required.||Stencil required unless small lot, simple PCB.|
|Vias in pads not possible.||Vias in pads possible.|
|Standard temperature (130C Tg) laminate.||High-temperature (170C Tg) laminate.|
|Through-hole test points.||Through-hole or SMT test points.|
|Lower component and circuit density.||Greatly increased component density.|
|Larger PCB footprint.||Minimal PCB footprint.|
|Rework relatively simple.||Some rework more involved.|
|Moderate warp and twist tolerable.||Warp and twist more critical for assembly.|
|Fiducial pads not required for component placement.||Fiducial pads required for automated pick-and-place equipment.|
Stability of the component when exposed to external stress
Ease of thermal management/ heat dissipation
Availability of the part and its alternative
Cost-effectiveness of assembly
High performance and life-span of the package
Facilitate rework in case of board failure
Surface mount technology is a widely used method for manufacturing electronic circuits in which components are mounted directly onto the surface of printed circuit boards (PCBs). Here are the general steps involved in the SMT manufacturing process:
1. Material preparation and examination
Prepare the SMC and PCB and exam if there’s any flaws. The PCB normally has flat, usually tin-lead, silver, or gold plated copper pads without holes, called solder pads.
2. Stencil preparation
Stencil is used to provide fixed position for solder paste printing. It’s produced according to the designed positions of solder pads on PCB.
3. Solder paste printing
Solder paste, usually a mixture of flux and tin, is used to connect the SMC and solder pads on PCB. It’s applied to PCB with the stencil using a squeegee on a angle range from 45°-60°.
4. SMC placement
The printed PCB then proceed to the pick-and-place machines, where they are carried on a conveyor belt and the electronic components are placed on them.
5. Reflow soldering
Soldering oven: after SMC was placed, the boards are conveyed into the reflow soldering oven.
Pre-heat zone: the first zone in oven is a pre-heat zone, where the temperature of the board and all the components is raised simultaneously and gradually. Temperature ramp up rate in this section is 1.0℃-2.0℃ per second until it reaches 140℃-160℃.
Soak zone: the boards will be kept in this zone on temperature from 140℃-160℃ for 60-90 seconds.
Reflow zone: the boards then enter a zone where the temperature ramp up at 1.0℃-2.0℃ per second to the peak of 210℃-230℃ to melt the tin in the solder paste, bonding the component leads to the pads on the PCB. The surface tension of the molten solder helps keep the components in place.
Cooling zone: a section to ensure solder freezes at exit of heating zone to avoid joint defect.
5. Clean and inspection
Clean the boards after soldering, and check if there’s any flaws. Rework or repair the defects and store the products. Common equipment related to SMT include magnifying lens, AOI (Automated Optical Inspection), flying probe tester, X-ray machine, etc.
When applying surface-mount components onto a board, you have to use great care. Soldering such small items is an arduous and delicate task, so specialized equipment is necessary if you’d like to do so. Application is possible in different ways, as was done in the past. However, manufacturing them in this way is done due to the significant cost savings associated with the process.
Surface-mount technology is a welcome process that has incorporated incredible benefits to many different areas of life. This new technology has afforded the world to propel forward in ways that were previously impossible.
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